UT Testing-Self Study Notes- Rev1

Preparatory Notes for 

ASNT NDT Level III Examination 

- Ultrasonic Testing, UT 

My pre-exam self study note - 2014 

Charlie Chong/ Fion Zhang



http://en.wikipedia.org/wiki/Greek_alphabet

Numerical Prefix 

• Micro - (µ) a prefix in the SI and other systems of units denoting a factor of 

10 -6 (one millionth) 

• Nano - a prefix in the SI and other systems of units denoting a factor of 10 -9 

(one billionth) 

• Pico - a prefix in the International System of Units (SI) denoting a factor of 

10 -12

Fion Zhang 

2014/July/31 

http://meilishouxihu.blog.163.com/

Contents: 

1. ASNT Level III Exam Topical Outline 

2. AE Codes and Standards 

■ ASTM. 

■ ASME V. 

3. Reading 01 

Introduction to UT by ndt-ed.org with thanks (always) 

1. Others reading. 

2. Addendum 1 – Equipment Calibrations 

3. Addendum 2 – Equations & Calculations. 

4. Addendum 3 – Questions & Answers I 

5. Addendum 4 – Questions & Answers II 


ASNT UT Level III Examination Topical Outline 

This examination is 4 hours in length, having 135 questions of equal value. 

1. Principles/Theory 

2. Equipment/Materials 

3. Techniques/Calibrations 

• Contact 

• Immersion 

• Comparison of contact and immersion methods 

• Remote monitoring 

• Calibration (electronic and functional) 

https://www.asnt.org/MajorSiteSections/Certification/ASNT%20NDT%20Level%20I 

II%20Program/NDT%20Level%20III%20Examinations 


4. Interpretation/Evaluations 

• Evaluation of base metal product forms 

• Evaluation of weldments 

• Evaluation of bonded structures 

• Variables affecting test results 

• Evaluation (general) 

5. Procedures 

• Specific applications 

• Codes/Standards/Specifications 

6. Safety and Health 


References 

1. Level III Study Guide: Ultrasonic Testing (2261) 

2. NDT Handbook: Volume 7, Ultrasonic Testing (147) 

3. Supplement to Recommended Practice No. SNT-TC-1A (Q&A Book) - 

Ultrasonic Testing Method (2028) 

4. Ultrasonics: Fundamentals, Technology, Applications (341) 

5. Refresher Course: ASNT offers a UT Refresher Course based on the Body 

of Knowledge outlined above. 

The number in parentheses following each reference is the ASNT catalog 

number. 


UT - Ultrasonic Testing 

Length: 4 hours Questions: 135 

1. Principles/Theory 

• Nature of sound waves 

• Modes of sound wave generation 

• Velocity, frequency, and wavelength of sound waves 

• Attenuation of sound waves 

• Acoustic impedance 

• Reflection 

• Refraction and mode conversion 

• Snell’s law and critical angles 

• Fresnel and Fraunhofer effects 


2. Equipment/Materials 

• Pulse/echo instrumentation 

• Digital thickness instrumentation 

• Transducer operation and theory 

• Transducer operation/manipulations 

• Resonance testing equipment 

• Couplants 

• Calibration blocks 

• Cables/connectors 

• Test specimen 

• Miscellaneous materials 


3. Techniques/Calibrations 

•Contact 

• Immersion 

• Comparison of contact and immersion methods 

• Remote monitoring 

• Calibration (electronic and functional) 


4. Interpretation/Evaluations 

• Evaluation of base metal product forms 

• Evaluation of weldments 

• Evaluation of bonded structures 

• Variables affecting test results 

• Evaluation (general) 

5. Procedures 

• Specific applications 

• Codes/Standards/Specifications 

Reference Catalog Number 

NDT Handbook, Second Edition: Volume 7, 

Ultrasonic Testing 132 

ASNT Level III Study Guide: Ultrasonic Testing 2261A 

Ultrasonics: Fundamentals, Technology, 

Applications 341 


ASME V Article Numbers: 

Gen Article 1 

RT Article 2 

Nil Article 3 

UT Article 4 for welds 

UT Article 5 for materials 

PT Article 6 

MT Article 7 

ET Article 8 

Visual Article 9 

LT Article 10 

AE Article 11 (FRP) /Article 12 (Metallic) / Article 13 (Continuous) 

Qualif. Article 14 

ACFM Article 15

ASTM/ AWS Standards 

• ASTM E494 – 10: Practice for Measuring Ultrasonic Velocity in Materials. 

• ASTM standard E-164, "Standard Practice for Contact Examination of 

Weldments“. 

• AWS Structural Welding Code, section 6. 

• Annual Book of the American Society of Testing and Materials, 

ASTM. Volume 03.03, Nondestructive Testing

Other Reading 

• http://techcorr.com/services/Inspection-and-Testing/Ultrasonic-Shear-Wave.cfm 

• http://www.cnde.iastate.edu/faacasr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultraprinciples.html 

• http://www.ndt.net/article/v05n09/berke/berke1.htm#0 

• http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.htm 

l&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_si 

de_menu=s2 

• https://www.nde-ed.org/GeneralResources/Glossary/letter/d.htm 

• http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/general/ 

• http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/ 

• http://www.olympus-ims.com/en/knowledge/ 

• http://wenku.baidu.com/view/3cf257781711cc7931b716e0.html 

• http://www.docin.com/p-148566003.html 

• http://www.studyblue.com/notes/note/n/ut-asnt-level-ii/deck/6278710


Study Note 1: 

Ultrasonic Testing 

Charlie Chong/ Fion Zhang 

Source: http://www.ndted.org/EducationResources/CommunityCollege/Ultra 

sonics/cc_ut_index.htm

Content: 

Section 1: Introduction 

1.1: Basic Principles of Ultrasonic Testing 

1.2: Advantages and Disadvantages 

1.3: Limitations

Content: Section 2: Physics of Ultrasound 

2.1: Wave Propagation 

2.2: Modes of Sound Wave Propagation 

2.3: Properties of Acoustic Plane Wave 

2.4: Wavelength and Defect Detection 

2.5: Sound Propagation in Elastic Materials 

2.6: Attenuation of Sound Waves 

2.7: Acoustic Impedance 

2.8: Reflection and Transmission Coefficients (Pressure) 

2.9: Refraction and Snell's Law 

2.10: Mode Conversion 

2.11: Signal-to-Noise Ratio 

2.12: Wave Interaction or Interference 

2.13: Inverse Square Rule/ Inverse Rule 

2.14: Resonance 

2.15 Measurement of Sound 

2.16 Practice Makes Perfect

Content: Section 3: Equipment & Transducers 

3.1: Piezoelectric Transducers 

3.2: Characteristics of Piezoelectric Transducers 

3.3: Radiated Fields of Ultrasonic Transducers 

3.4: Transducer Beam Spread 

3.5: Transducer Types 

3.6: Transducer Testing I 

3.7: Transducer Testing II 

3.8: Transducer Modeling 

3.9: Couplants 

3.10: Electromagnetic Acoustic Transducers (EMATs) 

Continues Next Page

3.11: Pulser-Receivers 

3.12: Tone Burst Generators In Research 

3.13: Arbitrary Function Generators 

3.14: Electrical Impedance Matching and Termination 

3.15: Data Presentation 

3.16: Error Analysis 

3.17: Transducer Quality Factor “Q” 

3.18: Testing Techniques 

3.19: UT Equipment Circuitry 

3.20: Further Reading on Sub-Section 3

Content: Section 4: Calibration Methods 

4.1: Calibration Methods 

4.2: The Calibrations 

4.2.1: Distance Amplitude Correction (DAC) 

4.2.2: Finding the probe index 

4.2.3: Checking the probe angle 

4.2.4: Calibration of shear waves for range V1 Block 

4.2.5: Dead Zone 

4.2.7: Transfer Correction 

4.2.8: Linearity Checks (Time Base/ Equipment Gain/ Vertical Gain) 

4.2.9: TCG-Time Correction Gain 

4.3: Curvature Correction 

4.4: Calibration References & Standards 

4.5: Exercises 

4.6: Video Time

Content: Section 5: Measurement Techniques 

5.1: Normal Beam Inspection 

5.2: Angle Beams 

5.3: Reflector Sizing 

5.4: Automated Scanning 

5.5: Precision Velocity Measurements 

5.6: Attenuation Measurements 

5.7: Spread Spectrum Ultrasonics 

5.8: Signal Processing Techniques 

5.9: Scanning Methods 

5.10: Scanning Patterns 

5.11: Pulse Repetition Rate and Penetration 

5.12: Interferences & Non Relevant Indications 

5.13: Entry Surface Variables 

5.14: The Concept of Effective Distance 

5.15: Exercises

Content: Section 6: Selected Applications & Techniques 

6.1: Defects & Discontinuities 

6.2: Rail Inspection 

6.3: Weldments (Welded Joints) 

6.4: Pipe & Tube 

6.5: Echo Dynamic 

6.6: Technique Sheets 

6.7: Material Properties-Elastic Modulus Measurements 

6.8: High Temperature Ultrasonic Testing 

6.9: TOFD Introduction

Content: Section 7: Reference Material 

7.1: UT Material Properties 

7.2: General References & Resources 

7.3: Video Time 

Content: Section 8: Ultrasonic Inspection Quizzes 

8.1: Ultrasonic Inspection Quizzes 

8.2: Online UT Quizzes





1.2: Advantages and Disadvantages 

1.3: Limitations


ULTRASONIC INSPECTION is a nondestructive method in which beams of 

high-frequency sound waves are introduced into materials for the detection of 

surface and subsurface flaws in the material. The sound waves travel through 

the material with some attendant loss of energy (attenuation) and are 

reflected at interfaces. The reflected beam is displayed and then analyzed to 

define the presence and location of flaws or discontinuities. The degree of 

reflection depends largely on the physical state of the materials forming the 

interface and to a lesser extent on the specific physical properties of the 

material.

For example, sound waves are almost completely reflected at metal/gas 

interfaces. Partial reflection occurs at metal/liquid or metal/solid interfaces, 

with the specific percentage of reflected energy depending mainly on the 

ratios of certain properties of the material on opposing sides of the interface. 

Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and 

other discontinuities that produce reflective interfaces can be easily detected. 

Inclusions and other in-homogeneities can also be detected by causing partial 

reflection or scattering of the ultrasonic waves or by producing some other 

detectable effect on the ultrasonic waves.

In ultrasonic testing, the reflected wave signal is transformed into an electrical 

signal by the transducer and is displayed on a screen. In the applet below, the 

reflected signal strength is displayed versus the time from signal generation to 

when a echo was received. Signal travel time can be directly related to the 

distance that the signal traveled. From the signal, information about the 

reflector location, size, orientation and other features can sometimes be 

gained.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ultrasoundInspection.swf

Basics of Ultrasonic Test- Contact Pulse Echo Method 

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultra-principles.html

Immersion Method- Figure below shows an immersion UT setup with CRT 

or computer screen display. IP indicates the initial pulse while FW and BW 

indicate the front and back wall of the specimen, respectively. 

Display / CRT 

Amplitude 

Water path 

Time / Distance

Basics of Ultrasonic Test- A-Scan

1.2: Source-1: The advantages of ultrasonic testing include 

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the 

advantages of ultrasonic inspection that are often cited include: 

• It is sensitive to both surface and subsurface discontinuities. 

• The depth of penetration for flaw detection or measurement is superior to 

other NDT methods. 

• Only single-sided access is needed when the pulse-echo technique is 

used. 

• It is highly accurate in determining reflector position and estimating size 

and shape. 

• Minimal part preparation is required. 

• Electronic equipment provides instantaneous results. 

• Detailed images can be produced with automated systems. 

• It has other uses, such as thickness measurement, in addition to flaw 

detection.

Source-2: The advantages of ultrasonic testing include 

• It can be used to determine mechanical properties and microstructure. 

• It can be used for imaging and microscopy. 

• It is portable and cost effective. 

• It can be used with all states of matter except plasma and vacuum. 

• It is not affected by optical density.

Source-3: Advantages and Disadvantages 

The principal advantages of ultrasonic inspection as compared to other 

methods for nondestructive inspection of metal parts are: 

• Superior penetrating power, which allows the detection of flaws deep in 

the part. Ultrasonic inspection is done routinely to thicknesses of a few 

meters on many types of parts and to thicknesses of about 6 m (20 ft) in 

the axial inspection of parts such as long steel shafts or rotor forgings 

• High sensitivity, permitting the detection of extremely small flaws 

• Greater accuracy than other nondestructive methods in determining the 

position of internal flaws, estimating their size, and characterizing their 

orientation, shape, and nature 

• Only one surface needs to be accessible

• Operation is electronic, which provides almost instantaneous indications of 

flaws. This makes the method suitable for immediate interpretation, 

automation, rapid scanning, in-line production monitoring, and process 

control. With most systems, a permanent record of inspection results can 

be made for future reference 

• Volumetric scanning ability, enabling the inspection of a volume of metal 

extending from front surface to back surface of a part 

• Nonhazardous to operations or to nearby personnel and has no effect on 

equipment and materials in the vicinity 

• Portability 

• Provides an output that can be processed digitally by a computer to 

characterize defects and to determine material properties

The disadvantages of ultrasonic inspection include the following: 

• Manual operation requires careful attention by experienced technicians. 

• Extensive technical knowledge is required for the development of 

inspection procedures. 

• Parts that are rough, irregular in shape, very small or thin, or not 

homogeneous are difficult to inspect. 

• Discontinuities that are present in a shallow layer immediately beneath the 

surface may not be detectable. 

• Couplants are needed to provide effective transfer of ultrasonic wave 

energy between transducers and parts being inspected. 

• Reference standards are needed, both for calibrating the equipment and 

for characterizing flaws.

1.3: Limitations (Disadvantages) 

As with all NDT methods, ultrasonic inspection also has its limitations, which 

include: 

• Surface must be accessible to transmit ultrasound. 

• Skill and training is more extensive than with some other methods. 

• It normally requires a coupling medium to promote the transfer of sound 

energy into the test specimen. 

• Materials that are rough, irregular in shape, very small, exceptionally thin 

or not homogeneous are difficult to inspect. 

• Cast iron and other coarse grained materials are difficult to inspect due to 

low sound transmission and high signal noise. 

• Linear defects oriented parallel to the sound beam may go undetected. 

• Reference standards are required for both equipment calibration and the 

characterization of flaws.

Section 2: Physics of Ultrasound 



Content: Section 2: Physics of Ultrasound 

2.0: Ultrasound Formula 












2.12: The Sound Fields- Dead / Fresnel & Fraunhofer Zones 



2.15 Measurement of Sound 



http://www.ndt-ed.org/GeneralResources/Calculator/calculator.htm

Ultrasonic Formula

Ultrasonic Formula

Parameters of Ultrasonic Waves


Ultrasonic testing is based on time-varying deformations or vibrations in 

materials, which is generally referred to as acoustics. All material substances 

are comprised of atoms, which may be forced into vibration motion about their 

equilibrium positions. Many different patterns of vibration motion exist at the 

atomic level, however, most are irrelevant to acoustics and ultrasonic testing. 

Acoustics is focused on particles that contain many atoms that move in 

unison to produce a mechanical wave. When a material is not stressed in 

tension or compression beyond its elastic limit, its individual particles perform 

elastic oscillations. When the particles of a medium are displaced from their 

equilibrium positions, internal (electrostatic) restoration forces arise. It is these 

elastic restoring forces between particles, combined with inertia of the 

particles, that leads to the oscillatory motions of the medium. 

Keywords: 

■ internal (electrostatic) restoration forces 

■ inertia of the particles

Acoustic Spectrum

Acoustic Spectrum

Acoustic Spectrum

Acoustic Wave – Node and Anti-Node 

The points where the two waves constantly cancel each other are called 

nodes, and the points of maximum amplitude between them, antinodes. 

http://www.physicsclassroom.com/Class/waves/u10l4c.cfm 

http://www.physicsclassroom.com/Class/waves/h4.gif

Acoustic Wave – Node and Anti-Node 

Formation of a standing wave by two waves from opposite directions

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.html

Q151 A point, line or surface of a vibration body marked by absolute or 

relative freedom from vibratory motion (momentarily?) is referred to as: 

a) a node 

b) an antinode 

c) rarefaction 

d) compression 



2.2.1 Modes of Ultrasound 

In solids, sound waves can propagate in four principle modes that are based 

on the way the particles oscillate. Sound can propagate as; 

• longitudinal waves, 

• shear waves, 

• surface waves, 

• and in thin materials as plate waves. 

Longitudinal and shear waves are the two modes of propagation most widely 

used in ultrasonic testing. The particle movement responsible for the 

propagation of longitudinal and shear waves is illustrated below.

2.2.2 Propagation & Polarization Vectors 

• Propagation Vector- The direction of wave propagation 

• Polarization Vector- The direction of particle motion

Longitudinal and shear waves

Longitudinal and shear waves- Defined the Vectors 




2.2.3 Longitudinal Wave 

Also Knows as: 

• longitudinal waves, 

• pressure wave 

• compressional waves. 

• density waves 

can be generated in (1) liquids, as well as (2) solids 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/longitudinal.swf

In longitudinal waves, the oscillations occur in the longitudinal direction or the 

direction of wave propagation. Since compressional and dilational forces are 

active in these waves, they are also called pressure or compressional waves. 

They are also sometimes called density waves because their particle density 

fluctuates as they move. Compression waves can be generated in liquids, as 

well as solids because the energy travels through the atomic structure by a 

series of compressions and expansion (rarefaction) movements.

Longitudinal wave: Longitudinal waves (L-Waves) compress and decompress 

the material in the direction of motion, much like sound waves in air.

Longitudinal Wave

2.2.4 Shear waves (S-Waves) 

In air, sound travels by the compression and rarefaction of air molecules in 

the direction of travel. However, in solids, molecules can support vibrations in 

other directions, hence, a number of different types of sound waves are 

possible. Waves can be characterized in space by oscillatory patterns that 

are capable of maintaining their shape and propagating in a stable 

manner. The propagation of waves is often described in terms of what are 

called “wave modes.” 

As mentioned previously, longitudinal and transverse (shear) waves are most 

often used in ultrasonic inspection. However, at surfaces and interfaces, 

various types of elliptical or complex vibrations of the particles make other 

waves possible. Some of these wave modes such as (1) Rayleigh and (2) 

Lamb waves are also useful for ultrasonic inspection. 

Keywords: 

Compression 

Rarefaction

Shear waves vibrate particles at right angles compared to the motion of the 

ultrasonic wave. The velocity of shear waves through a material is 

approximately half that of the longitudinal waves. The angle in which the 

ultrasonic wave enters the material determines whether longitudinal, shear, or 

both waves are produced.

Shear waves

In the transverse or shear wave, the particles oscillate at a right angle or 

transverse to the direction of propagation. Shear waves require an 

acoustically solid material for effective propagation, and therefore, are not 

effectively propagated in materials such as liquids or gasses. Shear waves 

are relatively weak when compared to longitudinal waves. In fact, shear 

waves are usually generated in materials using some of the energy from 

longitudinal waves. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/transverse.swf

Q10: For a shear wave travelling from steel to water incident on the boundary 

at 10 degrees will give a refracted shear wave in water with an angle of: 

A. 0 degrees 

B. 5 degrees 

C. 20 degrees 

D. none of the above 


2.2.5 Rayleigh Characteristics 

Rayleigh waves are a type of surface wave that travel near the surface of 

solids. Rayleigh waves include both longitudinal and transverse motions that 

decrease exponentially in amplitude as distance from the surface increases. 

There is a phase difference between these component motions. In isotropic 

solids these waves cause the surface particles to move in ellipses in planes 

normal to the surface and parallel to the direction of propagation – the major 

axis of the ellipse is vertical. At the surface and at shallow depths this motion 

is retrograde 逆行 , that is the in-plane motion of a particle is counterclockwise 

when the wave travels from left to right. 

http://en.wikipedia.org/wiki/Rayleigh_wave

Rayleigh waves are a type of surface acoustic wave that travel on solids. 

They can be produced in materials in many ways, such as by a localized 

impact or by piezo-electric transduction, and are frequently used in nondestructive 

testing for detecting defects. They are part of the seismic waves 

that are produced on the Earth by earthquakes. When guided in layers they 

are referred to as Lamb waves, Rayleigh–Lamb waves, or generalized 

Rayleigh waves.

Rayleigh waves

Q29: The longitudinal wave incident angle which results in formation of a 

Rayleigh wave is called: 

A. Normal incidence 

B. The first critical angle 

C. The second critical angle 

D. Any angle above the first critical angle 


Surface (or Rayleigh) waves travel the surface of a relatively thick solid 

material penetrating to a depth of one wavelength. 

Surface waves combine both (1) a longitudinal and (2) transverse motion to 

create an elliptic orbit motion as shown in the image and animation below. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/rayleigh.swf

The major axis of the ellipse is perpendicular to the surface of the solid. As 

the depth of an individual atom from the surface increases the width of its 

elliptical motion decreases. Surface waves are generated when a 

longitudinal wave intersects a surface near the second critical angle and 

they travel at a velocity between .87 and .95 of a shear wave. Rayleigh 

waves are useful because they are very sensitive to surface defects (and 

other surface features) and they follow the surface around curves. 

Because of this, Rayleigh waves can be used to inspect areas that other 

waves might have difficulty reaching. 

Wave velocity: 

• Longitudinal wave velocity =1v, 

• The velocity of shear waves through a material is approximately half that 

of the longitudinal waves, (≈0.5v) 

• Surface waves are generated when a longitudinal wave intersects a 

surface near the second critical angle and they travel at a velocity 

between .87 and .95 of a shear wave. ≈(0.87~0.95)x0.5v

The major axis of the ellipse is perpendicular to the surface of the solid.

Surface wave

Surface wave or Rayleigh wave are formed when shear waves refract to 90. 

The whip-like particle vibration of the shear wave is converted into elliptical 

motion by the particle changing direction at the interface with the surface. The 

wave are not often used in industrial NDT although they do have some 

application in aerospace industry. Their mode of propagation is elliptical along 

the surface of material, penetrating to a depth of one wavelength. They will 

follow the contour of the surface and they travel at approximately 90% of the 

velocity of the shear waves. 

Depth of penetration of 

about one wavelength 

Direction of wave propagation

Surface wave has the ability to follow surface contour, until it meet a sharp 

change i.e. a surface crack/seam/lap. However the surface waves could be 

easily completely absorbed by excess couplant of simply touching the part 

ahead of the waves. 

Transducer 

Wedge 

Surface discontinuity 

Specimen

Surface wave - Following Contour 

Surface wave

Surface wave – One wavelength deep 

λ 

λ

Rayleigh Wave 

http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif

Love Wave 

http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif

Love Wave

Other Reading: Rayleigh Waves 

Surface waves (Rayleigh waves) are another type of ultrasonic wave used in 

the inspection of materials. These waves travel along the flat or curved 

surface of relatively thick solid parts. For the propagation of waves of this type, 

the waves must be traveling along an interface bounded on one side by the 

strong elastic forces of a solid and on the other side by the practically 

negligible elastic forces between gas molecules. Surface waves leak energy 

into liquid couplants and do not exist for any significant distance along the 

surface of a solid immersed in a liquid, unless the liquid covers the solid 

surface only as a very thin film. Surface waves are subject to attenuation in a 

given material, as are longitudinal or transverse waves. They have a velocity 

approximately 90% of the transverse wave velocity in the same material. The 

region within which these waves propagate with effective energy is not much 

thicker than about one wavelength beneath the surface of the metal.

At this depth, wave energy is about 4% of the wave energy at the surface, 

and the amplitude of oscillation decreases sharply to a negligible value at 

greater depths. Surface waves follow contoured surfaces. For example, 

surface waves traveling on the top surface of a metal block are reflected from 

a sharp edge, but if the edge is rounded off, the waves continue down the 

side face and are reflected at the lower edge, returning to the sending point. 

Surface waves will travel completely around a cube if all edges of the cube 

are rounded off. Surface waves can be used to inspect parts that have 

complex contours.

Q110: What kind of wave mode travel at a velocity slightly below the shear 

wave and their modes of propagation are both longitudinal and transverse 

with respect to the surface? 

a) Rayleigh wave 

b) Transverse wave 

c) L-wave 

d) Longitudinal wave 


Q: Which of the following modes of vibration exhibits the shortest wavelength 

at a given frequency and in a given material? 

A. longitudinal wave 

B. compression wave 

C. shear wave 

D. surface wave 


2.2.6 Lamb Wave: 

Lamb waves propagate in solid plates. They are elastic waves whose 

particle motion lies in the plane that contains the direction of wave 

propagation and the plate normal (the direction perpendicular to the plate). In 

1917, the english mathematician horace lamb published his classic analysis 

and description of acoustic waves of this type. Their properties turned out to 

be quite complex. An infinite medium supports just two wave modes traveling 

at unique velocities; but plates support two infinite sets of lamb wave modes, 

whose velocities depend on the relationship between wavelength and plate 

thickness.

Since the 1990s, the understanding and utilization of lamb waves has 

advanced greatly, thanks to the rapid increase in the availability of computing 

power. Lamb's theoretical formulations have found substantial practical 

application, especially in the field of nondestructive testing. 

The term rayleigh–lamb waves embraces the rayleigh wave, a type of wave 

that propagates along a single surface. Both rayleigh and lamb waves are 

constrained by the elastic properties of the surface(s) that guide them. 

http://en.wikipedia.org/wiki/Lamb_wave 

http://pediaview.com/openpedia/Lamb_waves

Types of Wave 

New! 

• Plate wave- Love 

• Stoneley wave 

• Sezawa

Plate or Lamb waves are the most commonly used plate waves in 

NDT. Lamb waves are complex vibrational waves that propagate parallel to 

the test surface throughout the thickness of the material. Propagation of Lamb 

waves depends on the density and the elastic material properties of a 

component. They are also influenced a great deal by the test frequency and 

material thickness. Lamb waves are generated at an incident angle in which 

the parallel component of the velocity of the wave in the source is equal to the 

velocity of the wave in the test material. Lamb waves will travel several 

meters in steel and so are useful to scan plate, wire, and tubes. 

Lamb wave influenced by: (Dispersive Wave) 

■ 

■ 

■ 

■ 

Density 

Elastic material properties 

Frequencies 

Material thickness

Plate or Lamb waves are similar to surface waves except they can only be 

generated in materials a few wavelengths thick. 

http://www.ndt.net/ndtaz/files/lamb_a.gif

Plate wave or Lamb wave are formed by the introduction of surface wave 

into a thin material. They are a combination of (1) compression and surface or 

(2) shear and surface waves causing the plate material to flex by totally 

saturating the material. The two types of plate waves:

With Lamb waves, a number of modes of particle vibration are possible, but 

the two most common are symmetrical and asymmetrical. The complex 

motion of the particles is similar to the elliptical orbits for surface 

waves. Symmetrical Lamb waves move in a symmetrical fashion about the 

median plane of the plate. This is sometimes called the extensional mode 

because the wave is “stretching and compressing” the plate in the wave 

motion direction. Wave motion in the symmetrical mode is most efficiently 

produced when the exciting force is parallel to the plate. The asymmetrical 

Lamb wave mode is often called the “flexural mode” because a large portion 

of the motion moves in a normal direction to the plate, and a little motion 

occurs in the direction parallel to the plate. In this mode, the body of the plate 

bends as the two surfaces move in the same direction. 

The generation of waves using both piezoelectric transducers and 

electromagnetic acoustic transducers (EMATs) are discussed in later sections. 

Keywords: 

Symmetrical = extensional mode 

Asymmetrical = flexural mode

When guided in layers they are referred to as Lamb waves, Rayleigh–Lamb 

waves, or generalized Rayleigh waves. 

Lamb waves – 2 modes





Symmetrical = extensional mode

Other Reading: Lamb Wave 

Lamb waves, also known as plate waves, are another type of ultrasonic wave 

used in the nondestructive inspection of materials. Lamb waves are 

propagated in plates (made of composites or metals) only a few wavelengths 

thick. A Lamb wave consists of a complex vibration that occurs throughout the 

thickness of the material. The propagation characteristics of Lamb waves 

depend on the density, elastic properties, and structure of the material as well 

as the thickness of the test piece and the frequency. Their behavior in general 

resembles that observed in the transmission of electromagnetic waves 

through waveguides. 

There are two basic forms of Lamb waves: 

• Symmetrical, or dilatational 

• Asymmetrical, or bending

The form is determined by whether the particle motion is symmetrical or 

asymmetrical with respect to the neutral axis of the test piece. Each form is 

further subdivided into several modes having different velocities, which can 

be controlled by the angle at which the waves enter the test piece. 

Theoretically, there are an infinite number of specific velocities at which Lamb 

waves can travel in a given material. Within a given plate, the specific 

velocities for Lamb waves are complex functions of plate thickness and 

frequency. 

In symmetrical (dilatational) Lamb waves, there is a compressional 

(longitudinal) particle displacement along the neutral axis of the plate and an 

elliptical particle displacement on each surface (Fig. 4a). In asymmetrical 

(bending) Lamb waves, there is a shear (transverse) particle displacement 

along the neutral axis of the plate and an elliptical particle displacement on 

each surface (Fig. 4b). The ratio of the major to minor axes of the ellipse is a 

function of the material in which the wave is being propagated.

Fig. 4 Diagram of the basic patterns of (a) symmetrical (dilatational) and (b) 

asymmetrical (bending) Lamb waves. The wavelength, , is the distance 

corresponding to one complete cycle.

Q1: The wave mode that has multiple or varying wave velocities is: 

A. Longitudinal waves 

B. Shear waves 

C. Transverse waves 

D. Lamb waves 


2.2.7 Dispersive Wave: 

Wave modes such as those found in Lamb wave have a velocity of 

propagation dependent upon the operating frequency, sample thickness and 

elastic moduli. They are dispersive (velocity change with frequency) in that 

pulses transmitted in these mode tend to become stretched or dispersed.

Dispersion refers to the fact that in a real medium such as water, air, or glass, 

a wave traveling through that medium will have a velocity that depends upon 

its frequency. Dispersion occurs for any form of wave, acoustic, 

electromagnetic, electronic, even quantum mechanical. Dispersion is 

responsible for a prism being able to resolve light into colors and defines the 

maximum frequency of broadband pulses one can send down an optical fiber 

or through a copper wire. Dispersion affects wave and swell forecasts at 

sea and influences the design of sound equipment. Dispersion is a physical 

property of the medium and can combine with other properties to yield very 

strange results. For example, in the propagation of light in an optical fiber, the 

glass introduces dispersion and separates the wavelengths of light according 

to frequency, however if the light is intense enough, it can interact with the 

electrons in the material changing its refractive index. The combination of 

dispersion and index change can cancel each other leading to a wave that 

can propagate indefinitely maintaining a constant shape. Such a wave has 

been termed a soliton. 

http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html

Plate or Lamb waves are generated at an incident angle in which the parallel 

component of the velocity of the wave in the source is equal to the velocity of 

the wave in the test material.

Thickness Limitation: 

One can not generate shear / surface (or Lamb?) wave on a plate that is 

thinner than ½ the wavelength.


In the previous pages, it was pointed out that sound waves propagate due to 

the vibrations or oscillatory motions of particles within a material. An 

ultrasonic wave may be visualized as an infinite number of oscillating masses 

or particles connected by means of elastic springs. Each individual particle is 

influenced by the motion of its nearest neighbor and both (1) inertial and (2) 

elastic restoring forces act upon each particle. 

A mass on a spring has a single resonant frequency determined by its spring 

constant k and its mass m. The spring constant is the restoring force of a 

spring per unit of length. Within the elastic limit of any material, there is a 

linear relationship between the displacement of a particle and the force 

attempting to restore the particle to its equilibrium position. This linear 

dependency is described by Hooke's Law.

Spring model- A mass on a spring has a single resonant frequency 

determined by its spring constant k and its mass m.

Spring model- A mass on a spring has a single resonant frequency 

determined by its spring constant k and its mass m.

In terms of the spring model, Hooke's Law says that the restoring force due to 

a spring is proportional to the length that the spring is stretched, and acts in 

the opposite direction. Mathematically, Hooke's Law is written as F =-kx, 

where F is the force, k is the spring constant, and x is the amount of particle 

displacement. Hooke's law is represented graphically it the bottom. Please 

note that the spring is applying a force to the particle that is equal and 

opposite to the force pulling down on the particle.

Elastic Model

Elastic Model / Longitudinal Wave

Elastic Model / Longitudinal Wave

Elastic Model / Shear Wave

Elastic Model / Shear Wave

The Speed of Sound 

Hooke's Law, when used along with Newton's Second Law, can explain a few 

things about the speed of sound. The speed of sound within a material is a 

function of the properties of the material and is independent of the amplitude 

of the sound wave. Newton's Second Law says that the force applied to a 

particle will be balanced by the particle's mass and the acceleration of the 

particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's 

Law then says that this force will be balanced by a force in the opposite 

direction that is dependent on the amount of displacement and the spring 

constant (F = -kx). Therefore, since the applied force and the restoring force 

are equal, ma = -kx can be written. The negative sign indicates that the force 

is in the opposite direction. 

F= ma = -kx

Since the mass m and the spring constant k are constants for any given 

material, it can be seen that the acceleration a and the displacement x are the 

only variables. It can also be seen that they are directly proportional. For 

instance, if the displacement of the particle increases, so does its acceleration. 

It turns out that the time that it takes a particle to move and return to its 

equilibrium position is independent of the force applied. So, within a given 

material, sound always travels at the same speed no matter how much force 

is applied when other variables, such as temperature, are held constant. 

a ∝ x

What properties of material affect its speed of sound? 

Of course, sound does travel at different speeds in different materials. This is 

because the (1) mass of the atomic particles and the (2) spring constants are 

different for different materials. The mass of the particles is related to the 

density of the material, and the spring constant is related to the elastic 

constants of a material. The general relationship between the speed of sound 

in a solid and its density and elastic constants is given by the following 

equation:

Elastic constant 

→ spring constants 

Density 

→ mass of the atomic particles

Where V is the speed of sound, C is the elastic constant, and p is the material 

density. This equation may take a number of different forms depending on the 

type of wave (longitudinal or shear) and which of the elastic constants that are 

used. The typical elastic constants of a materials include: 

• Young's Modulus, E: a proportionality constant between uniaxial stress 

and strain. 

• Poisson's Ratio, n: the ratio of radial strain to axial strain 

• Bulk modulus, K: a measure of the incompressibility of a body subjected to 

hydrostatic pressure. 

• Shear Modulus, G: also called rigidity, a measure of a substance's 

resistance to shear. 

• Lame's Constants, l and m: material constants that are derived from 

Young's Modulus and Poisson's Ratio.

Q163 Acoustic velocity of materials are primary due to the material's: 

a) density 

b) elasticity 

c) both a and b 

d) acoustic impedance 


Q50: The principle attributes that determine the differences in ultrasonic 

velocities among materials are: 

A. Frequency and wavelength 

B. Thickness and travel time 

C. Elasticity and density 

D. Chemistry and permeability 


When calculating the velocity of a longitudinal wave, Young's Modulus and 

Poisson's Ratio are commonly used. 

When calculating the velocity of a shear wave, the shear modulus is used. It 

is often most convenient to make the calculations using 

Lame's Constants, which are derived from Young's Modulus and Poisson's 

Ratio.

E/N/G

It must also be mentioned that the subscript ij attached to C (C ij ) in the above 

equation is used to indicate the directionality of the elastic constants with 

respect to the wave type and direction of wave travel. In isotropic materials, 

the elastic constants are the same for all directions within the material. 

However, most materials are anisotropic and the elastic constants differ with 

each direction. For example, in a piece of rolled aluminum plate, the grains 

are elongated in one direction and compressed in the others and the elastic 

constants for the longitudinal direction are different than those for the 

transverse or short transverse directions. 

V longitudinal 

V transverse

Examples of approximate compressional sound velocities in materials are: 

Aluminum - 0.632 cm/microsecond 

1020 steel - 0.589 cm/microsecond 

Cast iron - 0.480 cm/microsecond. 

Examples of approximate shear sound velocities in materials are: 

Aluminum - 0.313 cm/microsecond 

1020 steel - 0.324 cm/microsecond 

Cast iron - 0.240 cm/microsecond. 

When comparing compressional and shear velocities, it can be noted that 

shear velocity is approximately one half that of compressional velocity. The 

sound velocities for a variety of materials can be found in the ultrasonic 

properties tables in the general resources section of this site.

Longitudinal Wave Velocity: V L 

The velocity of a longitudinal wave is described by the following equation: 

V L 

E 

μ 

P 

= Longitudinal bulk wave velocity 

= Young’s modulus of elasticity 

= Poisson ratio 

= Material density

Shear Wave Velocity: V S 

The velocity of a shear wave is described by the following equation: 

V s 

E 

μ 

P 

G 

= Shear wave velocity 

= Young’s modulus of elasticity 


= Material density 

= Shear modulus


Wavelength, Frequency and Velocity 

Among the properties of waves propagating in isotropic solid materials are 

wavelength, frequency, and velocity. The wavelength is directly proportional 

to the velocity of the wave and inversely proportional to the frequency of the 

wave. This relationship is shown by the following equation.

The applet below shows a longitudinal and transverse wave. The direction of 

wave propagation is from left to right and the movement of the lines indicate 

the direction of particle oscillation. The equation relating ultrasonic 

wavelength, frequency, and propagation velocity is included at the bottom of 

the applet in a reorganized form. The values for the wavelength, frequency, 

and wave velocity can be adjusted in the dialog boxes to see their effects on 

the wave. Note that the frequency value must be kept between 0.1 to 1 MHz 

(one million cycles per second) and the wave velocity must be between 0.1 

and 0.7 cm/us.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_4/applet_2_4.htm


Java don’t work? Uninstalled → Reinstalled → Then → 

http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

Java don’t work? 








As can be noted by the equation, a change in frequency will result in a 

change in wavelength. Change the frequency in the applet and view the 

resultant wavelength. At a frequency of .2 and a material velocity of 0.585 

(longitudinal wave in steel) note the resulting wavelength. Adjust the material 

velocity to 0.480 (longitudinal wave in cast iron) and note the resulting 

wavelength. Increase the frequency to 0.8 and note the shortened wavelength 

in each material. 

In ultrasonic testing, the shorter wavelength resulting from an increase in 

frequency will usually provide for the detection of smaller discontinuities. This 

will be discussed more in following sections. 

Keywords: 

the shorter wavelength resulting from an increase in frequency will usually 

provide for the detection of smaller discontinuities

The velocities sound waves 

The velocities of the various kinds of sound waves can be calculated from the 

elastic constants of the material concerned, that is the modulus of elasticity E 

(measured in N/m2), the density p in kg/m 3 , and Poisson's ratio μ (a 

dimensionless number). 

for longitudinal waves: 

for transverse waves:

The two velocities of sound are linked by the following relation: 

For all solid materials Poisson's ratio μ lies between 0 and 0.5, so that the 

numerical value of the expression 

always lies between 0 and 0.707. In steel and aluminum, μ= 0.28 and 0.34, 

respectively, 

-- 

= 0.55 and 0.49 respectIvely.


2.5.1 Sensitivity & Resolution 

In ultrasonic testing, the inspector must make a decision about the frequency 

of the transducer that will be used. As we learned on the previous page, 

changing the frequency when the sound velocity is fixed will result in a 

change in the wavelength of the sound. 

The wavelength of the ultrasound used has a significant effect on the 

probability of detecting a discontinuity. A general rule of thumb is that a 

discontinuity must be larger than one-half the wavelength to stand a 

reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic 

inspection to describe a technique's ability to locate flaws. Sensitivity is the 

ability to locate small discontinuities. Sensitivity generally increases with 

higher frequency (shorter wavelengths). Resolution is the ability of the system 

to locate discontinuities that are close together within the material or located 

near the part surface. Resolution also generally increases as the frequency 

increases.

Keywords: 

• Discontinuity must be larger than one-half the wavelength to stand a 

reasonable chance of being detected. 

• Sensitivity is the ability to locate small discontinuities. Sensitivity generally 

increases with higher frequency (shorter wavelengths). 

• Resolution is the ability of the system to locate discontinuities that are 

close together within the material or located near the part surface. 

• Resolution also generally increases as the frequency increases, pulse 

length decrease, bandwidth increase (highly damp)

2.5.2 Grain Size & Frequency Selection 

The wave frequency can also affect the capability of an inspection in adverse 

ways. Therefore, selecting the optimal inspection frequency often involves 

maintaining a balance between the favorable and unfavorable results of the 

selection. Before selecting an inspection frequency, the material's grain 

structure and thickness, and the discontinuity's type, size, and probable 

location should be considered. 

As frequency increases, sound tends to scatter from large or course grain 

structure and from small imperfections within a material. Cast materials often 

have coarse grains and other sound scatters that require lower frequencies to 

be used for evaluations of these products. 

(1) Wrought and (2) forged products with directional and refined grain 

structure can usually be inspected with higher frequency transducers.

Keywords: 

• Coarse grains →Lower frequency to avoid scattering and noise, 

• Fine grains →Higher frequency to increase sensitivity & resolution.

Since more things in a material are likely to scatter a portion of the sound 

energy at higher frequencies, the penetrating power (or the maximum depth 

in a material that flaws can be located) is also reduced. Frequency also has 

an effect on the shape of the ultrasonic beam. Beam spread, or the 

divergence of the beam from the center axis of the transducer, and how it is 

affected by frequency will be discussed later. 

It should be mentioned, so as not to be misleading, that a number of other 

variables will also affect the ability of ultrasound to locate defects. These 

include the pulse length, type and voltage applied to the crystal, properties of 

the crystal, backing material, transducer diameter, and the receiver circuitry of 

the instrument. These are discussed in more detail in the material on signalto-noise 

ratio.

Coarse grains →Lower frequency to avoid scattering and noise, 

Fine grains →Higher frequency to increase sensitivity & resolution. 

http://www.cnde.iastate.edu/ultrasonics/grain-noise

Detectability variable: 

• pulse length, 

• type and voltage applied to the crystal, 

• properties of the crystal, 

• backing material, 

• transducer diameter, and 

• the receiver circuitry of the instrument.

Keywords: 

• Higher the frequency, greater the scattering, thus less penetrating. 

• Higher the frequency better sensitivity and better resolution 

• If the grain size is 1/10 the wavelength, the ultrasound will be significantly 

scattered.

Q7: When a material grain size is on the order of ______ wavelength or 

larger, excessive scattering of the ultrasonic beam affect test result. 

A. 1 

B. ½ 

C. 1/10 

D. 1/100

2.5.3 Further Reading 

Detectability variable: 

• pulse length, 

• type and voltage applied to the crystal, 

• properties of the crystal, 

• backing material, 

• transducer diameter (focal length → Cross sectional area), and 

• the receiver circuitry of the instrument. 

Investigating on: Sonic pulse volume ∝ pulse length, transducer Φ

Pulse Length: 

A sound pulse traveling through a 

metal occupies a physical 

volume. This volume changes 

with depth, being smallest in the 

focal zone. The pulse volume, a 

product of a pulse length L and a 

cross-sectional area A, can be 

fairly easily measured by 

combining ultrasonic A-scans and 

C-scans, as will be seen shortly. 

For many cases of practical interest, the inspection simulation models predict 

that S/N (signal to noise ratio) is inversely proportional to the square root of the 

pulse volume at the depth of the defect. This is known as the “pulse volume 

rule-of-thumb” and has become a guiding principle for designing 

inspections. Generally speaking, it applies when both the grain size and the 

lateral size of the defect are smaller than the sound pulse diameter. 

http://www.cnde.iastate.edu/ultrasonics/grain-noise

Determining cross sectional area using reflector- A Scan (6db drop)

Determining cross sectional area using reflector- C Scan

“Sonic pulse volume” and S/N (defect resolution)

Pulse volume rule-of-thumb: 

Competing grain noise ∝√(pulse volume)


2.6.1 Material Attenuation: 

Attenuation by definition is the rate of decrease of sound energy when a 

ultrasound wave id propagating in a medium. The sound attenuation in 

material depends on heat treatment, grain size, viscous friction, crystal 

stricture (anisotropy or isotropy), porosity, elastic hysteresis, hardness, 

Young’s modulus, etc. 

Sound attenuations are affected by; (1) Geometric beam spread, (2) 

Absorption, (3) Scattering. 

Material attenuation affects item (2) & (3).

When sound travels through a medium, its intensity diminishes with distance. 

In idealized materials, sound pressure (signal amplitude) is only reduced by 

the (1) spreading of the wave. Natural materials, however, all produce an 

effect which further weakens the sound. This further weakening results from 

(2) scattering and (3) absorption. Scattering is the reflection of the sound in 

directions other than its original direction of propagation. Absorption is the 

conversion of the sound energy to other forms of energy. The combined 

effect of scattering and absorption (spreading?) is called attenuation. 

Ultrasonic attenuation is the decay rate of the wave as it propagates through 

material. 

Attenuation of sound within a material itself is often not of intrinsic interest. 

However, natural properties and loading conditions can be related to 

attenuation. Attenuation often serves as a measurement tool that leads to the 

formation of theories to explain physical or chemical phenomenon that 

decreases the ultrasonic intensity.

Absorption: 

Sound attenuations are affected by; (1) Geometric beam spread, (2) Absorption, 

(3) Scattering. 

Absorption processes 

1. Mechanical hysteresis 

2. Internal friction 

3. Others (?) 

For relatively non-elastic material, these soft and pliable material include lead, 

plastid, rubbers and non-rigid coupling materials; much of the energy is loss as 

heat during sound propagation and absorption is the main reason that the 

testing of these material are limit to relatively thin section/

Scattering: 

Grain Size and Wave Frequency 

The relative impact of scattering source of a material depends upon their 

grain sizes in comparison with the Ultrasonic sound wave length. As the 

scattering size approaches that of a wavelength, scattering by the grain is a 

concern. The effects from such scattering could be compensated with the use 

of increasing wavelength ultrasound at the cost of decreasing sensitivity and 

resolution to detection of discontinuities. 

Other effect are anisotropic columnar grain with different elastic behavior at 

different grain direction. In this case the internal incident wave front becomes 

distorted and often appear to change direction (propagate better in certain 

preferred direction) in respond to material anisotropy.

Anisotropic Columnar Grains 

with different elastic behavior at different grain direction.

Spreading/ Scattering / adsorption (reflection is a form of scattering) 

Adsorption 

Scattering 

Spreading 

Scatterbrain

The amplitude change of a decaying plane wave can be expressed as: 

In this expression A o is the unattenuated amplitude of the propagating wave 

at some location. The amplitude A is the reduced amplitude after the wave 

has traveled a distance z from that initial location. The quantity α is the 

attenuation coefficient of the wave traveling in the z-direction. The α 

dimensions of are nepers/length, where a neper is a dimensionless 

quantity. The term e is the exponential (or Napier's constant) which is equal 

to approximately 2.71828.

The units of the attenuation value in Nepers per meter (Np/m) can be 

converted to decibels/length by dividing by 0.1151. Decibels is a more 

common unit when relating the amplitudes of two signals.

Attenuation is generally proportional to the square of sound frequency. 

Quoted values of attenuation are often given for a single frequency, or an 

attenuation value averaged over many frequencies may be given. Also, the 

actual value of the attenuation coefficient for a given material is highly 

dependent on the way in which the material was manufactured. Thus, quoted 

values of attenuation only give a rough indication of the attenuation and 

should not be automatically trusted. Generally, a reliable value of attenuation 

can only be obtained by determining the attenuation experimentally for the 

particular material being used. 

Attenuation ∝ Frequency (f ) 2

Attenuation can be determined by evaluating the multiple back wall reflections 

seen in a typical A-scan display like the one shown in the image at the bottom. 

The number of decibels between two adjacent signals is measured and this 

value is divided by the time interval between them. This calculation produces 

a attenuation coefficient in decibels per unit time Ut. This value can be 

converted to nepers/length by the following equation. 

Where v is the velocity of sound in meters per 

second and Ut is in decibels per second.

Amplitude at distance Z 

where: 



A o 

Ut 

A 


2.6.2 Factors Affecting Attenuation: 

1. Testing Factors 

• Testing frequency 

• Boundary conditions 

• Wave form geometry 

2. Base Material Factors 

• Material type 

• Chemistry 

• Integral constituents (fiber, voids, water content, inclusion, anisotropy) 

• Forms (casting, wrought) 

• Heat treatment history 

• Mechanical processes(Hot or cold working; forging, rolling, extruding, 

TMCP, directional working)

2.6.3 Frequency selection 

There is no ideal frequency; therefore, frequency selection must be made with 

consideration of several factors. Frequency determines the wavelength of the 

sound energy traveling through the material. Low frequency has longer 

wavelengths and will penetrate deeper than higher frequencies. To penetrate 

a thick piece, low frequencies should be used. Another factor is the size of the 

grain structure in the material. High frequencies with shorter wavelengths 

tend to reflect off grain boundaries and become lost or result in ultrasonic 

noise that can mask flaw signals. Low frequencies must be used with coarse 

grain structures. However, test resolution decreases when frequency is 

decreased. Small defects detectable at high frequencies may be missed at 

lower frequencies. In addition, variations in instrument characteristics and 

settings as well as material properties and coupling conditions play a major 

role in system performance. It is critical that approved testing procedures be 

followed.

2.6.4 Further Reading on Attenuation

Q94: In general, which of the following mode of vibration would have the 

greatest penetrating power in a coarse grain material if the frequency of 

the wave are the same? 

a) Longitudinal wave 

b) Shear wave 

c) Transverse wave 

d) All the above modes would have the same penetrating power 

Q: The random distribution of crystallographic direction in alloys with large 

crystalline structures is a factor in determining: 

A. Acoustic noise levels 

B. Selection of test frequency 

C. Scattering of sound 

D. All of the above 


Q168: Heat conduction, viscous friction, elastic hysteresis, and scattering are 

four different mechanism which lead to: 

A. Attenuation 

B. Refraction 

C. Beam spread 

D. Saturation 


Q7: When the material grain size is in the order of ____ wavelength or larger, 

excessive scattering of the ultrasound beam may affect test result: 

A. 1 

B. ½ 

C. 1/10 

D. 1/100 



Acoustic impedance is a measured of resistance of sound propagation 

through a part. 

From the table air has lower acoustic impedance than steel and for a given 

energy Aluminum would travel a longer distance than steel before the same 

amount of energy is attenuated.

Transmission & Reflection Animation: 

http://upload.wikimedia.org/wikipedia/commons/3/30/Partial_transmittance.gif

Sound travels through materials under the influence of sound pressure. 

Because molecules or atoms of a solid are bound elastically to one another, 

the excess pressure results in a wave propagating through the solid. 

The acoustic impedance (Z) of a material is defined as the product of its 

density (p) and acoustic velocity (V). 

Z = pV 

Acoustic impedance is important in: 

1. the determination of acoustic transmission and reflection at the boundary 

of two materials having different acoustic impedances. 

2. the design of ultrasonic transducers. 

3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any 

material, so long as its density (p) and acoustic velocity (V) are known. The 

applet also shows how a change in the impedance affects the amount of 

acoustic energy that is reflected and transmitted. The values of the reflected 

and transmitted energy are the fractional amounts of the total energy incident 

on the interface. Note that the fractional amount of transmitted sound energy 

plus the fractional amount of reflected sound energy equals one. The 

calculation used to arrive at these values will be discussed on the next page. 


Reflection/Transmission Energy as a function of Z


Q2.8: The acoustic impedance of material used to determined: 

A. Angle of refraction at the interface 

B. Attenuation of material 

C. Relative amount of sound energy coupled through and reflected at an 

interface 

D. Beam spread within the material 



Ultrasonic waves are reflected at boundaries where there is a difference in 

acoustic impedances (Z) of the materials on each side of the boundary. (See 

preceding page for more information on acoustic impedance.) This difference 

in Z is commonly referred to as the impedance mismatch. The greater the 

impedance mismatch, the greater the percentage of energy that will be 

reflected at the interface or boundary between one medium and another. 

The fraction of the incident wave intensity that is reflected can be derived 

because particle velocity and local particle pressures must be continuous 

across the boundary.

When the acoustic impedances of the materials on both sides of the boundary 

are known, the fraction of the incident wave intensity that is reflected can be 

calculated with the equation below. The value produced is known as the 

reflection coefficient. Multiplying the reflection coefficient by 100 yields the 

amount of energy reflected as a percentage of the original energy.

Since the amount of reflected energy plus the transmitted energy must equal 

the total amount of incident energy, the transmission coefficient is calculated 

by simply subtracting the reflection coefficient from one. 

Formulations for acoustic reflection and transmission coefficients (pressure) 

are shown in the interactive applet below. Different materials may be 

selected or the material velocity and density may be altered to change the 

acoustic impedance of one or both materials. The red arrow represents 

reflected sound and the blue arrow represents transmitted sound. 


Reflection Coefficient:

Note that the reflection and transmission coefficients are often expressed in 

decibels (dB) to allow for large changes in signal strength to be more easily 

compared. To convert the intensity or power of the wave to dB units, take the 

log of the reflection or transmission coefficient and multiply this value times 

10. However, 20 is the multiplier used in the applet since the power of sound 

is not measured directly in ultrasonic testing. The transducers produce a 

voltage that is approximately proportionally to the sound pressure. The power 

carried by a traveling wave is proportional to the square of the pressure 

amplitude. Therefore, to estimate the signal amplitude change, the log of the 

reflection or transmission coefficient is multiplied by 20.

Using the above applet, note that the energy reflected at a water-stainless 

steel interface is 0.88 or 88%. The amount of energy transmitted into the 

second material is 0.12 or 12%. The amount of reflection and transmission 

energy in dB terms are -1.1 dB and -18.2 dB respectively. The negative sign 

indicates that individually, the amount of reflected and transmitted energy is 

smaller than the incident energy.

If reflection and transmission at interfaces is 

followed through the component, only a small 

percentage of the original energy makes it back 

to the transducer, even when loss by attenuation 

is ignored. For example, consider an immersion 

inspection of a steel block. The sound energy 

leaves the transducer, travels through the water, 

encounters the front surface of the steel, 

encounters the back surface of the steel and 

reflects back through the front surface on its way 

back to the transducer. At the water steel 

interface (front surface), 12% of the energy is 

transmitted. At the back surface, 88% of the 

12% that made it through the front surface is 

reflected. This is 10.6% of the intensity of the 

initial incident wave. As the wave exits the part 

back through the front surface, only 12% of 10.6 

or 1.3% of the original energy is transmitted back 

to the transducer.

Incident Wave other than Normal? – Oblique Incident 

http://www.slideshare.net/crisevelise/fundamentals-ofultrasound?related=1&utm_campaign=related&utm_medium=1&utm_sourc 

e=29

Incident Wave other than Normal? – Oblique Incident

Q: The figure above shown the partition of incident and reflected wave at 

water-Aluminum interface at an incident angle of 20, the reflected and 

transmitted wave are: 

A. 60% and 40% 

B. 40% and 60% 

C. 1/3 and 2/3 

D. 80% and 20% 

Note: if normal incident the reflected 70% Transmitted 30% 


Further Reading (Olympus Technical Note) 

The boundary between two materials of different acoustic impedances is 

called an acoustic interface. When sound strikes an acoustic interface at 

normal incidence, some amount of sound energy is reflected and some 

amount is transmitted across the boundary. The dB loss of energy on 

transmitting a signal from medium 1 into medium 2 is given by: 

dB loss of transmission = 10 log 10 [ 4Z 1 Z 2 / (Z 1 +Z 2 ) 2 ] 

The dB loss of energy of the echo signal in medium 1 reflecting from an 

interface boundary with medium 2 is given by: 

dB loss of Reflection = 10 log 10 [ (Z 1 -Z 2 ) 2 / (Z 1 +Z 2 ) 2 ]

For example: The dB loss on transmitting from water (Z = 1.48) into 1020 

steel (Z = 45.41) is -9.13 dB; this also is the loss transmitting from 1020 steel 

into water. The dB loss of the backwall echo in 1020 steel in water is -0.57 

dB; this also is the dB loss of the echo off 1020 steel in water. The waveform 

of the echo is inverted when Z2

Further Reading: Reflection & Transmission for Normal Incident 

http://www.slideshare.net/crisevelise/fundamentals-ofultrasound?related=1&utm_campaign=related&utm_me 

dium=1&utm_source=29

Q6: For an ultrasonic beam with normal incidence the transmission coefficient 

is given by: 

http://webpages.ursinus.edu/lriley/courses/p212/lectures/node19.html#eq:acousticR 

http://sepwww.stanford.edu/sep/prof/waves/fgdp8/paper_html/node2.html 




Refraction and Snell's Law 

When an ultrasonic wave passes through an 

interface between two materials at an oblique 

angle, and the materials have different indices 

of refraction, both reflected and refracted waves 

are produced. This also occurs with light, which 

is why objects seen across an interface appear 

to be shifted relative to where they really are. 

For example, if you look straight down at an 

object at the bottom of a glass of water, it looks 

closer than it really is. A good way to visualize 

how light and sound refract is to shine a 

flashlight into a bowl of slightly cloudy water 

noting the refraction angle with respect to the 

incident angle.

V s1 

Only If this medium support shear wave i.e. Solid 

V L1 

V L1 

V S2 

V L2

Refraction takes place at an interface due to the different velocities of the 

acoustic waves within the two materials. The velocity of sound in each 

material is determined by the material properties (elastic modulus and density) 

for that material. In the animation below, a series of plane waves are shown 

traveling in one material and entering a second material that has a higher 

acoustic velocity. Therefore, when the wave encounters the interface between 

these two materials, the portion of the wave in the second material is moving 

faster than the portion of the wave in the first material. It can be seen that this 

causes the wave to bend. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/waveRefraction.swf

http://www.ni.com/white-paper/3368/en/

Snell's Law describes the relationship between the angles and the velocities 

of the waves. Snell's law equates the ratio of material velocities V1 and V2 to 

the ratio of the sine's of incident (Ɵ 1 °) and refracted (Ɵ 2 °) angles, as shown in 

the following equation. 

Where: 

V L1 is the longitudinal wave velocity 

in material 1. 

V L2 is the longitudinal wave velocity 

in material 2.

Note that in the diagram, there is a reflected longitudinal wave (V L1' ) shown. 

This wave is reflected at the same angle as the incident wave because the 

two waves are traveling in the same material, and hence have the same 

velocities. This reflected wave is unimportant in our explanation of Snell's Law, 

but it should be remembered that some of the wave energy is reflected at the 

interface. In the applet below, only the incident and refracted longitudinal 

waves are shown. The angle of either wave can be adjusted by clicking and 

dragging the mouse in the region of the arrows. Values for the angles or 

acoustic velocities can also be entered in the dialog boxes so the that applet 

can be used as a Snell's Law calculator.

Snell Law 


Snell Law

When a longitudinal wave moves from a slower to a faster material, there is 

an incident angle that makes the angle of refraction for the wave 90 o . This is 

know as the first critical angle. The first critical angle can be found from 

Snell's law by putting in an angle of 90° for the angle of the refracted ray. At 

the critical angle of incidence, much of the acoustic energy is in the form of an 

inhomogeneous compression wave, which travels along the interface and 

decays exponentially with depth from the interface. This wave is sometimes 

referred to as a "creep wave." Because of their inhomogeneous nature and 

the fact that they decay rapidly, creep waves are not used as extensively as 

Rayleigh surface waves in NDT. However, creep waves are sometimes more 

useful than Rayleigh waves because they suffer less from surface 

irregularities and coarse material microstructure due to their longer 

wavelengths.

Snell Law

Refraction and mode conversion occur 

because of the change in L-wave 

velocity as it passes the boundary from 

one medium to another. The higher the 

difference in the velocity of sound 

between two materials, the larger the 

resulting angle of refraction. L-waves 

and S-waves have different angles of 

refraction because they have dissimilar 

velocities within the same material. 

s the angle of the ultrasonic transducer 

continues to increase, L-waves move 

closer to the surface of the UUT. 

The angle at which the L-wave is parallel with the surface of the UUT is 

referred to as the first critical angle. This angle is useful for two reasons. Only 

one wave mode is echoed back to the transducer, making it easy to interpret 

the data. Also, this angle gives the test system the ability to look at surfaces 

that are not parallel to the front surface, such as welds.

Example: Snell’s Law 

L-wave and S-wave refraction angles are calculated using Snell’s law. You 

also can use this law to determine the first critical angle for any combination 

of materials. 

Where: 

Ɵ 2 ° = angle of the refracted beam in the UUT 

Ɵ 1 ° = incident angle from normal of beam in the wedge or liquid 

V 1 = velocity of incident beam in the liquid or wedge 

V 2 = velocity of refracted beam in the UUT

For example, calculate the first critical angle for a transducer on a plastic 

wedge that is examining aluminum. 

V 1 = 0.267 cm/µs (for L-waves in plastic) 

V 2 = 0.625 cm/µs (for L-waves in aluminum) 

Ɵ 2 ° = 90 degree (angle of L-wave for first critical angle) 

Ɵ 1 ° = unknown 

The plastic wedge must have a minimum angle of 25.29 ° to transmit only S- 

waves into the UUT. When the S-wave angle of refraction is greater than 90°, 

all ultrasonic energy is reflected by the UUT.

Snell Law: First critical angle

Snell Law: 1 st / 2 nd Critical Angles

Q155 Which of the following can occur when an ultrasound beam reaches the 

interface of 2 dissimilar materials? 

a) Reflection 

b) refraction 

c) mode conversion 

d) all of the above

Q. Both longitudinal and shear waves may be simultaneously generated in a 

second medium when the angle of incidence is: 

a) between the normal and the 1st critical angle 

b) between the 1st and 2nd critical angle 

c) past the second critical angle 

d) only at the second critical angle 


Q: When angle beam contact testing a test piece, increasing the incident 

angle until the second critical angle is reached results in: 

A. Total reflection of a surface wave 

B. 45 degree refraction of the shear wave 

C. Production of a surface wave 

D. None of the above 


Typical angle beam assemblies make use of mode conversion and Snell's 

Law to generate a shear wave at a selected angle (most commonly 30°, 45°, 

60°, or 70°) in the test piece. As the angle of an incident longitudinal wave 

with respect to a surface increases, an increasing portion of the sound energy 

is converted to a shear wave in the second material, and if the angle is high 

enough, all of the energy in the second material will be in the form of shear 

waves. There are two advantages to designing common angle beams to take 

advantage of this mode conversion phenomenon. 

• First, energy transfer is more efficient at the incident angles that generate 

shear waves in steel and similar materials. 

• Second, minimum flaw size resolution is improved through the use of 

shear waves, since at a given frequency, the wavelength of a shear wave 

is approximately 60% the wavelength of a comparable longitudinal wave.

Snell Law: 

http://techcorr.com/services/Inspection-and-Testing/Ultrasonic-Shear-Wave.cfm

Depth & Skip

More on Snell Law 

Like light, when an incident ultrasonic wave encounters an interface to an 

adjacent material of a different velocity, at an angle other than normal to the 

surface, then both reflected and refracted waves are produced. 

Understanding refraction and how ultrasonic energy is refracted is especially 

important when using angle probes or the immersion technique. It is also the 

foundation formula behind the calculations used to determine a materials first 

and second critical angles. 

First Critical Angle 

Before the angle of incidence reaches the first critical angle, both longitudinal 

and shear waves exist in the part being inspected. The first critical angle is 

said to have been reached when the longitudinal wave no longer exists within 

the part, that is, when the longitudinal wave is refracted to greater or equal 

than 90°, leaving only a shear wave remaining in the part.

Second Critical Angle 

The second critical angle occurs when the angle of incidence is at such an 

angle that the remaining shear wave within the part is refracted out of the part. 

At this angle, when the refracted shear wave is at 90° a surface wave is 

created on the part surface 

Beam angles should always be plotted using the appropriate industry 

standard, however, knowing the effect of velocity and angle on refraction will 

always benefit an NDT technician when working with angle inspection or the 

immersion technique. 

The above calculator uses the following equation: 

ultrasonic snells law formula 

Where: 

A1 = The angle of incidence. 

V1 = The incident material velocity 

A2 = The angle of refraction 

V2 = The refracted material velocity

http://www.ndtcalc.com/calculators.html


When sound travels in a solid material, one form of wave energy can be 

transformed into another form. For example, when a longitudinal waves hits 

an interface at an angle, some of the energy can cause particle movement in 

the transverse direction to start a shear (transverse) wave. Mode conversion 

occurs when a wave encounters an interface between materials of different 

acoustic impedances and the incident angle is not normal to the interface. 

From the ray tracing movie below, it can be seen that since mode conversion 

occurs every time a wave encounters an interface at an angle, ultrasonic 

signals can become confusing at times.

Mode Conversion 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ModeConversion/ModeConv.swf

In the previous section, it was pointed out 

that when sound waves pass through an 

interface between materials having different 

acoustic velocities, refraction takes place at 

the interface. The larger the difference in 

acoustic velocities between the two 

materials, the more the sound is refracted. 

Notice that the shear wave is not refracted 

as much as the longitudinal wave. This 

occurs because shear waves travel slower 

than longitudinal waves. Therefore, the 

velocity difference between the incident 

longitudinal wave and the shear wave is not 

as great as it is between the incident and 

refracted longitudinal waves. 

Also note that when a longitudinal wave is reflected inside the material, the 

reflected shear wave is reflected at a smaller angle than the reflected 

longitudinal wave. This is also due to the fact that the shear velocity is less 

than the longitudinal velocity within a given material.

Snell's Law holds true for shear waves as well as longitudinal waves and can 

be written as follows 

= 

Where: 

VL1 is the longitudinal wave velocity in material 1. 


VS1 is the shear wave velocity in material 1. 

VS2 is the shear wave velocity in material 2.

Snell's Law

In the applet below, the shear (transverse) wave ray path has been added. 

The ray paths of the waves can be adjusted by clicking and dragging in the 

vicinity of the arrows. Values for the angles or the wave velocities can also be 

entered into the dialog boxes. It can be seen from the applet that when a 

wave moves from a slower to a faster material, there is an incident angle 

which makes the angle of refraction for the longitudinal wave 90 degrees. As 

mentioned on the previous page, this is known as the first critical angle and 

all of the energy from the refracted longitudinal wave is now converted to a 

surface following longitudinal wave. This surface following wave is sometime 

referred to as a creep wave and it is not very useful in NDT because it 

dampens out very rapidly.

Reflections

Creep wave

V S1 

V S2

Beyond the first critical angle, only the shear wave propagates into the 

material. For this reason, most angle beam transducers use a shear wave so 

that the signal is not complicated by having two waves present. In many 

cases there is also an incident angle that makes the angle of refraction for the 

shear wave 90 degrees. This is known as the second critical angle and at this 

point, all of the wave energy is reflected or refracted into a surface following 

shear wave or shear creep wave. Slightly beyond the second critical angle, 

surface waves will be generated. 

Keywords: 

■ 

■ 

Longitudinal creep wave 

Shear creep wave

Snell Law- 1 st & 2 nd Critical Angles

Note that the applet defaults to compressional velocity in the second material. 

The refracted compressional wave angle will be generated for given 

materials and angles. To find the angle of incidence required to generate a 

shear wave at a given angle complete the following: 

1. Set V1 to the longitudinal wave velocity of material 1. This material could 

be the transducer wedge or the immersion liquid. 

2. Set V2 to the shear wave velocity (approximately one-half its 

compressional velocity) of the material to be inspected. 

3. Set Q2 to the desired shear wave angle. 

4. Read Q1, the correct angle of incidence.

Transverse wave can be introduced into the test material by various methods: 

1. Inclining the incident L-wave at an angle beyond the first critical angle, yet 

short of second critical angle using a wedge. 

2. In immersion method, changing the angle of the normal search unit 

manipulator, 

3. Off-setting the normal transducer from the center-line for round bar or pipe. 

for 45° refracted transverse wave, the rule 

of thumb is the offset d= 1/6 of rod diameter

Offset of Normal probe above circular object 

θ 1 

θ 1R 

θ 2

Calculate the offset for following conditions: 

Aluminum rod being examined is 6" diameter, what is the off set needed for (a) 

45 refracted shear wave (b) Logitudinal wave to be generated? 

(L-wave velocity for AL=6.3x10 5 cm/s, T-wave velocity for AL=3.1x10 5 cm/s, 

Wave velocity in water=1.5X10 5 cm/s) 

Question (a)

Refraction and mode conversion at non-perpendicular boundaries

Refraction and mode conversion at non-perpendicular boundaries 

http://static4.olympus-ims.com/data/Flash/HTML5/incident_angle/IncidentAngle.html?rev=5E62




Q1. From the above figures, if the incident angle is 50 Degree, what are the 

sound wave in the steel? 

Answer: 65 Degree Shear wave in steel. 

Q2. If 50 Degree longitudinal wave in steel is used what is the possible 

problem? 

Answer: If 50 degree Longitudinal wave is generated in steel, shear wave at 

28 degree is also generated and this may cause fault indications. 


Q118: At the water-steel interface, the angle of incidence in water is 7 degree. 

The principle mode of vibration that exist in steel is: 

A. Longitudinal 

B. Shear 

C. Both A & B (Possible incorrect answer) 

D. Surface 

Hint: The keyword is “the principle mode” 


Q: On Calculation: 

Incident angle= 7° 

Refracted longitudinal wave = 29.11° 

Refracted shear wave = 15.49° 


Q72. In a water immersion test, ultrasonic energy is transmitted into steel at 

an incident angle of 14. What is the angle of refracted shear wave within 

the material? V s = 3.2 x 10 5 cm/s, V w = 1.5 x 10 5 cm/s 

a) 45° 

b) 23° 

c) 31° 

d) 13° 


Q1. If you were requested to design a plastid shoe to generate Rayleigh wave 

in aluminum, what would be the incident angle of the ultrasonic energy? 

VA = 3.1 x 105 cm/s, Vp = 2.6 x 105 cm/s 

a) 37° 

b) 57° 

c) 75° 

d) 48° 


Q53. The term used to determined the relative transmittance and reflectance 

of an ultrasonic energy at an interface is called: 

a) Acoustic attenuation 

b) Interface reflection 

c) Acoustic impedance ratio 

d) Acoustic frequency 



In a previous page, the effect that frequency and wavelength have on flaw 

detectability was discussed. However, the detection of a defect involves many 

factors other than the relationship of wavelength and flaw size. For example, 

the amount of sound that reflects from a defect is also dependent on the 

acoustic impedance mismatch between the flaw and the surrounding material. 

A void is generally a better reflector than a metallic inclusion because the 

impedance mismatch is greater between air and metal than between two 

metals. 

Often, the surrounding material has competing reflections. Microstructure 

grains in metals and the aggregate of concrete are a couple of examples. A 

good measure of detectability of a flaw is its signal-to-noise ratio (S/N). The 

signal-to-noise ratio is a measure of how the signal from the defect compares 

to other background reflections (categorized as "noise"). A signal-to-noise 

ratio of 3 to 1 is often required as a minimum.

The absolute noise level and the absolute strength of an echo from a "small" 

defect depends on a number of factors, which include: 

1. The probe size and focal properties. 

2. The probe frequency, bandwidth and efficiency. 

3. The inspection path and distance (water and/or solid). 

4. The interface (surface curvature and roughness). 

5. The flaw location with respect to the incident beam. 

6. The inherent noisiness of the metal microstructure. 

7. The inherent reflectivity of the flaw, which is dependent on its acoustic 

impedance, size, shape, and orientation. 

8. Cracks and volumetric defects can reflect ultrasonic waves quite differently. 

Many cracks are "invisible" from one direction and strong reflectors from 

another. 

9. Multifaceted flaws will tend to scatter sound away from the transducer.

The following formula relates some of the variables affecting the signal-tonoise 

ratio (S/N) of a defect:

Sound Volume: Area x pulse length 

Material properties 

Flaw geometry: Figure of merit 

FOM and amplitudes responds

Rather than go into the details of this formulation, a few fundamental 

relationships can be pointed out. The signal-to-noise ratio (S/N), and 

therefore, the detectability of a defect: 

1. Increases with increasing flaw size (scattering amplitude). The detectability 

of a defect is directly proportional to its size. 

2. Increases with a more focused beam. In other words, flaw detectability is 

inversely proportional to the transducer beam width. 

3. Increases with decreasing pulse width (delta-t). In other words, flaw 

detectability is inversely proportional to the duration of the pulse (∆t) 

produced by an ultrasonic transducer. The shorter the pulse (often higher 

frequency), the better the detection of the defect. Shorter pulses 

correspond to broader bandwidth frequency response. See the figure 

below showing the waveform of a transducer and its corresponding 

frequency spectrum.

Acoustic Volume: w x w y ∆t

Determining cross sectional area using reflector- A Scan (6db drop)

Determining cross sectional area using reflector- C Scan

“Sonic pulse volume” and S/N (defect resolution)

4. Decreases in materials with high density and/or a high ultrasonic velocity. 

The signal-to-noise ratio (S/N) is inversely proportional to material density 

and acoustic velocity. 

5. Generally increases with frequency. However, in some materials, such as 

titanium alloys, both the "A flaw " and the "Figure of Merit (FOM)" terms in the 

equation change at about the same rate with changing frequency. So, in 

some cases, the signal-to-noise ratio (S/N) can be somewhat independent 

of frequency.

Pulse Length

Pulse Length Affect Resolution

2.12: The Sound Fields 

2.12.1 Wave Interaction or Interference 

Before we move into the next section, the subject of wave interaction must 

be covered since it is important when trying to understand the performance 

of an ultrasonic transducer. On the previous pages, wave propagation was 

discussed as if a single sinusoidal wave was propagating through the 

material. However, the sound that emanates from an ultrasonic transducer 

does not originate from a single point, but instead originates from many 

points along the surface of the piezoelectric element. This results in a 

sound field with many waves interacting or interfering with each other.

Transducer cut-out 

http://ichun-chen.com/ultrasonic-transducer

When waves interact, they superimpose on each other, and the amplitude of 

the sound pressure or particle displacement at any point of interaction is the 

sum of the amplitudes of the two individual waves. First, let's consider two 

identical waves that originate from the same point. When they are in phase 

(so that the peaks and valleys of one are exactly aligned with those of the 

other), they combine to double the displacement of either wave acting alone. 

When they are completely out of phase (so that the peaks of one wave are 

exactly aligned with the valleys of the other wave), they combine to cancel 

each other out. When the two waves are not completely in phase or out of 

phase, the resulting wave is the sum of the wave amplitudes for all points 

along the wave.

UT Transducer

UT Transducer 

http://www.fhwa.dot.gov/publications/research/infrastructure/structures/04042/index.cfm#toc

UT Transducer- Surface creep wave transducer



Wave Interaction 

Complete in-phase Complete out of-phase not in-phase

When the origins of the two interacting waves are not the same, it is a little 

harder to picture the wave interaction, but the principles are the same. Up 

until now, we have primarily looked at waves in the form of a 2D plot of wave 

amplitude versus wave position. However, anyone that has dropped 

something in a pool of water can picture the waves radiating out from the 

source with a circular wave front. If two objects are dropped a short distance 

apart into the pool of water, their waves will radiate out from their sources and 

interact with each other. At every point where the waves interact, the 

amplitude of the particle displacement is the combined sum of the amplitudes 

of the particle displacement of the individual waves. 

With an ultrasonic transducer, the waves propagate out from the transducer 

face with a circular wave front. If it were possible to get the waves to 

propagate out from a single point on the transducer face, the sound field 

would appear as shown in the upper image to the right. Consider the light 

areas to be areas of rarefaction and the dark areas to be areas of 

compression.

With an ultrasonic transducer, the waves propagate out from the transducer 

face with a circular wave front. If it were possible to get the waves to 

propagate out from a single point on the transducer face, the sound field 

would appear as shown in the upper image to the right. Consider the light 

areas to be areas of rarefaction and the dark areas to be areas of 

compression.

However, as stated previously, sound waves originate from multiple points 

along the face of the transducer. The lower image to the right shows what the 

sound field would look like if the waves originated from just two points. It can 

be seen that where the waves interact, there are areas of constructive and 

destructive interference. The points of constructive interference are often 

referred to as nodes. 

The points of constructive interference 

are often referred to as nodes

29. It is possible for a discontinuity smaller than the transducer to produce 

indications of fluctuating amplitude as the search unit is moved laterally if 

testing is being performed in the: 

(a) Fraunhofer zone 

(b) Near field 

(c) Snell field 

(d) Shadow zone 


Q5: Acoustic pressure along the beam axis moving away from the probe has 

various maxima and minima due to interference. At the end of the near field 

pressure is: 

a) a maximum 

b) a minimum 

c) the average of all maxima and minima 

d) none of the above 

Q4: For a plane wave, sound pressure is reduced by attenuation in a 

_______ fashion. 

a) linear 

b) exponential 

c) random 

d) none of the above 


2.12.2 Variations in sound intensity 

Intensity 

Distance

Of course, there are more than two points of origin along the face of a 

transducer. The image below shows five points of sound origination. It can be 

seen that near the face of the transducer, there are extensive fluctuations or 

nodes and the sound field is very uneven. In ultrasonic testing, this in known 

as the near field (near zone) or Fresnel zone. The sound field is more 

uniform away from the transducer in the far field, or Fraunhofer zone, where 

the beam spreads out in a pattern originating from the center of the 

transducer. It should be noted that even in the far field, it is not a uniform 

wave front. However, at some distance from the face of the transducer and 

central to the face of the transducer, a uniform and intense wave field 

develops.

The sound wave exit from a transducer can be separated into 2 zones or 

areas; The Near Field (Fresnel) and the Far Field (Fraunhofer).

2.12.3 Fresnel & Fraunhofer Zone 

Fresnel Field, the Near Field are region directly adjacent to the transducer 

and characterized as a collection of symmetrical high and low pressure 

regions cause by interference wave fronts emitting from the continuous or 

near continuous sound sources. 

http://blog.3bscientific.com/science_education_insight/2013/04/3b-scientific-makes-waves-with-new-physics-education-kit.html

The Near Field (Fresnel) and the Far Field (Fraunhofer).

The Near Field (Fresnel)– Wave Interference (Maxima & Minima) 

The sound field of a transducer is divided into two zones; the near field and 

the far field. The near field is the region directly in front of the transducer 

where the echo amplitude goes through a series of maxima and minima and 

ends at the last maximum, at distance N from the transducer.

Amplitude ← 

Near Field Effect: Because of the variations within the near field it can be 

difficult to accurately evaluate flaws using amplitude based techniques. 

Near Field Y o 

+ 

Far Field 

Distance from Transducer face →

Fresnel / Fraunhofer Zone 


Near field (near zone) 

or Fresnel zone 

far field (far zone) 

or Fraunhofer zone 

Z f 


Near field (near zone) 

or Fresnel zone 

Crystal Focus 

Accoustical axis 

far field (far zone) 

or Fraunhofer zone 

Angle of divergence 

D 0 

6 

N 

Near field 

Far field 

Z f

Near/ Far Fields 

http://miac.unibas.ch/PMI/05-UltrasoundImaging.html

Near/ Far Fields

where α is the radius of the 

transducer and λ the wavelength.

where D is the diameter of the transducer 

and λ the wavelength. 

K= is the spread factor 

K=1.22 for null edges 

K=1.08 for 20dB down point (10% of peak) 

K=0.88 for 10dB down point (32% of peak) 

K=0.7(0.56?) for 6dB down point (50% of peak) 

Source for K, ASNT Study Guide UT by Matthew J Golis

The curvature and the area over which the sound is being generated, the 

speed that the sound waves travel within a material and the frequency of the 

sound all affect the sound field. Use the Java applet below to experiment with 

these variables and see how the sound field is affected. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/appletUltrasoundPropagation/Applet.html

Fresnel & Fraunhofer Zone 

10dB, K-0.88 

6dB, K=0.7? Or 0.56?

Fresnel & Fraunhofer Zone

Fresnel & Fraunhofer Zone

Fresnel & Fraunhofer Zone 

http://static1.olympus-ims.com/data/Flash/HTML5/beamSpread/BeamSpread.html?rev=6C43 

http://www.olympus-ims.com/en/ndt-tutorials/transducers/wave-front/

Q: Where does beam divergence occur? 

A. Near field 

B. Far field 

C. At the crystal 



Q4: A transducer has a near field in water of 35 mm. When used in contact 

on steel the near zone will be about: 

a) 47 mm 

b) 35 mm 

c) 18 mm 

d) 9 mm 

Q8: A rectangular probe, 4 mm X 8 mm, will have its maximum half angle of 

divergence: 

a) in the 4 mm direction 

b) in the 8 mm direction 

c) in no particular orientation 

d) constant in all directions 


Q160 Beam divergence is a function of the dimensions of the crystal and the 

wavelength of the beam transmitted through a medium, and it: 

A. increase if the frequency or the crystal diameter is decrease 

B. Decrease if the frequency or the crystal diameter is decrease 

C. increase if the frequency is increase and the diameter is decrease 

D. decrease if the frequency is increase and the crustal diameter is decrease 

Q52: What is the transducer half-angle beam spread of a 1.25cm diameter 

2.25 MHz transducer in water (VL= 1.5 x 10 5 cm/s)? 

A. 2.5 degree 

B. 3.75 degree 

C. 37.5 degree 

D. 40.5 degree

2.12.4 Dead Zone 

In ultrasonic testing, the interval following the initial pulse where the 

transducer ring time of the crystal that prevents detection or interpretation of 

reflected energy (echoes). In contact ultrasonic testing, the area just below 

the surface of a test object that can not be inspected because of the 

transducer is still ringing down and not yet ready to receive signals. The dead 

is minimized by the damping medium behind the crystal. The dead zone 

increase when the probe frequency decrease and it only found in single 

crystal contact techniques.

Dead Zone - The interval following the surface of a test object to the nearest 

inspectable depth. Any interval following a reflected signal where no direct 

echoes from discontinuities cannot be detected, due to characteristics of the 

equipment. 

dead zone after echo and dead zone after initial pulse, both are common 

phenomena. Actually the dead zone cannot be determined as a single figure 

without additional parameters, hence the echo can be recognized, however, 

signal quality is important. Useful parameters are linearity or signal in a nice 

ratio that can describe the echo amplitude quality within a dead zone. For this 

reason standards such as GE specifications are needed to check equipment 

capability. The appearance of inference effects, within the dead zone, has to 

be considered as well. 

Definition by: http://www.ndt.net/ndtaz/content.php?id=103

Dead Zone -The initial pulse is a technical necessity. It limits the detectability 

of near-surface discontinuities. Reflectors in the dead zone, the nonresolvable 

area immediately beneath the surface, cannot be detected (Figure 

8-10). The dead zone is a function of the width of the initial pulse which is 

influenced by the probe type, test instrument discontinuities and quality of the 

interface. 

The dead zone can be verified with an International Institute of Welding (IIW) 

calibration block. With the time base calibrated to 50 mm, and the transducer 

on position A (Figure 8-11), the extent of the dead zone can be inferred to be 

either less than or greater than 5 mm. With the probe at position B, the dead 

zone can be said to be either less than or greater than 10 mm. 

This is done by ensuring that the peak from the Perspex insert appears 

beyond the trailing edge of the initial pulse start. Excessive dead zones are 

generally attributable to a probe with excessive ringing in the crystal.

Dead Zone -The initial pulse is a technical necessity. It limits the detectability 

of near-surface discontinuities. Reflectors in the dead zone, the nonresolvable 

area immediately beneath the surface, cannot be detected. The 

dead zone is a function of the width of the initial pulse which is influenced by 

the probe type, test instrument discontinuities and quality of the interface. 

This is done by ensuring that the peak from the Perspex insert appears 

beyond the trailing edge of the initial pulse start. Excessive dead zones are 

generally attributable to a probe with excessive ringing in the crystal.

Dead Zone Illustration 

http://www.ndt.net/ndtaz/content.php?id=103

Dead Zone 

http://www.ni.com/white-paper/5369/en/

Q: On an A-scan display, the “dead zone” refers to: 

A. The distance contained within the near field 

B. The area outside the beam spread 

C. The distance covered by the front surface pulse with and recovery 

time 

D. The area between the near field and the far field 


Q36: To eliminate the decrease in sensitivity close to a wall which is parallel 

to the beam direction, the transducer used should be: 

A. As small as possible 

B. As low frequency as possible 

C. Both A & B 

D. Large and with a frequency as high as possible 

Q45: The length of the near field for a 2.5cm diameter, 5MHz transducer 

placed in oil with V=1.4 x 10 5 cm/s is approximately: 

A. 0.028 cm 

B. 6.25 cm 

C. 22.3 cm 

D. 55.8 cm 



Large Reflector, a reflector larger than the extreme edge of beam / 3D away 

from the Near Zone- Inverse Rule

Large Reflector Inverse Rule

Small Reflector, a reflector smaller than the extreme edge of beam / 3D away 

from the Near Zone – Inverse Square Rule

Small Reflector Inverse Square Rule


Another form wave interference occurred when the normal incidence and 

reflected plane wave interact within a narrow parallel interface. When the 

phase of the reflected wave match that of incoming incident wave, the 

amplitude of the superimposed wave doubling, creating a standing wave. 

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/string.html#c3 

Resonance occurred when the thickness of the material is equal to half the 

wave length or multiple of it. It also occur when longitudinal wave travel thru 

a thin sheet of materials during immersion testing.

Fundamental Frequency 

The lowest resonant frequency of a vibrating object is called its fundamental 

frequency. 

Most vibrating objects have more than one resonant frequency and those 

used in musical instruments typically vibrate at harmonics of the fundamental. 

A harmonic is defined as an integer (whole number) multiple of the 

fundamental frequency. 

Vibrating strings, open cylindrical air columns, and conical air columns will 

vibrate at all harmonics of the fundamental. Cylinders with one end closed will 

vibrate with only odd harmonics of the fundamental. Vibrating membranes 

typically produce vibrations at harmonics, but also have some resonant 

frequencies which are not harmonics. It is for this class of vibrators that the 

term overtone becomes useful - they are said to have some non-harmonic 

overtones. 

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/funhar.html

Thickness of Crystal at Fundamental Frequency 

Fundamental resonance frequency 

Harmonic resonance frequency 

= V/f 

= N. V/f (N = integer) 

Piezoelectric crystal will has the greatest sensitivity when it is driven at its 

fundamental frequency, this occurs when the thickness of the crystal is at ½ 

λ. 

If the thickness is given, the fundamental frequency could be calculated:

Transducers Piezoelectric Thickness: 

The resonant phenomenon occurred when piezoelectric are electrically 

excited at their characteristic (fundamental resonance) frequency. 

http://bme240.eng.uci.edu/students/09s/patelnj/Ultrasound_for_Nerves/Ultrasound_Background.html

Resonance UT Testing- The diagram below shown how resonance is used 

to measured thickness and detect defect. However pulse-echo methods have 

been refined to perform most of function of flaw detections and resonant 

instruments are rarely used.

Application Case#1: 

The specimen's geometry determines the number of its natural frequencies: a 

rod has few whilst a complex work-piece has many such frequencies. 

Typically, the information that can be obtained by acoustic resonance 

analysis includes cracks, structural properties, cavities, layer separation, 

chipping, density fluctuations etc. Damping behaviour depends firstly on the 

material, and secondly, on how the specimen is positioned during its 

excitation. In order to achieve high frequency resolution, signal duration 

("ringing duration") should be as long as possible (> 50 ms).

From the natural frequencies it is possible to calculate specimen-specific 

characteristics and assign them to quality attributes, e. g. pass / OK, cracked, 

material structure, hardness deviation / partly hardened etc. 

Application 

Acoustic resonance testing can be applied to all work pieces that "sound". 

Summary 

Resonance analysis is a qualitative method, i.e. it can differentiate between 

defective and non-defective parts, so that it is especially suitable for quality 

assurance in the series production cycle. It compares the actual oscillatory 

situation with the target one derived from a learning base. This learning base 

is established by using defined standard parts. The number of self-resonant 

frequencies is determined by the geometry of the object under test. For 

instance a bar has few resonant frequencies, while a complex lattice-type 

object has many natural resonances. After a systematic engineering 

approach, it is possible to compensate the influence of the production scatter. 

http://ndttechnologies.com/products/AcousticResonance.html

Application Case#2: 

Electromagnetic Acoustic Resonance Nondestructive Testing (NDT) 

Equipment Datasheets 

Coating Thickness Gauge -- DTG-500 

from OMEGA Engineering, Inc. 

Digital coating thickness gauge with a range of 0 to 40.0 mils (0 to 1000 

micrometers). SPECIFICATIONS. Display: 3-digit LCD with max readout of 

1999 counts. Range: 0 to 40 mils/0 to 1000 µm. Resolution: 0. 

Instrument Information 

Instrument Type: Coating Thickness 

Instrument Technology: Electromagnetic Acoustic Resonance 

Form Factor: Portable / Handheld / Mobile 

http://www.globalspec.com/specsearch/PartSpecs?partid={0DBF141D-6832-4F31-9AB8- 

B87F063BFDC4}&vid=99786&comp=2975

Q: The formula used to determine the fundamental resonance frequency is: 

A. F= V/T 

B. F= V/2T 

C. F= T/V 

D. F= VT 

Q: When maximum sensitivity is required from a transducer: 

A. A straight beam unit should be used 

B. A large diameter crystal should be used 

C. The piezoelectric element should be driven at its fundamental 

frequency 

D. The bandwidth of the transducer should be as large as possible 


Q7: The resonance frequency of 2cm thick plate of Naval Brass (V=4.43 x 10 5 

cm/s) is: 

A. 0,903 MHz 

B. 0.443 MHz 

C. 0.222 MHz 

D. 0.111 MHz 

Q35: Resonance testing equipment generally utilized: 

A. Pulsed longitudinal; waves 

B. Continuous longitudinal waves 

C. Pulsed shear wave 

D. Continuous shear waves 


2.15 Measurement of Sound

dB is a measures of ratio of 2 values in a logarithmic scale given by following 

equation: 

Unlike the SPL (standard pressure level) used in noise measurement, in UT testing, 

we do not know the exactly ultrasonic sound level energy generated by the probe 

(neither is it necessary). The used of the ratio of 2 values given by the above equation 

is used .

Ultrasonic Formula - Signal Amplitude Gain/Loss Expressed in dB 

The dB is a logarithmic unit that describes a ratio of two measurements. The 

equation used to describe the difference in intensity between two ultrasonic or 

other sound measurements is: 

where: ∆I is the difference in sound intensity expressed in decibels (dB), P1 

and P2 are two different sound pressure amplitude measurements, and the 

log is to base 10.

The Decibel 

The equation used to describe the difference in intensity between two 

ultrasonic or other sound measurements is: 

where: ∆I is the difference in sound intensity expressed in decibels (dB), P1 

and P2 are two different sound pressure measurements, and the log is to 

base 10. 

What exactly is a decibel? 

The decibel (dB) is one tenth of a Bel, which is a unit of measure that was 

developed by engineers at Bell Telephone Laboratories and named for 

Alexander Graham Bell. The dB is a logarithmic unit that describes a ratio of 

two measurements. The basic equation that describes the difference in 

decibels between two measurements is:

where: delta X is the difference in some quantity expressed in decibels, X1 

and X2 are two different measured values of X, and the log is to base 10. 

(Note the factor of two difference between this basic equation for the dB and 

the one used when making sound measurements. This difference will be 

explained in the next section.)

Why is the dB unit used? 

Use of dB units allows ratios of various sizes to be described using easy to 

work with numbers. For example, consider the information in the table.

From this table it can be seen that ratios from one up to ten billion can be 

represented with a single or double digit number. Ease to work with numbers 

was particularly important in the days before the advent of the calculator or 

computer. The focus of this discussion is on using the dB in measuring sound 

levels, but it is also widely used when measuring power, pressure, voltage 

and a number of other things.

Use of the dB in Sound Measurements 

Sound intensity is defined as the sound power per unit area perpendicular to 

the wave. Units are typically in watts/m2 or watts/cm2. For sound intensity, 

the dB equation becomes: 

However, the power or intensity of sound is generally not measured directly. 

Since sound consists of pressure waves, one of the easiest ways to quantify 

sound is to measure variations in pressure (i.e. the amplitude of the pressure 

wave). When making ultrasound measurements, a transducer is used, which 

is basically a small microphone. Transducers like most other microphones 

produced a voltage that is approximately proportionally to the sound pressure 

(P). The power carried by a traveling wave is proportional to the square of the 

amplitude. Therefore, the equation used to quantify a difference in sound 

intensity based on a measured difference in sound pressure becomes:

However, the power or intensity of sound is generally not measured directly. 

Since sound consists of pressure waves, one of the easiest ways to quantify 

sound is to measure variations in pressure (i.e. the amplitude of the pressure 

wave). When making ultrasound measurements, a transducer is used, which 

is basically a small microphone. Transducers like most other microphones 

produced a voltage that is approximately proportionally to the sound pressure 

(P). The power carried by a traveling wave is proportional to the square of the 

amplitude. 

I α P 2 , I α V 2 where I=intensity, P=amplitude, V=voltage 

Therefore, the equation used to quantify a difference in sound intensity based 

on a measured difference in sound pressure becomes: 

(The factor of 2 is added to the equation because the logarithm of the square of a 

quantity is equal to 2 times the logarithm of the quantity.)

Since transducers and microphones produce a voltage that is proportional to 

the sound pressure, the equation could also be written as: 

where: ∆I is the change in sound intensity incident on the transducer and 

V1 and V2 are two different transducer output voltages.

Revising the table to reflect the relationship between the ratio of the measured 

sound pressure and the change in intensity expressed in dB produces 

From the table it can be seen that 6 dB equates to 

a doubling of the sound pressure. Alternately, 

reducing the sound pressure by 2, results in a – 6 

dB change in intensity.

Sound Levels- Relative

Sound Levels- Relative dB

Practice:

“Absolute" Sound Levels 

Sound pressure level (SPL) or sound level is a logarithmic measure of the 

effective sound pressure of a sound relative to a reference value. It is 

measured in decibels (dB) above a standard reference level. The standard 

reference sound pressure in air or other gases is 20 µPa, which is usually 

considered the threshold of human hearing (at 1 kHz). 

http://en.wikipedia.org/wiki/DB_SPL#Sound_pressure_level

“Absolute" Sound Levels 

Whenever the decibel unit is used, it always represents the ratio of two values. 

Therefore, in order to relate different sound intensities it is necessary to 

choose a standard reference level. The reference sound pressure 

(corresponding to a sound pressure level of 0 dB) commonly used is that at 

the threshold of human hearing, which is conventionally taken to be 2×10 −5 

Newton per square meter, or 20 micropascals (20μPa). To avoid confusion 

with other decibel measures, the term dB(SPL) is used.


dB meter 

97.3dB against standards sound pressure level 

20log(P/20X10 -6 )=97.3 

Absolute level =10 97.3/20 x 20 X 10 -6 

=1.46564 N/M 2 

Actual Sound pressure → 

↖ Standard reference pressure 20 μMpa

Absolute: 

The standard reference sound pressure in air or other gases is 20 µPa, which 

is usually considered the threshold of human hearing (at 1 kHz). 

Actual Sound pressure → 

↖ Standard reference pressure 20 μMpa 

Absolute: 

Sound pressure level in dB as a ratio to 

standard reference in logarithmic scale. 

76db= 20log(P/20 μPa) 

Log(P/20 μPa)=3.8dB 

P= 10 3.8 x 20 μPa 

=126191 μPa 

http://www.ncvs.org/ncvs/tutorials/voiceprod/equation/chapter9/index.html

Exercise: 

Find the absolute sound level in μPa for the following measurement of air 

traffic noise.

Exercise: ANSWER 

Find the absolute sound level in μPa for the following measurement of air 

traffic noise. 

SPL= 95.8 dB= 20log(P/20x10 -6 ) 

log(P/20x10-6)= 95.8/20 

P= 10 95.8/20 x 20x10 -6 

P= 1.233 N/M 2 #

Practice: 

dB

Relative dB: Example Calculation 1 

Two sound pressure measurements are made using an ultrasonic 

transducer. The output voltage from the transducer is 600 mv for the first 

measurement and 100 mv for the second measurement. Calculate the 

difference in the sound intensity, in dB, between the two measurements? 

The sound intensity changed by -15.56dB. In other words, the sound 

intensity decreased by 15.56 dB

Example Calculation 2 

If the intensity between two ultrasonic measurements increases by 6 dB, and 

the first measurement produces a transducer output voltage of 30 mv, what 

was the transducer output voltage for the second measurement?

Example Calculation 3 

Consider the sound pressure difference between the threshold of human 

hearing, 0 dB, and the level of sound often produce at a rock concert, 120 dB. 

How much is the rock concert sound greater than that of the threshold of 

human hearing.

What is the absolute rock concert sound pressure?


Practice Makes Perfect 

28. An advantage of using lower frequencies during ultrasonic testing is that: 

(a) Near surface resolution is improved 

(b) Sensitivity to small discontinuities is improved 

(c) Beam spread is reduced 

(d) Sensitivity to unfavorable oriented flaws is improved 


Q104: If an ultrasonic wave is transmitted through an interface of two 

materials in which the first material has a higher acoustic impedance value 

but the same velocity value as the secong material, the angle of refraction 

will be: 

a) A greater than the incidence 

b) Less than the angle of incidence 

c) The same as the angle of incidence 

d) Beyond the critical angle. 


学习总是开心事 




学习总是开心事













Section 3: Equipment & Transducers 


Typical sound velocities

Wavelength in mm for Steel

Content: Section 3: Equipment & Transducers 






3.6: Transducer Testing I 




Continues Next Page





3.14: Transducer Quality Factor “Q” 


3.16: Testing Techniques 

3.17: UT Equipment Circuitry 

3.18: Further Reading on Sub-Section 3 

3.19: Questions & Answers


The Definitions: 

• Nominal frequency (F) - nominal operating frequency of the transducer 

(usually stamped on housing) 

• Peak frequency (PF) - the highest frequency response measured from 

the frequency spectrum 

• Bandwidth center frequency (BCF) - the average of the lowest and 

highest points at a -6 dB level of the frequency spectrum 

• Bandwidth (BW) - the difference between the highest and lowest 

frequencies at the -6 dB level of the frequency spectrum; also % of BCF or 

of PF 

• Pulse width (PW) - the time duration of the time domain envelope that is 

20 dB above the rising and decaying cycles of a transducer response

• Sensitivity is the ability of the search unit to detect reflections or echoes 

from small defects or flaws. 

• The acoustic impedance of a transducer is the product of its density and 

the velocity of sound within it. 

• Resolution is the resolving power includes the ability to separate 

reflections from two closely spaced flaws or reflectors. 

• Front surface pulse (at crystal face), Initial pulse, or “Main Bang” - the 

first indication on the screen, represents the emission of ultrasonic energy 

from the crystal face. 

• Front surface pulse (at interface) - ?

Pulse width (PW) - the time duration of the time domain envelope that is 20 

dB above the rising and decaying cycles of a transducer response

Bandwidth (BW) - the difference between the highest and lowest frequencies 

at the -6 dB level of the frequency spectrum; also % of BCF or of PF

Piezoelectric Properties 

The conversion of electrical pulses to mechanical vibrations and the 

conversion of returned mechanical vibrations back into electrical energy is the 

basis for ultrasonic testing. The active element is the heart of the transducer 

as it converts the electrical energy to acoustic energy, and vice versa. The 

active element is basically a piece of polarized material (i.e. some parts of the 

molecule are positively charged, while other parts of the molecule are 

negatively charged) with electrodes attached to two of its opposite faces. 

When an electric field is applied across the material, the polarized molecules 

will align themselves with the electric field, resulting in induced dipoles within 

the molecular or crystal structure of the material. 

The effectiveness of the search unit for a particular application depends on 

Q factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving 

power.

This alignment of molecules will cause the material to change dimensions. 

This phenomenon is known as electrostriction. In addition, a permanentlypolarized 

material such as quartz (SiO2) or barium titanate (BaTiO3) will 

produce an electric field when the material changes dimensions as a result of 

an imposed mechanical force. This phenomenon is known as the 

piezoelectric effect. Additional information on why certain materials produce 

this effect can be found in the linked presentation material, which was 

produced by the Valpey Fisher Corporation. 

Keyword: 

SiO2- Quartz 

BaTiO3- Barium Titanate 

Electric field is applied causing dimensional change: electrostriction 

Electric field is generated by dimensional change: piezoelectric effect

Fig. 5.10: Basic design of a single 

transducer Ultrasound head 

Piezoelectric materials have two nice 

properties: 

1. Piezoelectric materials change their 

shape upon the application of an 

electric field as the orientation of the 

dipoles changes. 

2. Conversely, if a mechanical forces 

is applied to the crystal a the 

electric field is changed producing a 

small voltage signal. 

The piezoelectric crystals thus function 

as the transmitter as well as the 

receiver!

Transducer Effectiveness 

The effectiveness of the search unit for a particular application depends on 

Q factor, bandwidth, frequency, sensitivity, acoustic impedance, and resolving 

power.

Piezoelectric crystals 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricEffect.ppt 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/PiezoelectricElements.ppt

Piezoelectric crystals 

http://www.ndt-kits.com/blog/wp-content/uploads/2013/05/What-is-piezoelectric-transducer.gif 

http://www.ndt-kits.com/blog/?cat=7

Piezoelectric crystals




The active element of most acoustic transducers used today is a 

piezoelectric ceramic, which can be cut in various ways to produce different 

wave modes. A large piezoelectric ceramic element can be seen in the image 

of a sectioned low frequency transducer. Preceding the advent of 

piezoelectric ceramics in the early 1950's, piezoelectric crystals made from 

quartz crystals and magnetostrictive materials were primarily used. The active 

element is still sometimes referred to as the crystal by old timers in the NDT 

field. When piezoelectric ceramics were introduced, they soon became the 

dominant material for transducers due to their good piezoelectric properties 

and their ease of manufacture into a variety of shapes and sizes. They also 

operate at low voltage and are usable up to about 300°C. The first 

piezoceramic in general use was (1) barium titanate, and that was followed 

during the 1960's by (2) lead Zirconate Titanate compositions, which are now 

the most commonly employed ceramic for making transducers. New materials 

such as piezo-polymers and composites are also being used in some 

applications. 

Keywords: 

(1) Barium Titanate 

(2) Lead Zirconate Titanate

The thickness of the active element is determined by the desired frequency of 

the transducer. A thin wafer element vibrates with a wavelength that is twice 

its thickness. Therefore, piezoelectric crystals are cut to a thickness that is ½ 

the desired radiated wavelength. The higher the frequency of the transducer, 

the thinner the active element. The primary reason that high frequency 

contact transducers are not produced is because the element is very thin and 

too fragile.

The fundamental frequency of the transducer is determined by its thickness: 

From the equation, it can be seen that for high frequency transducer, the 

thickness is very thin , thus fragile; making its only suitable for immersion 

techniques only.

At Interface: Reflection & Transmittance 

1,87 

Incoming wave 

1,0 

0,87 

Transmitted wave 

Reflected wave 

Perspex 

Steel


Incoming wave 

1,0 

Transmitted wave 

0,13 

Reflected wave 

Perspex 

-0,87 

Steel

At Interface: Reflection & Transmittance


At first glance a sound pressure exceeding 

100 % seems paradoxical and one suspects 

a contradiction of the energy law. However, 

according to Eq. (1.4) the intensity, i.e. the 

energy per unit time and unit area, is not 

calculated from the sound pressure 

(squared) only but also from the acoustic 

impedance of the material in which the wave 

travels. However, since this impedance in 

steel is very much greater than in water, the 

calculation shows that the intensity of the 

transmitted wave is very much smaller there 

than in water in spite of the higher sound 

pressure.

Piezoelectric crystals may be X or Y cut depending on which orientation 

they are sliced. The crystals used in UT testing are X cut, due to the mode of 

vibration they produced (longitudinal wave). This means that the crystal is 

sliced with it main axis perpendicular with the X axis.


Q153 A quartz crystal cut so that its major faces are parallel to the X, Y axes 

and perpendicular to the X axis is called: 

a) a Y-cut crystal/ longitudinal wave 

b) a Y-cut crystal/ shear wave 

c) a X-cut crystal/ longitudinal wave 

d) a X-cut crystal/ shear wave 

e) a XY-cut crystal/ longitudinal wave 

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html




3.1.1: Type of Piezoelectric Crystal 

■ Quartz is a Silicon Oxide (SiO 3 ) 

■ Lithium Sulphate LiSO 4 Decomposed 130°C 

■ Barium Titanate (BaTiO 3 ) Curies point 120°C 

■ Lead Metaniobate (PBNbO 6 ) 

■ Lead Zirconate Titanate (PBZrO 3 . PbTiO 3 )* Curies point 350°C 

*Pb[Zr x Ti 1-x ]O 3 (0≤x≤1).

■ Quartz is a Silicon Oxide (SiO 3 ) crystal found naturally and X cut across 

the crustal give compression wave, a Y cut produces shear wave. 

Advantages: 

1. Resistance to wear 

2. insoluble in water 

3. resistance to ageing 

4. easy to cut to give the required frequency 

Disadvantage 

1. It is inefficient, needs a lot of energy to 

produce small amount of ultrasound 

2. Quart crystals are susceptible to 

damages (nor robust) 

3. High voltage to produce low frequency 

sound

Quartz

SiO3-Silicon Quartz

■ 

Lithium Sulphate LiSO 4 , grows from Lithium Sulphate solution by 

evaporation. 

Advantages: 

1. Lithium Sulphate is the most efficient receiver of ultrasound 

2. It has low electric impedance 

3. Operate well at low voltage 

4. it does not age 

5. it has very good resolution 

6. crystals are easily damp and give a short pulse length 

Disadvantage 

1. It dissolves in water 

2. It breaks easily 

3. It decomposed at temperature above 130°C (what is Curie temperature?) 

All of which make it unsuitable for industrial used, except for medical 

ultrasonic where the temperature restriction is not a concern.

Lithium Sulphate LiSO 4 硫酸锂

Followings are Piezoelectric crystals- Polarized crystals made by heating up 

powders to high temperatures, pressing them into shape and allow them to 

cool in a very strong electric fields. 

Heat applied 

Pressed Powders 

Fused polarized PZT 

Heat applied

■ Barium Titanate (BaTiO 3 ) are polarized crystals made by baking Barium 

Titanate at 1250C and cooling in a 2KV/mm electric field. 

Advantages 

It is efficient ultrasound generator 

It requires low voltage 

It has good sensitivity 

Disadvantages 

Its curies point is about only 120°C, above which it loss it functionality 

It deteriorated over time

BaTiO 3

BaTiO 3

■ Lead Metaniobate (PBNbO 6 ) crystals are made the similar way as 

Barium Titanate 

Advantages 

It has high internal damping 

It gives narrow pulse of ultrasound, which gives good resolution 

Disadvantage 

It has much less sensitivity than Lead Zirconate Titanate PZT

Fig. 3: Comparison between PZT (left) and 1-3 piezocomposite transducer 

(right) on a prospect wedge

Fig. 4: Comparison between lead Metaniobate (left) and 1-3 piezocomposite 

transducer (right) for a WSY70-4 probe 

http://www.ndt.net/article/splitt/splitt_e.htm

■ Lead Zirconate Titanate (PBZrO 3 . PbTiO 3 )* is the best all round crystal 

for industrial use. 

Advantages 

■ It has high Curies point 350°C 

■ It has good resolution 

■ It does not dissolved in water 

■ It is tough 

■ It does not dissolve in water 

■ It is easily damp. 

Other Transducer> Polyvinylchloride probe for high frequency 15MHz, giving 

high resolution and very high sensitivity. 

*Pb[Zr x Ti 1-x ]O 3 (0≤x≤1).

■ Lead Zirconate Titanate PZT Curies point 350°C 

350°C

350°C is also goof for:



Curie Temperature: In physics and materials science, the Curie temperature 

(Tc), or Curie point, is the temperature where a material's permanent 

magnetism changes to induced magnetism. The force of magnetism is 

determined by magnetic moments. The Curie temperature is the critical point 

where a material's intrinsic magnetic moments change direction. Magnetic 

moments are permanent dipole moments within the atom which originate from 

electrons' angular momentum and spin. Materials have different structures of 

intrinsic magnetic moments that depend on temperature. At a material's Curie 

Temperature those intrinsic magnetic moments change direction. 

Permanent magnetism is caused by the alignment of magnetic moments and 

induced magnetism is created when disordered magnetic moments are forced 

to align in an applied magnetic field. For example, the ordered magnetic 

moments (ferromagnetic, figure 1) change and become disordered 

(paramagnetic, figure 2) at the Curie Temperature. Higher temperatures make 

magnets weaker as spontaneous magnetism only occurs below the Curie 

Temperature. Magnetic susceptibility only occurs above the Curie 

Temperature and can be calculated from the Curie-Weiss Law which is 

derived from Curie's Law.

Lead zirconium Titanate is an intermetallic inorganic compound with the 

chemical formula Pb[Zr x Ti 1-x ]O 3 (0≤x≤1). Also called PZT, it is a ceramic 

perovskite material that shows a marked piezoelectric effect, which finds 

practical applications in the area of electroceramics. It is a white solid that is 

insoluble in all solvents.

Lead zirconium Titanate PZT 

http://en.wikipedia.org/wiki/Lead_zirconate_titanate

http://www.ndt.net/article/platte2/platte2.htm

Properties of Piezoelectric Materials

Ceramic Transducer

Q67: Which of the following transducer materials is the most efficient receiver 

of ultrasonic energy? 

(a) Lead metaniobate 

(b) Quartz 

(c) Lithium sulphate 

(d) Barium titanate 

Q69: An advantage of using lithium sulphate in search units it that: 

(a) It is one of the most efficient generators of ultrasonic energy 

(b) It is one of the most efficient receivers of ultrasonic energy 

(c) It is insoluble 

(d) It can withstand temperatures as high as 700ºC

Q68: Which of the following transducer materials is the most efficient 

transmitter of ultrasonic energy? 

(a) Lead metaniobate 

(b) Quartz 

(c) Lithium sulphate 

(d) Barium titanate 

Q17: Which of the following is the least efficient receiver of ultrasonic Energy? 

(a) Quartz 

(b) Lithium sulphate 

(c) Lead metaniobate 

(d) Barium titanate

Q21: An advantage of using a ceramic transducer in search units is that: 



(c) It has a very low mechanical impedance 

(d) It can withstand temperatures as high as 700 o C

Q73: Which of the following is the most durable piezoelectric material? 

A. Barium titanate 

B. Quartz 

C. Dipotassoium tartrate 

D. Rochelle salt 

Q12: The 1 MHz transducer that should normally have the best time or 

distance resolution is a: 

A. Quartz transducer with air backing 

B. Quartz transducer with phenolic backing 

C. Barium titanate transducer with phenolic backing 

D. Lithium Sulphate transducer with epoxy backing


The transducer is a very important part of the ultrasonic instrumentation 

system. As discussed on the previous page, the transducer incorporates a 

piezoelectric element, which converts electrical signals into mechanical 

vibrations (transmit mode) and mechanical vibrations into electrical signals 

(receive mode). Many factors, including material, mechanical and electrical 

construction, and the external mechanical and electrical load conditions, 

influence the behavior of a transducer. Mechanical construction includes 

parameters such as the radiation surface area, mechanical damping, housing, 

connector type and other variables of physical construction. As of this writing, 

transducer manufacturers are hard pressed when constructing two 

transducers that have identical performance characteristics.

Transducer

Transducer PZT & Matching Layer Thicknesses

3.2.1 Transducer Cut-Out 

A cut away of a typical contact transducer is shown above. It was previously 

learned that the piezoelectric element is cut to ½ the desired wavelength. To 

get as much energy out of the transducer as possible, an impedance 

matching is placed between the active element and the face of the transducer. 

Optimal impedance matching is achieved by sizing the matching layer so that 

its thickness is ¼ of the desired wavelength. This keeps waves that were 

reflected within the matching layer in phase when they exit the layer (as 

illustrated in the image to the top). (HOW?) 

For contact transducers, the matching layer is made from a material that has 

an acoustical impedance “Z” between the active element and steel. 

Immersion transducers have a matching layer with an acoustical impedance 

“Z” between the active element and water. 

Contact transducers also incorporate a wear plate to protect the matching 

layer and active element from scratching.

Contact Transducer Types: 

socket 

crystal 

Damping 

Delay / protecting face 

Electrical matching 

Cable 

Straight beam probe 

TR-probe 

Angle beam probe

Transducer

Transducer: Straight Beam

Transducer: Angle Beam

Transducer Cut-Out

3.2.2 The Active Element (Crystal) 

The active element, which is piezo or ferroelectric material, converts 

electrical energy such as an excitation pulse from a flaw detector into 

ultrasonic energy. The most commonly used materials are polarized 

ceramics which can be cut in a variety of manners to produce different wave 

modes. New materials such as piezo polymers and composites are also 

being employed for applications where they provide benefit to transducer 

and system performance.

3.2.3 Design of Matching Layer 

The matching layer consists of a layer of material with acoustic impedance 

that of intermediate between the top & bottom mediums. The thickness its 

thickness is ¼ of the desired wavelength , determined from the center 

operating frequency of the transducer and the speed of sound of the matching 

layer.

Matching Layer: Immersion & Delay Transducers 

Backing 

As wear plate 

λ /2 

λ /4 

Active Element 

Matching Layer

3.2.4 Backing (Damping) 

The backing is usually a highly attenuative, high density material that is used 

to control the vibration of the transducer by absorbing the energy radiating 

from the back face of the active element. When the acoustic impedance 

of the backing matches the acoustic impedance of the active element, 

the result will be a heavily damped transducer that displays good range 

resolution but may be lower in signal amplitude. If there is a mismatch in 

acoustic impedance between the element and the backing, more sound 

energy will be reflected forward into the test material. The end result is a 

transducer that is lower in resolution due to a longer waveform duration, but 

may be higher in signal amplitude or greater in sensitivity.

Note on Backing: 

The backing material supporting the crystal has a great influence on the 

damping characteristics of a transducer. 

Using a backing material with an impedance similar to that of the active 

element will produce the most effective damping. Such a transducer will have 

a wider bandwidth resulting in higher sensitivity. 

As the mismatch in impedance between the active element and the backing 

material increases, material penetration increases but transducer sensitivity is 

reduced. 

Keywords: 

Backing impedance mismatch small: Higher sensitivity 

Backing impedance mismatch high: Higher penetration.

3.2.5 Wear Plate 

The basic purpose of the transducer wear plate is to protect the transducer 

element from the testing environment. In the case of contact transducers, the 

wear plate must be a durable and corrosion resistant material in order to 

withstand the wear caused by use on materials such as steel.

Matching Layer (Wear Plate) 

For immersion, angle beam, and delay line transducers the wear plate has 

the additional purpose of serving as an acoustic transformer between the 

high acoustic impedance of the active element and the water, the wedge 

or the delay line all of which are of lower acoustic impedance. 

This is accomplished by selecting a 

matching layer that is ¼ λ 

wavelength thick and of the desired 

acoustic impedance (the active 

element is nominally ½ λ wavelength). 

The choice of the wear surface 

thickness is based upon the idea of 

superposition that allows waves 

generated by the active element to be 

in phase with the wave reverberating 

in the matching layer as shown in 

Figure (4).

When signals are in phase, their amplitudes are additive, thus a greater 

amplitude wave enters the test piece. Figure (12) shows the active element 

and the wear plate, and when they are in phase. If a transducer is not tightly 

controlled or designed with care and the proper materials, and the sound 

waves are not in phase, it causes a disruption in the wave front.

Transducers

Transducers 

http://www.ndt-kits.com/Angle-Beam-Ultrasonic-Transducer-UT0013-s-381-428.html

3.2.6 Transducer Efficiency, Bandwidth and Frequency 

3.2.6.1 Resolution 

Some transducers are specially fabricated to be more efficient transmitters 

and others to be more efficient receivers. A transducer that performs well in 

one application will not always produce the desired results in a different 

application. For example, sensitivity to small defects is proportional to the 

product of the efficiency of the transducer as a transmitter and a receiver. 

Resolution, the ability to locate defects near the surface or in close proximity 

in the material, requires a highly damped transducer.

Resolution: BS4331 Pt 3. the 

recommended resolution should 

be able to distinguished two 

discrete echoes less than two 

wavelength apart. By discrete 

echoes mean they are split by 

more than 6dB. 

(Vertical spatial resolution) 

50% Amplitude or 

6dB line.

2 λ 

50% Amplitude or 

6dB line. 

2 λ

In the early days of ultrasonic testing we used the 100, 91 and 85mm steps, at the radius end of 

the V1 block to test resolving power. However, today this is regarded as too crude a test and BS 

4331 Part 3 (now obsolete) recommended that we should be able to recognise two discrete 

echoes less than two wavelengths apart. By discrete echoes they mean split by more than 6dB, 

or to more than half the total height of the signals.

3.2.6.2 Transducer Damping 

It is also important to understand the concept of bandwidth, or range of 

frequencies, associated with a transducer. The frequency noted on a 

transducer is the central or center frequency and depends primarily on the 

backing material. 

Highly damped transducers will respond to frequencies above and below the 

central frequency. The broad frequency range provides a transducer with high 

resolving power. Less damped transducers will exhibit a narrower frequency 

range and poorer resolving power, but greater penetration. 

The central frequency will also define the capabilities of a transducer. Lower 

frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in a 

material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced 

penetration but greater sensitivity to small discontinuities. High frequency 

transducers, when used with the proper instrumentation, can improve flaw 

resolution and thickness measurement capabilities dramatically. Broadband 

transducers with frequencies up to 150 MHz are commercially available.

Transducer Damping (illustration with X-axis frequency domain) 

Less damped transducers will 

exhibit a narrower frequency range 

and poorer resolving power, but 

greater penetration. 

Highly damped transducers will 

respond to frequencies above and 

below the central frequency. The 

broad frequency range provides a 

transducer with high resolving 

power.

Transducer (Backing) Damping: 

• Highly damped transducers will respond to frequencies above and below 

the central frequency. The broad frequency range provides a transducer 

with high resolving power. 

• Less damped transducers will exhibit a narrower frequency range and 

poorer resolving power, but greater penetration.

Transducer Damping 

Narrow 

bandwidth 

X-axis time domain 

Wide 

bandwidth 

X-axis time domain

Transducer Damping

Transducer Damping

Transducer Damping at -20dB

Transducer Damping at -14dB

Transducer Damping

Transducer Damping- Pulse Length

Wave form Duration at -10dB

Transducer Damping- Low Damping (X-axis time domain)

Transducer Damping- High Damping (X-axis time domain)

48. A more highly damped transducer crystal results in: 

(a) Better resolution 

(b) Better sensitivity (mistake) 

(c) Lower sensitivity 

(d) Poorer resolution

3.2.6.3 Bandwidth: 

It is also important to understand the concept of bandwidth, or range of 

frequencies, associated with a ultrasonic transducer. The frequency noted on 

a transducer is the central or center frequency and depends primarily on the 

backing material. 

Highly damped ultrasonic transducers will respond to frequencies above and 

below the central frequency. The broad frequency range provides a 

transducer with high resolving power. 

Less damped transducers will exhibit a narrower frequency range and poorer 

resolving power, but greater penetration. 

The central frequency will also define the capabilities of a transducer. Lower 

frequencies (0.5MHz-2.25MHz) provide greater energy and penetration in 

material, while high frequency crystals (15.0MHz-25.0MHz) provide reduced 



resolution and thickness measurement capabilities dramatically. Broadband 

transducers with frequencies up to 150 MHz are commercially available.

Bandwidth: 

• The unit for bandwidth is MHz 

• The unit for pulse length is mm or time

The central frequency will also define the capabilities of a transducer. 

1. Lower frequencies (0.5MHz-2.25MHz) provide greater energy and 

penetration in a material, 

2. while high frequency crystals (15.0MHz-25.0MHz) provide reduced 



resolution and thickness measurement capabilities dramatically.





The relation between MHz bandwidth and waveform duration is shown 

in Figure below. The scatter is wider at -40 dB because the 1% trailing end of 

the waveform contains very little energy and so has very little effect on the 

analysis of bandwidth. Because of the scatter it is most appropriate to specify 

waveforms in the time domain (microseconds) and spectra in the frequency 

domain.

Transducers are constructed to withstand some abuse, but they should be 

handled carefully. Misuse, such as dropping, can cause cracking of the wear 

plate, element, or the backing material. Damage to a transducer is often 

noted on the A-scan presentation as an enlargement of the initial pulse.

The approximate relations shown in Figure (6) above, can be used to assist in 

transducer selection. For example, if a -14 dB waveform duration of one 

microsecond is needed, what frequency transducer should be selected? 

From the graph, a bandwidth of approximately 1 to 1.2 MHz corresponds 

to approximately 1 microsecond -14 dB waveform duration. Assuming a 

nominal 50% fractional bandwidth transducer, this calculates to a nominal 

center frequency of 2 to 2.4 MHz. Therefore, a transducer of 2.25 MHz or 

3.5 MHz may be applicable. 

http://olympus-ims.com/data/File/panametrics/UT-technotes.en.pdf

Instrumentation Filtered Band Width: 

1. Broad band instrument means a wide array of frequencies could be 

processed by the instrument. The frequencies shown will be a close 

representation of the actual electrical signal measured by the receiver 

transducer. The S/N may not be very good, the shape of the amplitude 

tend to be the actual representation. 

2. Narrow band instrument, suppressed a portion of frequencies above and 

below the center frequency. With the high frequencies noise suppressed, 

gain could be increase, leading to improved sensitivity. However the shape 

and relative amplitude of pulse frequency components often altered

Instrumentation Band Width:

Q8: Receiver noise must often be filtered out of a test system. Receiver 

amplifier noise increases proportionally to: 

A. the square root of the amplifier bandwidth 

B. the inverse square of the amplifier bandwidth 

C. attenuation 

D. temperature

Q164: The resolving power of a transducer is directly proportional to its: 

A. Diameter 

B. Bandwidth 

C. Pulse repetition rate 


Bandwidth is the frequency range of the pulse, it is not the pulse length

Q48: The approximate bandwidth of the transducer with the frequency 

response shown in figure 1 (-3dB) is: 

A. 4 MHz (standard answer) 

B. 8 MHz 

C. 10 MHz 

D. 12 MHz 

≈6.5MHz


The sound that emanates from a piezoelectric transducer does not originate 

from a point, but instead originates from most of the surface of the 

piezoelectric element. Round transducers are often referred to as piston 

source transducers because the sound field resembles a cylindrical mass in 

front of the transducer. The sound field from a typical piezoelectric transducer 

is shown below. The intensity of the sound is indicated by color, with lighter 

colors indicating higher intensity. 

Ɵ

Since the ultrasound originates from a number of points along the transducer 

face, the ultrasound intensity along the beam is affected by constructive and 

destructive wave interference as discussed in a previous page on wave 

interference. These are sometimes also referred to as diffraction effects. This 

wave interference leads to extensive fluctuations in the sound intensity near 

the source and is known as the near field. Because of acoustic variations 

within a near field, it can be extremely difficult to accurately evaluate flaws in 

materials when they are positioned within this area.

The pressure waves combine to form a relatively uniform front at the end of 

the near field. The area beyond the near field where the ultrasonic beam is 

more uniform is called the far field. In the far field, the beam spreads out in a 

pattern originating from the center of the transducer. The transition between 

the near field and the far field occurs at a distance, N, and is sometimes 

referred to as the "natural focus" of a flat (or unfocused) transducer. The 

near/far field distance, N, is significant because amplitude variations that 

characterize the near field change to a smoothly declining amplitude at this 

point. The area just beyond the near field is where the sound wave is well 

behaved and at its maximum strength. Therefore, optimal detection results 

will be obtained when flaws occur in this area.

Near Field

Angular characteristics for large distances from the oscillator. 

a: Values of the sound pressure in a linear plot; 

b: the same plotted in dB

Angular characteristics: Lines of equal sound pressure, plotted in dB. Also 

the distance from the radiator is plotted in a logarithmic measure

Angular characteristics: Spatial distribution of the sound pressure plotted in 

linear values on a half plane through the radiator

Angular characteristics: Sound-pressure mountain measured in a plane 

parallel to the oscillator

Angular characteristics: Sound pressure on the axis of a piston oscillator

For a piston source transducer of radius (a), frequency (f), and velocity (V) in 

a liquid or solid medium, the applet below allows the calculation of the 

near/far field transition point. In the Java applet below, the radius (a) and the 

near field/far field distance can be in metric or English units (e.g. mm or inch), 

the frequency (f) is in MHz and the sound velocity (V) is in metric or English 

length units per second (e.g. mm/sec or inch/sec). Just make sure the length 

units used are consistent in the calculation.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_3/applet_3_3.htm

Spherical or cylindrical focusing changes the structure of a transducer field by 

"pulling" the N point nearer the transducer. It is also important to note that the 

driving excitation normally used in NDT applications are either spike or 

rectangular pulsars, not a single frequency. This can significantly alter the 

performance of a transducer. Nonetheless, the supporting analysis is widely 

used because it represents a reasonable approximation and a good starting 

point.

Beam Spreads 

http://www.eclipsescientific.com/Software/ESBeamToolAScan/index.html

Probe Dimension & Spread angle 

探子小 , 近场杂波短 , 声扩张度较大 .

Probe Dimension & Spread angle 

探子大 , 近场杂波长 , 声扩张度较小 .

Probe dimension & Z f , , Ɵ 


Probe dimension & Z f, , Ɵ 



As discussed on the previous page, round transducers are often referred to 

as piston source transducers because the sound field resembles a cylindrical 

mass in front of the transducer. However, the energy in the beam does not 

remain in a cylinder, but instead spreads out as it propagates through the 

material. The phenomenon is usually referred to as beam spread but is 

sometimes also referred to as beam divergence or ultrasonic diffraction. It 

should be noted that there is actually a difference between beam spread and 

beam divergence. Beam spread is a measure of the whole angle from side to 

side of the main lobe of the sound beam in the far field. Beam divergence is a 

measure of the angle from one side of the sound beam to the central axis of 

the beam in the far field. Therefore, beam spread is twice the beam 

divergence. 

Far field, or Fraunhofer zone

Although beam spread must be considered when performing an ultrasonic 

inspection, it is important to note that in the far field, or Fraunhofer zone, the 

maximum sound pressure is always found along the acoustic axis (centerline) 

of the transducer. Therefore, the strongest reflections are likely to come from 

the area directly in front of the transducer. 

Beam spread occurs because the vibrating particle of the material (through 

which the wave is traveling) do not always transfer all of their energy in the 

direction of wave propagation. Recall that waves propagate through the 

transfer of energy from one particle to another in the medium. If the particles 

are not directly aligned in the direction of wave propagation, some of the 

energy will get transferred off at an angle. (Picture what happens when one 

ball hits another ball slightly off center). In the near field, constructive and 

destructive wave interference fill the sound field with fluctuation. At the start of 

the far field, however, the beam strength is always greatest at the center of 

the beam and diminishes as it spreads outward.

As shown in the applet below, beam spread is largely determined by the 

frequency and diameter of the transducer. Beam spread is greater when 

using a low frequency transducer than when using a high frequency 

transducer. As the diameter of the transducer increases, the beam spread will 

be reduced. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/toplinks-rev2.swf

Near/ Far Fields 

Near field, constructive and 

destructive wave interference fill the 

sound field with fluctuation 

- reverberence 

Far field, however, the 

beam strength is always 

greatest at the center of the 

beam and diminishes as it 

spreads outward.

Beam angle is an important consideration in transducer selection for a couple 

of reasons. First, beam spread lowers the amplitude of reflections since 

sound fields are less concentrated and, thereby weaker. Second, beam 

spread may result in more difficulty in interpreting signals due to reflections 

from the lateral sides of the test object or other features outside of the 

inspection area. Characterization of the sound field generated by a transducer 

is a prerequisite to understanding observed signals. 

Numerous codes exist that can be used to standardize the method used for 

the characterization of beam spread. American Society for Testing and 

Materials ASTM E-1065, addresses methods for ascertaining beam shapes in 

Section A6, Measurement of Sound Field Parameters. However, these 

measurements are limited to immersion probes. In fact, the methods 

described in E-1065 are primarily concerned with the measurement of beam 

characteristics in water, and as such are limited to measurements of the 

compression mode only. Techniques described in E-1065 include pulse-echo 

using a ball target and hydrophone receiver, which allows the sound field of 

the probe to be assessed for the entire volume in front of the probe.

For a flat piston source transducer, an approximation of the beam spread may 

be calculated as a function of the transducer diameter (D), frequency (F), and 

the sound velocity (V) in the liquid or solid medium. The applet below allows 

the beam divergence angle (1/2 the beam spread angle) to be calculated. 

This angle represents a measure from the center of the acoustic axis to the 

point where the sound pressure has decreased by one half (-6 dB) to the side 

of the acoustic axis in the far field. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/EquipmentTrans/applet_3_4/applet_3_4.htm


Ultrasonic transducers are manufactured for a variety of applications and can 

be custom fabricated when necessary. Careful attention must be paid to 

selecting the proper transducer for the application. A previous section on 

Acoustic Wavelength and Defect Detection gave a brief overview of factors 

that affect defect detectability. From this material, we know that it is important 

to choose transducers that have the desired; 

■ 

■ 

■ 

frequency, (thickness of piezoelectric material) 

bandwidth, (Back damping) 

Focusing (curvature probe) 

to optimize inspection capability. Most often the transducer is chosen either to 

enhance the sensitivity or resolution of the system. Transducers are classified 

into groups according to the application.

3.5.1 Contact transducers 

are used for direct contact inspections, and are generally hand manipulated. 

They have elements protected in a rugged casing to withstand sliding contact 

with a variety of materials. These transducers have an ergonomic design so 

that they are easy to grip and move along a surface. They often have 

replaceable wear plates to lengthen their useful life. Coupling materials of 

water, grease, oils, or commercial materials are used to remove the air gap 

between the transducer and the component being inspected.

Contact Transducers

Contact probe

Contact Transducer 

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/ 

http://static2.olympus-ims.com/data/Flash/dual.swf?rev=6C5C


43. Which of the following is a disadvantage of contact testing? 

(a) Ability to maintain uniform coupling on rough surface 

(b) Ease of field use 

(c) Greater penetrating power than immersion testing 

(d) Less penetrating power than immersion testing

3.5.2 Immersion transducers 

In immersion testing, the transducer do not contact the component. These 

transducers are designed to operate in a liquid environment and all 

connections are watertight. Immersion transducers usually have an 

impedance matching layer that helps to get more sound energy into the water 

and, in turn, into the component being inspected. Immersion transducers can 

be purchased with a (1) planer, (2) cylindrically focused or (3) spherically 

focused lens. A focused transducer can improve the sensitivity and axial 

resolution by concentrating the sound energy to a smaller area. Immersion 

transducers are typically used inside a water tank or as part of a squirter or 

bubbler system in scanning applications.

Unfocused & Focused

Focusing Ration in water/steel (F=4) 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/beam-characteristics/

Focused Transducer (Olympus) 

Z B 

F Z 

Z E 

D 

= Beginning of the Focal Zone 

= Focal Zone 

= End of the Focal Zone 

= Element Diameter

Focal Length Equation: 

The focal length F is determined by following equation; 

Where: 

F = Focal Length in water 

R = Curvature of the focusing lens 

n = Ration of L-velocity of epoxy to L-velocity of water 

F

Focal Length Variations 

Focal Length Variations due to Acoustic Velocity and Geometry of the Test 

Part. The measured focal length of a transducer is dependent on the material 

in which it is being measured. This is due to the fact that different materials 

have different sound velocities. When specifying a transducer’s focal length it 

is typically specified for water. Since most materials have a higher velocity 

than water, the focal length is effectively shortened. This effect is caused by 

refraction (according to Snell’s Law) and is illustrated in Figure (18).

Focal Length Variations

This change in the focal length can be predicted by Equation (13). 

For example, given a particular focal length and material path, this equation 

can be used to determine the appropriate water path to compensate for the 

focusing effect in the test material. 

Eqn. 13 

WP = F – MP.(C tm /C w ) 

WP 

MP 

F 

C tm 

C w 

= Water Path 

= Material Depth 

= Focal Length in Water 

= Sound Velocity in the Test Material 

= Sound Velocity in the water 

In addition, the curvature of surface of the test piece can affect focusing. 

Depending on whether the entry surface is concave or convex, the sound 

beam may converge more rapidly than it would in a flat sample or it may 

spread and actually defocus.

Cylindrical & Spherical Focused

Cylindrical & Spherical Focused

Q79: What type of search unit allows the greatest resolving power with 

standard ultrasonic testing equipment? 

a) Delay tip 

b) Focused 

c) Highly damped 

d) High Q 

Q165: Acoustic lens elements with which of the following permit focusing the 

sound energy to enter cylindrical surface normally or along a line of focus. 

a) Cylindrical curvature 

b) Spherical lens curvatures 

c) Convex shapes 

d) Concave shapes

Q18: Which of the following is an advantage of a focused transducer? 

(a) Extended useful range 

(b) Reduced sensitivity in localised area 

(c) Improved signal to noise ratio over an extended range 

(d) Higher resolution over a limited range 

Q67: A divergent sound beam is produced by: 

(a) Concave mirror 

(b) Convex mirror 

(c) Convex lens 

(d) None of the above

Q78: Which of the following is not an advantage of a focused transducer? 

(a) High sensitivity to small flaws 

(b) Deep penetration 

(c) High resolving power 

(d) Not much affected by surface roughness 

Q79: What type of search unit allows the greatest resolving power with 

standard ultrasonic testing equipment? 

(a) Delay tip 

(b) Focused 

(c) Highly damped 

(d) High Q

3.5.3 Dual element transducers 

contain two independently operated elements in a single housing. One of the 

elements transmits and the other receives the ultrasonic signal. Active 

elements can be chosen for their sending and receiving capabilities to provide 

a transducer with a cleaner signal, and transducers for special applications, 

such as the inspection of course grained material. Dual element transducers 

are especially well suited for making measurements in applications where 

reflectors are very near the transducer since this design eliminates the ring 

down effect that single-element transducers experience (when single-element 

transducers are operating in pulse echo mode, the element cannot start 

receiving reflected signals until the element has stopped ringing from its 

transmit function). Dual element transducers are very useful when making 

thickness measurements of thin materials and when inspecting for near 

surface defects. The two elements are angled towards each other to create a 

crossed-beam sound path in the test material. 

Keywords: For near surface effects 

■ Fresnel zone (near zone) 

■ Ring down effect

For a single crystal probe the length of the initial pulse is the dead zone and 

any signal from a reflector at a shorter distance than this will be concealed 

in the initial pulse. We deliberately delay the initial pulse beyond the left of 

the time base, by mounting the transducers of a twin (or double) crystal 

probe onto plastic wedges. This and the focusing of the crystals reduces the 

dead zone considerably and it is only where the transmission and receptive 

beams do not overlap that we cannot assess flaws. 

A twin or double crystal probe is designed to minimise the problem of dead 

zone. A twin crystal probe has two crystals mounted on Perspex shoes 

angled inwards slightly to focus at a set distance in the test material. Were 

the crystals not angled, the pulse would be reflected straight back into the 

transmitting crystal.

The Perspex shoes hold the crystals away from the test surface so that the 

initial pulse does not appear on the CRT screen. The dead zone is greatly 

reduced to the region adjoining the test surface, where the transmission and 

reception beams do not overlap. 

More on Dead Zone BS EN 12668-Part1 Section: 3.5 

Dead time after transmitter pulse 

time interval following the start of the transmitter pulse during which the amplifier is 

unable to respond to incoming signals, when using the pulse echo method, because of 

saturation by the transmitter pulse

There are other advantages 

1. Double crystal probes can be focused 

2. Can measure thin plate 

3. Can detect near surface flaws 

4. Has good near surface resolution 

Disadvantages 

1. Good contact is difficult with curved surfaces 

2. Difficult to size small defects accurately as the width of a double crystal 

3. probe is usually greater than that of a single crystal probe 

4. The amplitude of a signal decreases the further a reflector is situated 

5. from the focal distance - a response curve can be made out. 

Therefore single and twin crystal probes are complementary.

Other Reading (Olympus): Dual element transducers utilize separate 

transmitting and receiving elements, mounted on delay lines that are usually 

cut at an angle (see diagram on page 8). This configuration improves near 

surface resolution by eliminating main bang recovery problems. In addition, the 

crossed beam design provides a pseudo focus that makes duals more 

sensitive to echoes from irregular reflectors such as corrosion and pitting. 

One consequence of the dual element design is a sharply defined distance/ 

amplitude curve. In general, a decrease in the roof angle or an increase in 

the transducer element size will result in a longer pseudo-focal distance and 

an increase in useful range, as shown in Figure (13).

Advantages: 

Improves near surface resolution (sensitivity?) 

Provide a pseudo focus (improve sensitivity in the Far Zone?) 

Less affected by surface roughness due to the pseudo focus effect 

Disadvantage(?) 

The pseudo focus by tilting the active elements (roof angle?) reduces the 

useful range of transducer?

Figure (13).

Duo Elements Transducer 

Transmitting 

Crystal 

Acoustic 

Barrier 

Receiving 

Crystal 

Roof Angle 

Casing 

Cross Beam 

Sound path

Duo Elements Transducer

3.5.4 Delay line transducers 

provide versatility with a variety of replaceable options. Removable delay line, 

surface conforming membrane, and protective wear cap options can make a 

single transducer effective for a wide range of applications. As the name 

implies, the primary function of a delay line transducer is to introduce a time 

delay between the generation of the sound wave and the arrival of any 

reflected waves. This allows the transducer to complete its "sending" function 

before it starts its "listening" function so that near surface resolution is 

improved. They are designed for use in applications such as high precision 

thickness gauging of thin materials and delamination checks in composite 

materials. They are also useful in high-temperature measurement applications 

since the delay line provides some insulation to the piezoelectric element from 

the heat.

Delay Lined Transducer: 

Advantages: 

1. Heavily damped transducer combined with the use of a delay line provides 

excellent near surface resolution 

2. Higher transducer frequency improves resolution 

3. Improves the ability to measure thin materials or find small flaws while 

using the direct contact method 

4. Contouring available to fit curved parts 

Applications: 

1. Precision thickness gauging 

2. Straight beam flaw detection 

3. Inspection of parts with limited contact areas 

4. Replaceable Delay Line Transducers 

5. Each transducer comes with a standard delay line and retaining ring 

6. High temperature and dry couple delay lines are available 

7. Requires couplant between transducer and delay line tip

Other Reading (Olympus): Delay Line Transducers 

Delay line transducers are single element longitudinal wave transducers 

used in conjunction with a replaceable delay line. One of the reasons for 

choosing a delay line transducer is that near surface resolution can be 

improved. 

The delay allows the element to stop vibrating before a return signal from the 

reflector can be received. When using a delay line transducer, there will be 

multiple echoes from end of the delay line and it is important to take these 

into account. Another use of delay line transducers is in applications in 

which the test material is at an elevated temperature. The high 

temperature delay 

line options listed in this catalog (page 16, 17, 19) are not intended for 

continuous contact, they are meant for intermittent contact only. 

Advantages: 

■ Improve near surface resolution 

■ High temperature contact testing

Delay Lined Transducer

Delay lined Transducer

TR-Probe / Dual Crystal Probe- Transmitting Receiving Probe 

http://www.weldr.net/simple/skill/html/content_10802.htm

Probe Delay with TR-Probe

Cross Talk at High Gain

Probe Delay

Probe Delay

Delay Line UT 1 Lab 8 

www.youtube.com/embed/lelVZ9OGli8

3.5.5 Angle beam transducers 

Angle beam transducer and wedges are typically used to introduce a 

refracted shear wave into the test material. Transducers can be purchased in 

a variety of (1) fixed angles or in (2) adjustable versions where the user 

determines the angles of incidence and refraction. 

In the fixed angle versions, the angle of refraction that is marked on the 

transducer is only accurate for a particular material, which is usually steel. 

The angled sound path allows the sound beam to be reflected from the 

backwall to improve detectability of flaws in and around welded areas. They 

are also used to generate surface waves for use in detecting defects on the 

surface of a component.

Angle Beam Transducers- Angle beam transducers are typically used to 

locate and/or size flaws which are oriented non-parallel to the test surface.



















Angle Beam Transducers 

ϴ 1L 

ϴ 2L 

ϴ 2S

Angle Beam Transducers 

ϴ 1L 

ϴ 2L 

ϴ 2S

Angle Beam Transducers- Mode Conversion 

Figure (15) below shows the relationship between the incident angle and the 

relative amplitudes of the refracted or mode converted longitudinal, shear, 

and surface waves that can be produced from a plastic wedge into steel.

Angle Beam Transducers- Common Terms 

ϴ = Refracted angle T= Thickness LEG1=LEG2= T/Cos ϴ 

V PATH= 2x LEG= 2T/Cos ϴ 

SKIP= 2.T Tan ϴ 

ϴ

Angle Beam Transducers- Common Terms 

ϴ = Refracted angle T= Thickness Surface Distance= S.Sin ϴ 

Depth= S.Cos ϴ 

ϴ

Angle Beam Transducers- Longitudinal / Shear Wave Inspection 

Many AWS inspections are performed using refracted shear waves. 

However, grainy materials such as austenitic stainless steel may require 

refracted longitudinal waves or other angle beam techniques for successful 

inspections.

Angle Beam Transducer 

http://www.olympus-ims.com/en/ultrasonic-transducers/dualelement/ 

http://static4.olympus-ims.com/data/Flash/wedge_weld.swf?rev=EF60

3.5.6 Normal incidence shear wave transducers 

Normal Incidence Shear Wave transducers incorporate a shear wave crystal 

in a contact transducer case. These transducers are unique because they 

allow the introduction of shear waves directly into a test piece without the use 

of an angle beam wedge. Rather than using the principles of refraction, 

as with the angle beam transducers, to produce shear waves in a material, 

the crystal itself produces the shear wave (Y-cut). Careful design has enabled 

manufacturing of transducers with minimal longitudinal wave contamination. 

The ratio of the longitudinal to shear wave components is generally below - 

30dB. 

Because shear waves do not propagate in liquids, it is necessary to use a 

very viscous couplant when making measurements with these. When using 

this type of transducer in a through transmission mode application, it is 

important that direction of polarity of each of the transducers is in line with 

the other. If the polarities are 90° off, the receiver may not receive the signal 

from the transmitter.

Application of Normal incidence shear wave transducers 

Typically these transducers are used to make shear velocity measurements 

of materials. This measurement, along with a longitudinal velocity 

measurement can be used in the calculation of Poisson’s Ratio, Young’s 

Modulus, and Shear Modulus. These formulas are listed below for reference. 

Keys: 

S 

V L 

V T 

r 

E 

G 

= Poisson’s Ratio 

= Longitudinal Velocity 

= Shear Velocity 

= Material Density 

= Young’s Modulus 

= Shear Modulus

Normal incidence shear wave transducers 

http://static3.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970

Normal incidence shear wave transducers 

Advantages: 

1. Generate shear waves which propagate perpendicular to the test surface 

2. For ease of alignment, the direction of the polarization of shear wave is 

nominally in line with the right angle connector 

3. The ratio of the longitudinal to shear wave components is generally below 

-30 dB 

Applications: 

1. Shear wave velocity measurements 

2. Calculation of Young's Modulus of elasticity and shear modulus (see 

Technical Notes, page 46) 

3. Characterization of material grain structure 

http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

3.5.7 Paint brush transducers 

Paint brush transducers are used to scan wide areas. These long and narrow 

transducers are made up of an array of small crystals that are carefully 

matched to minimize variations in performance and maintain uniform 

sensitivity over the entire area of the transducer. Paint brush transducers 

make it possible to scan a larger area more rapidly for discontinuities. Smaller 

and more sensitive transducers are often then required to further define the 

details of a discontinuity.

Q: To evaluate and accurately locate discontinuities after scanning a part with 

paintbrush transducer, it is generally necessary to uae a: 

A. Transducer with a smaller crystal 

B. Scrubber 

C. Grid map 

D. Crystal collimator

3.5.8 Wheel Transducer 

Wheel Transducer Probe Features: 

The main driving advantage of this dry coupled solid contact wheel probe is 

that it works to overcome problems with couplant contamination (application 

& removal) as well as eliminating the practicalities of immersion systems. 

The "tyre" or delay material is constructed of hydrophilic polymers which have 

acoustic properties that lend themselves ideally to the implementation of 

ultrasonics. Applications include thickness measurement, composite 

inspection, delamination detection and general flaw detection.

Q: A special scanning device with the transducer mounted in a tire-like 

container filled with couplant is commonly called: 

A. A rotating scanner 

B. An axial scanner 

C. A wheel transducer 

D. A circular scanner 

Q: A wheel transducer scanning method is consider as: 

A. Contact method 

B. Immersion method 

C. Wheel method 

D. Not allowed

UT Technician At works- Salute!

3.6: Transducer Testing 

Some transducer manufacturers have lead in the development of transducer 

characterization techniques and have participated in developing the AIUM 

Standard Methods for Testing Single-Element Pulse-Echo Ultrasonic 

Transducers as well as ASTM-E 1065 Standard Guide for Evaluating 

Characteristics of Ultrasonic Search Units. 

Additionally, some manufacturers perform characterizations according to 

AWS, ESI, and many other industrial and military standards. Often, 

equipment in test labs is maintained in compliance with MIL-C-45662A 

Calibration System Requirements. As part of the documentation process, an 

extensive database containing records of the waveform and spectrum of each 

transducer is maintained and can be accessed for comparative or statistical 

studies of transducer characteristics.

Manufacturers often provide time and frequency domain plots for each 

transducer. The signals below were generated by a spiked pulser. The 

waveform image on the left shows the test response signal in the time domain 

(amplitude versus time). The spectrum image on the right shows the same 

signal in the frequency domain (amplitude versus frequency). The signal path 

is usually a reflection from the back wall (fused silica) with the reflection in the 

far field of the transducer.

TRANSDUCER EXCITATION 

As a general rule, all of our ultrasonic transducers are designed for negative 

spike excitation. The maximum spike excitation voltages should be limited to 

approximately 50 volts per mil of piezoelectric transducer thickness. Low 

frequency elements are thick, and high frequency elements are thin. 

A negative-going 600 volt fast rise time, short duration, spike excitation can 

be used across the terminals on transducers 5.0 MHz and lower in frequency. 

For 10 MHz transducers, the voltage used across the terminals should be 

halved to about 300 volts as measured across the terminals. 

Although negative spike excitation is recommended, continuous wave or tone 

burst excitations may be used. However there are limitations to consider 

when using these types of excitation. First, the average power dissipation to 

the transducer should not exceed 125 mW to avoid overheating the 

transducer and depoling the crystal. 

http://www.olympus-ims.com/en/5072pr/

Excitation: Spiked Pulser (negative spike excitation) 

0V 

10% 

Pulse Width @50% 

90% 

ΔT 

Time 

http://www.olympus-ims.com/en/5072pr/

Square Wave Spiked Pulser: (negative spike excitation) 

Square wave has controlled rise and fall times with directly adjustable voltage 

and pulse width. Precautions on the average power dissipation to the 

transducer should not exceed 125 mW to avoid overheating the transducer 

and depoling the crystal. 

0V 

Adjustable Voltage 

Adjustable Pulse width 

Time →

Pulse energy: Broad band versus Narrow band. 

Energy (dB) 

0 5 10 15 20 25 30 

Narrow band 

Broad band 

0.1 1.0 5.0 10 20 

Frequency MHz

UT Flaw Detector – Olympus EPOCH 600

Other tests may include the following: 

Electrical Impedance Plots provide important information about the design 

and construction of a transducer and can allow users to obtain electrically 

similar transducers from multiple sources. 

Beam Alignment Measurements provide data on the degree of alignment 

between the sound beam axis and the transducer housing. This information is 

particularly useful in applications that require a high degree of certainty 

regarding beam positioning with respect to a mechanical reference surface. 

Beam Profiles provide valuable information about transducer sound field 

characteristics. Transverse beam profiles are created by scanning the 

transducer across a target (usually either a steel ball or rod) at a given 

distance from the transducer face and are used to determine focal spot size 

and beam symmetry. Axial beam profiles are created by recording the pulseecho 

amplitude of the sound field as a function of distance from the 

transducer face and provide data on depth of field and focal length.

Effects of Probe Frequencies: 

1. Higher frequencies give better resolution 

2. Higher frequencies give better sensitivity 

3. Lower frequencies give better penetration 

4. Lower frequencies less attenuation 

5. Lower frequencies probe wider beam spread with more coverage to detect 

reflectors and reflectors with unfavorable orientation. 

6. Higher frequencies the beams are more focused and the sensitivity and 

resolution are better.

Effects of Probe Sizes: 

1. The larger the probe produce more energy thus more penetration 

2. Small probe small near zone 

3. The larger the probe the poorer the contacts on a curve substrate. 

Single or Double Crustal Probe Selection: 

1. Single crystal probe should be used for material thickness 15mm and 

above, according to the probe the near zone 

2. Single crystal probe should be used for thickness above 30mm 

3. Double crystal should be used for thin material

As noted in the ASTM E1065 Standard Guide for Evaluating Characteristics 

of Ultrasonic Transducers, the acoustic and electrical characteristics which 

can be described from the data, are obtained from specific procedures that 

are listed below: 

Frequency Response--The frequency response may be obtained from one 

of two procedures: shock excitation and sinusoidal burst. 

Sinusoidal excitation.

Shock excitation

Relative Pulse-Echo Sensitivity--The relative pulse-echo sensitivity may be 

obtained from the frequency response data by using a sinusoidal burst 

procedure. The value is obtained from the relationship of the amplitude of the 

voltage applied to the transducer and the amplitude of the pulse-echo signal 

received from a specified target. 

Time Response--The time response provides a means for describing the 

radio frequency (RF) response of the waveform. A shock excitation, pulseecho 

procedure is used to obtain the response. The time or waveform 

responses are recorded from specific targets that are chosen for the type of 

transducer under evaluation, for example, immersion, contact straight beam, 

or contact angle beam.

Frequency Response--The frequency response of the above transducer has 

a peak at 5 MHz and operates over a broad range of frequencies. Its 

bandwidth (4.1 to 6.15 MHz) is measured at the -6 dB points, or 70% of the 

peak frequency. The useable bandwidth of broadband transducers, especially 

in frequency analysis measurements, is often quoted at the -20 dB points. 

Transducer sensitivity and bandwidth (more of one means less of the other) 

are chosen based on inspection needs. 

Complex Electrical Impedance--The complex electrical impedance may be 

obtained with commercial impedance measuring instrumentation, and these 

measurements may provide the magnitude and phase of the impedance of 

the search unit over the operating frequency range of the unit. These 

measurements are generally made under laboratory conditions with minimum 

cable lengths or external accessories and in accordance with specifications 

given by the instrument manufacturer. The value of the magnitude of the 

complex electrical impedance may also be obtained using values recorded 

from the sinusoidal burst.

Sound Field Measurements--The objective of these measurements is to 

establish parameters such as the on-axis and transverse sound beam profiles 

for immersion, and flat and curved transducers. These measurements are 

often achieved by scanning the sound field with a hydrophone transducer to 

map the sound field in three dimensional space. An alternative approach to 

sound field measurements is a measure of the transducer's radiating surface 

motion using laser interferometry.


In high-technology manufacturing, part design and simulation of part 

inspection is done in the virtual world of the computer. Transducer modeling 

is necessary to make accurate predictions of how a part or component might 

be inspected, prior to the actual building of that part. Computer modeling is 

also used to design ultrasonic transducers. 

As noted in the previous section, an ultrasonic transducer may be 

characterized by detailed measurements of its electrical and sound radiation 

properties. Such measurements can completely determine the response of 

any one individual transducer.

There is ongoing research to develop general models that relate electrical 

inputs (voltage, current) to mechanical outputs (force, velocity) and vice-versa. 

These models can be very robust in giving accurate prediction of transducer 

response, but suffer from a lack of accurate modeling of physical variables 

inherent in transducer manufacturing. These electrical-mechanical response 

models must take into account the physical and electrical components in the 

figure below.

The Thompson-Gray Measurement Model, which makes very accurate 

predictions of ultrasonic scattering measurements made through liquid-solid 

interfaces, does not attempt to model transducer electrical-mechanical 

response. The Thompson-Gray Measurement Model approach makes use of 

reference data taken with the same transducer(s) to deconvolve electrophysical 

characteristics specific to individual transducers. See Section 5.4 

Thompson-Gray Measurement Model. 

The long term goal in ultrasonic modeling is to incorporate accurate models of 

the transducers themselves as well as accurate models of pulser-receivers, 

cables, and other components that completely describe any given inspection 

setup and allow the accurate prediction of inspection signals.


A couplant is a material (usually liquid) that facilitates the transmission of 

ultrasonic energy from the transducer into the test specimen. Couplant is 

generally necessary because the acoustic impedance mismatch between air 

and solids (i.e. such as the test specimen) is large. Therefore, nearly all of the 

energy is reflected and very little is transmitted into the test material. The 

couplant displaces the air and makes it possible to get more sound energy 

into the test specimen so that a usable ultrasonic signal can be obtained. In 

contact ultrasonic testing a thin film of oil, glycerin or water is generally used 

between the transducer and the test surface.

Couplant

Immersion Method - Water as a couplant 

When scanning over the part or making precise measurements, an immersion 

technique is often used. In immersion ultrasonic testing both the transducer 

and the part are immersed in the couplant, which is typically water. This 

method of coupling makes it easier to maintain consistent coupling while 

moving and manipulating the transducer and/or the part.

Squirter Column (bubbler)- Water as a couplant

Squirter Column (bubbler)- Water as a couplant 

https://www.youtube.com/user/UltrasonicSciences

Couplant

Couplant


As discussed on the previous page, one of the essential features of ultrasonic 

measurements is mechanical coupling between the transducer and the solid 

whose properties or structure are to be studied. This coupling is generally 

achieved in one of two ways. In immersion measurements, energy is coupled 

between the transducer and sample by placing both objects in a tank filled 

with a fluid, generally water. In contact measurements, the transducer is 

pressed directly against the sample, and coupling is achieved by the 

presence of a thin fluid layer inserted between the two. When shear waves 

are to be transmitted, the fluid is generally selected to have a significant 

viscosity.

Electromagnetic-acoustic transducers (EMAT) acts through totally different 

physical principles and do not need couplant. When a wire is placed near the 

surface of an electrically conducting object and is driven by a current at the 

desired ultrasonic frequency, eddy currents will be induced in a near surface 

region of the object. If a static magnetic field is also present, these eddy 

currents will experience Lorentz forces of the form 

F = I x B 

F the Lorentz force is the body force per unit volume, I is the induced 

dynamic current density, and B is the static magnetic induction. 

The most important application of EMATs has been in nondestructive 

evaluation (NDE) applications such as (1) flaw detection or (2) material 

property characterization. Couplant free transduction allows operation without 

contact at elevated temperatures and in remote locations. The coil and 

magnet structure can also be designed to excite complex wave patterns and 

polarizations that would be difficult to realize with fluid coupled piezoelectric 

probes. In the inference of material properties from precise velocity or 

attenuation measurements, using EMATs can eliminate errors associated 

with couplant variation, particularly in contact measurements.

F is the body force per unit volume, I is the induced dynamic current 

density, and B is the static magnetic induction.

EMAT

A number of practical EMAT configurations are shown below. In each, the 

biasing magnet structure, the coil, and the forces on the surface of the solid 

are shown in an exploded view. The first three configurations will excite 

beams propagating normal to the surface of the half-space and produce 

beams with radial, longitudinal, and transverse polarizations, respectively. 

The final two use spatially varying stresses to excite beams propagating at 

oblique angles or along the surface of a component. Although a great number 

of variations on these configurations have been conceived and used in 

practice, consideration of these three geometries should suffice to introduce 

the fundamentals. 

http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.html&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_side_menu=s2

Electromagnetic acoustic transducer 

http://en.wikipedia.org/wiki/Electromagnetic_acoustic_transducer 

Electromagnetic Acoustic Transducer (EMAT) is a transducer for non-contact 

sound generation and reception using electromagnetic mechanisms. EMAT is 

an ultrasonic nondestructive testing (NDT) method which does not require 

contact or couplant, because the sound is directly generated within the 

material adjacent to the transducer. Due to this couplant-free feature, EMAT 

is particularly useful for automated inspection, and hot, cold, clean, or dry 

environments. EMAT is an ideal transducer to generate Shear Horizontal (SH) 

bulk wave mode, Surface Wave, Lamb waves and all sorts of other guidedwave 

modes in metallic and/or ferromagnetic materials. As an emerging 

ultrasonic testing (UT) technique, EMAT can be used for thickness 

measurement, flaw detection, and material property characterization. After 

decades of research and development, EMAT has found its applications in 

many industries such as primary metal manufacturing and processing, 

automotive, railroad, pipeline, boiler and pressure vessel industries.

Comparison between EMAT and Piezoelectric Transducers 

As an Ultrasonic Testing (UT) method, EMAT has all the advantages of UT 

compared to other NDT methods. Just like piezoelectric UT probes, EMAT 

probes can be used in pulse echo, pitch-catch, and through-transmission 

configurations. EMAT probes can also be assembled into phased array 

probes, delivering focusing and beam steering capabilities. 

Advantages 

Compared to piezoelectric transducers, EMAT probes have the following 

advantages: 

1. No couplant is needed. Based on the transduction mechanism of EMAT, 

couplant is not required. This makes EMAT ideal for inspections at 

temperatures below the freezing point and above the evaporation point of 

liquid couplants. It also makes it convenient for situations where couplant 

handling would be impractical. 

2. EMAT is a non-contact method. Although proximity is preferred, a physical 

contact between the transducer and the specimen under test is not required.

3. Dry Inspection. Since no couplant is needed, the EMAT inspection can be 

performed in a dry environment. 

4. Less sensitive to surface condition. With contact-based piezoelectric 

transducers, the test surface has to be machined smoothly to ensure 

coupling. Using EMAT, the requirements to surface smoothness are less 

stringent; the only requirement is to remove loose scale and the like. 

5. Easier for sensor deployment. Using piezoelectric transducer, the wave 

propagation angle in the test part is affected by Snell’s law. As a result, a 

small variation in sensor deployment may cause a significant change in 

the refracted angle. 

6. Easier to generate SH-type waves. Using piezoelectric transducers, SH 

wave is difficult to couple to the test part. EMAT provide a convenient 

means of generating SH bulk wave and SH guided waves.

Challenges and Disadvantages 

The disadvantages of EMAT compared to piezoelectric UT can be 

summarized as follows: 

1. Low transduction efficiency. EMAT transducers typically produce raw 

signal of lower power than piezoelectric transducers. As a result, more 

sophisticated signal processing techniques are needed to isolate signal 

from noise. 

2. Limited to metallic or magnetic products. NDT of plastic and ceramic 

material is not suitable or at least not convenient using EMAT. 

3. Size constraints. Although there are EMAT transducers as small as a 

penny, commonly used transducers are large in size. Low-profile EMAT 

problems are still under research and development. Due to the size 

constraints, EMAT phased array is also difficult to be made from very 

small elements. 

4. Caution must be taken when handling magnets around steel products.

Applications of EMATs 

EMAT has been used in a broad range of applications and has potential to be 

used in many other applications. A brief and incomplete list is as follows. 

1. Thickness measurement for various applications 

2. Flaw detection in steel products 

3. Plate lamination defect inspection 

4. Bonded structure lamination detection 

5. Laser weld inspection for automotive components 

6. Various weld inspection for coil join, tubes and pipes. 

7. Pipeline in-service inspection. 

8. Railroad and wheel inspection 

9. Austenitic weld inspection for power industry 

10. Material characterization

http://mdienergy.com/emat.html

Cross-sectional view of a spiral coil EMAT exciting radially polarized shear 

waves propagating normal to the surface.

EMAT Transducer 

http://www-ndc.me.es.osakau.ac.jp/pmwiki_e/pmwiki.php?n=Research.EMATs

Cross-sectional view of a tangential field EMAT for exciting polarized 

longitudinal waves propagating normal to the surface.

Cross-sectional view of a normal field EMAT for exciting plane polarized 

shear waves propagating normal to the surface.

EMATS 

The bulk-shear-wave EMAT 

consists of a pair of permanent 

magnets and a spiral-elongated 

coil. Driving currents in the coil 

generate the electromagnet 

forces (Lorentz force and 

magnetostriction force) parallel 

to the surface to generate the 

shear waves propagating 

normal to the surface

Cross-sectional view of a meander coil EMAT for exciting obliquely 

propagating L or SV waves, Rayleigh waves, or guided modes (such as Lamb 

waves) in plates.

Cross-sectional view of a periodic permanent magnet EMAT for exciting 

grazing or obliquely propagating horizontally polarized (SH) waves or guided 

SH modes in plates.

Practical EMAT designs are relatively narrowband and require strong 

magnetic fields and large currents to produce ultrasound that is often weaker 

than that produced by piezoelectric transducers. Rare-earth materials such as 

Samarium-Cobalt and Neodymium-Iron-Boron are often used to produce 

sufficiently strong magnetic fields, which may also be generated by pulsed 

electromagnets. 

The EMAT offers many advantages based on its couplant-free operation. 

These advantages include the abilities to operate in remote environments at 

elevated speeds and temperatures, to excite polarizations not easily excited 

by fluid coupled piezoelectrics, and to produce highly consistent 

measurements. 

These advantages are tempered by low efficiencies, and careful electronic 

design is essential to applications.


Ultrasonic pulser-receivers are well suited to general purpose ultrasonic 

testing. Along with appropriate transducers and an oscilloscope, they can be 

used for flaw detection and thickness gauging in a wide variety of metals, 

plastics, ceramics, and composites. Ultrasonic pulser-receivers provide a 

unique, low-cost ultrasonic measurement capability

The pulser section of the instrument generates short, large amplitude electric 

pulses of controlled energy, which are converted into short ultrasonic 

pulses when applied to an ultrasonic transducer. Most pulser sections 

have very low impedance outputs to better drive transducers. Control 

functions associated with the pulser circuit include: 

1. Pulse length or damping (The amount of time the pulse is applied to the 

transducer.) 

2. Pulse energy (The voltage applied to the transducer. Typical pulser circuits 

will apply from 100 volts to 800 volts to a transducer.) 

100 volts to 800 volts (1KV~2KV could be used)

Transducer Cut-out

Pulse characteristics 

Pulse energy 

N= Pulse Rate 

Pulse length

Pulse Length: BS4331 Pt2. 

N= Pulse Rate 

Pulse length 

Pulse energy

Pulse Length: BS EN 12668- Part 1 Instrumentation 

3.22 

pulse duration 

time interval during which the modulus of the amplitude of a pulse is 10 % or 

more of its peak amplitude.

Pulse Length: A long pulse length may be 15 wavelength λ, a short pulse 

length may be only 2 λ and a normal pulse length usually about 5 λ. 

The longer the pulse length the more energy, thus more penetrating, however 

the resolution and sensitivity deteriorated.

Pulse Length

Pulse Length

Pulse Length

Pulse Length

Pulse Length and Wave form

Pulse-Length and Wave form Quality Factor 

Two different pulses with the same frequency, but different duration (pulse 

length), i.e. Number of oscillations. The shortest pulse has a wider dispersion 

of frequencies, i.e. a greater bandwidth.

Wave form Quality Factor

Pulse Length- x axis time domain 

Quality factor- x axis frequency domain 

Frequency 

Q Factor = f o /(f 1 -f 2 )

Pulse-Echo mode of operation, narrow band excitation (tone burst). 

Conventional air-coupled transducer with passive matching layers 

Two types of excitation: Sinusoidal/Shock. 

http://www.mdpi.com/1424-8220/13/5/5996/htm

Pulse-echo mode of operation, wideband excitation (spike). 1. (Red) Aircoupled 

transducer with active matching layer. 2. (Blue) Conventional aircoupled 

transducer with passive matching layers. 

λ /4 impedance 

matching layers

Modulus of the electrical impedance of the piezocomposite disk vs frequency. 

Circles: experimental measurements, solid red line: theoretical calculation. 

Z= pV

Sensitivity in pulse-echo mode of operation wideband excitation (spike). 1. 

(Red) Air-coupled transducer with active matching layer. 2. (Blue) 

Conventional air-coupled transducer with passive matching layers

Transducers

Damping: 

Shock wave transducer and low damped transducer : Shock wave 

transducers should always be used for wall thickness measurement. For 

smaller wall thicknesses this is as important for the pulse separation as is the 

frequency itself. For large wall thickness the shock wave is required also for a 

perfect start and stop trigger of the time measurement. Low damped 

transducers are not recommended. 

http://www.ndt.net/article/rohrext/us_pk/us_pk_e.htm

In the receiver section the voltage signals produced by the transducer, which 

represent the received ultrasonic pulses, are amplified. The amplified radio 

frequency (RF) signal is available as an output for display or capture for 

signal processing. Control functions associated with the receiver circuit 

include: 

1. Signal rectification (The RF signal can be viewed as positive half wave, 

negative half wave or full wave.) 

2. Filtering to shape and smooth return signals 

3. Gain, or signal amplification 

4. Reject control

The pulser-receiver is also used in material characterization work involving 

sound velocity or attenuation measurements, which can be correlated to 

material properties such as elastic modulus. In conjunction with a stepless 

gate and a spectrum analyzer, pulser-receivers are also used to study 

frequency dependent material properties or to characterize the performance 

of ultrasonic transducers.

Pulse/Beam Characteristics 

High frequency, short duration pulse exhibit better depth resolution but allow 

less penetration. A short time duration pulse only a few cycle is known as 

broad band pulse, because its frequency domain equivalent is large. Such 

pulse exhibit good depth resolution. 

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Transducers of the kind most commonly used for ultrasonic gauging will have 

these fundamental functional properties, which in turn affect the properties of 

the sound beam that they will generate in a given material: 

Type - The transducer will be identified according to its design and function 

as a contact, delay line, or immersion type. Physical characteristics of the test 

material such as surface roughness, temperature, and accessibility, as well 

as its sound transmission properties and the range of thickness to be 

measured, will all influence the selection of transducer type. 

Diameter - The diameter of the active transducer element, which is normally 

housed in a somewhat larger case. Smaller diameter transducers are often 

most easily coupled to the test material, while larger diameters may couple 

more efficiently into rough surfaces due to an averaging effect. Larger 

diameters are also required for design reasons as transducer frequency 

decreases.

Frequency - The number of wave cycles completed in one second, normally 

expressed in Kilohertz (KHz) or Megahertz (MHz). Most ultrasonic gauging is 

done in the frequency range from 500 KHz to 20 MHz, so most transducers 

fall within that range, although commercial transducers are available from 

below 50 KHz to greater than 200 MHz. Penetration increases with lower 

frequency, while resolution and focal sharpness increase with higher 

frequency. 

Waveform duration - The number of wave cycles generated by the 

transducer each time it is pulsed. A narrow bandwidth transducer has more 

cycles than a broader bandwidth transducer. Element diameter, backing 

material, electrical tuning and transducer excitation method all impact 

waveform duration. A short wave duration (broadband response) is desirable 

in most thickness gauging applications.

Bandwidth - Typical transducers for thickness gauging do not generate 

sound waves at a single pure frequency, but rather over a range of 

frequencies centered at the nominal frequency designation. Bandwidth is the 

portion of the frequency response that falls within specified amplitude limits. 

Broad bandwidth is usually desirable in thickness gauging applications 

involving contact, delay line, and immersion transducers.

Sensitivity - The relationship between the amplitude of the excitation pulse 

and that of the echo received from a designated target. This is a function of 

the energy output of the transducer. 

Beam profile - As a working approximation, the beam from a typical 

unfocused disk transducer is often thought of as a column of energy 

originating from the active element area that travels as a straight column for a 

while and then expands in diameter and eventually dissipates, like the beam 

from a spotlight.

In fact, the actual beam profile is complex, with pressure gradients in both the 

transverse and axial directions. In the beam profile illustration below, red 

represents areas of highest energy, while green and blue represent lower 

energy. 

The exact shape of the beam in a given case is determined by transducer 

frequency, transducer diameter, and material sound velocity. The area of 

maximum energy a short distance beyond the face of the transducer marks 

the transition between beam components known as the near field and the far 

field, each of which is characterized by specific types of pressure gradients. 

Near field length is an important factor in ultrasonic flaw detection, since it 

affects the amplitude of echoes from small flaws like cracks, but it is usually 

not a significant factor in thickness gauging applications.

Focusing - Immersion transducers can be focused with acoustic lenses to 

create an hourglass-shaped beam that narrows to a small focal zone and 

then expands. Certain types of delay line transducers can be focused as well. 

Beam focusing is very useful when measuring small diameter tubing or other 

test pieces with sharp radiuses, since it concentrates sound energy in a small 

area and improves echo response.

Attenuation - As it travels through a medium, the organized wave front 

generated by an ultrasonic transducer will begin to break down due to 

imperfect transmission of energy through the microstructure of any material. 

Organized mechanical vibrations (sound waves) turn into random mechanical 

vibrations (heat) until the wave front is no longer detectable. This process is 

known as sound attenuation. Attenuation varies with material, and increases 

proportionally to frequency. As a general rule, hard materials like metals are 

less attenuating than softer materials like plastics. Attenuation ultimately limits 

the maximum material thickness that can be measured with a given gage 

setup and transducer, since it determines the point at which an echo will be 

too small to detect. 

http://www.olympus-ims.com/en/ndt-tutorials/thickness_gage/transducers/beam_characteristics/

Q15: A significant limitation of a lower frequency, single element transducer is: 

a) Scatter of sound beam due to microstructure of test object 

b) Increased grain noise or ‘hash’ 

c) (Less beam spread 

d) Impaired ability to display discontinuities just below the entry surface 

How & Why ? 

Reasoning: Pulse/Beam Characteristics 

High frequency, short duration pulse exhibit better depth resolution but allow 

less penetration. 

Lower frequency, longer duration pulse.


Tone burst generators are often used in high power ultrasonic applications. 

They take low-voltage signals and convert them into high-power pulse trains 

for the most power-demanding applications. Their purpose is to transmit 

bursts of acoustic energy into a test piece, receive the resulting signals, and 

then manipulate and analyze the received signals in various ways. High 

power radio frequency (RF) burst capability allows researchers to work with 

difficult, highly attenuative materials or inefficient transducers such as EMATs. 

A computer interface makes it possible for systems to make high speed 

complex measurements, such as those involving multiple frequencies.

Tone burst generators

Tone burst generators 

http://www.seekic.com/circuit_diagram/Signal_Processing/SINGLE_TONE_BURST_GENERATOR.html


Arbitrary waveform generators permit the user to design and generate 

virtually any waveform in addition to the standard function generator signals 

(i.e. sine wave, square wave, etc.). Waveforms are generated digitally from a 

computer's memory, and most instruments allow the downloading of digital 

waveform files from computers. 

Ultrasonic generation pulses must be varied to accommodate different types 

of ultrasonic transducers. General-purpose highly damped contact 

transducers are usually excited by a wideband, spike-like pulse provided by 

many common pulser/receiver units. The lightly damped transducers used in 

high power generation, for example, require a narrowband tone-burst 

excitation from a separate generator unit. Sometimes the same transducer 

will be excited differently, such as in the study of the dispersion of a material's 

ultrasonic attenuation or to characterize ultrasonic transducers.

Section of biphase modulated spread spectrum ultrasonic waveform 

http://www.mpi-ultrasonics.com/content/mmm-signal-processing-examples

In spread spectrum ultrasonics (see spread spectrum page), encoded 

sound is generated by an arbitrary waveform generator continuously 

transmitting coded sound into the part or structure being tested. Instead of 

receiving echoes, spread spectrum ultrasonics generates an acoustic 

correlation signature having a one-to-one correspondence with the acoustic 

state of the part or structure (in its environment) at the instant of 

measurement. In its simplest embodiment, the acoustic correlation signature 

is generated by cross correlating an encoding sequence (with suitable cross 

and auto correlation properties) transmitted into a part (structure) with 

received signals returning from the part (structure).


When computer systems were first introduced decades ago, they were large, 

slow-working devices that were incompatible with each other. Today, national 

and international networking standards have established electronic control 

protocols that enable different systems to "talk" to each other. The Electronics 

Industries Associations (EIA) and the Institute of Electrical and Electronics 

Engineers (IEEE) developed standards that established common terminology 

and interface requirements, such as EIA RS-232 and IEEE 802.3. If a system 

designer builds equipment to comply with these standards, the equipment will 

interface with other systems. But what about analog signals that are used in 

ultrasonics?

Data Signals: Input versus Output 

Consider the signal going to and from ultrasonic transducers. When you 

transmit data through a cable, the requirement usually simplifies into 

comparing what goes in one end with what comes out the other. High 

frequency pulses degrade or deteriorate when they are passed through 

any cable. Both the height of the pulse (magnitude) and the shape of the 

pulse (wave form) change dramatically, and the amount of change 

depends on the data rate, transmission distance and the cable's electrical 

characteristics. Sometimes a marginal electrical cable may perform 

adequately if used in only short lengths, but the same cable with the same 

data in long lengths will fail. This is why system designers and industry 

standards specify precise cable criteria. 

1. Recommendation: Observe manufacturer's recommended practices for 

cable impedance, cable length, impedance matching, and any 

requirements for termination in characteristic impedance. 

2. Recommendation: If possible, use the same cables and cable dressing for 

all inspections.

Cable Electrical Characteristics 

The most important characteristics in an electronic cable are impedance, 

attenuation, shielding, and capacitance. In this page, we can only review 

these characteristics very generally, however, we will discuss capacitance in 

more detail. 

Impedance (Ohms) represents the total resistance that the cable presents to 

the electrical current passing through it. At low frequencies the impedance is 

largely a function of the conductor size, but at high frequencies conductor size, 

insulation material, and insulation thickness all affect the cable's impedance. 

Matching impedance is very important. If the system is designed to be 100 

Ohms, then the cable should match that impedance, otherwise errorproducing 

reflections are created. 

Attenuation is measured in decibels per unit length (dB/m), and provides an 

indication of the signal loss as it travels through the cable. Attenuation is very 

dependent on signal frequency. A cable that works very well with low 

frequency data may do very poorly at higher data rates. Cables with lower 

attenuation are better.

Shielding is normally specified as a cable construction detail. For example, 

the cable may be unshielded, contain shielded pairs, have an overall 

aluminum/mylar tape and drain wire, or have a double shield. Cable shields 

usually have two functions: to act as a barrier to keep external signals from 

getting in and internal signals from getting out, and to be a part of the 

electrical circuit. Shielding effectiveness is very complex to measure and 

depends on the data frequency within the cable and the precise shield design. 

A shield may be very effective in one frequency range, but a different 

frequency may require a completely different design. System designers often 

test complete cable assemblies or connected systems for shielding 

effectiveness.

Capacitance in a cable is usually measured as picofarads per foot (pf/m). It 

indicates how much charge the cable can store within itself. If a voltage signal 

is being transmitted by a twisted pair, the insulation of the individual wires 

becomes charged by the voltage within the circuit. Since it takes a certain 

amount of time for the cable to reach its charged level, this slows down and 

interferes with the signal being transmitted. Digital data pulses are a string of 

voltage variations that are represented by square waves. A cable with a high 

capacitance slows down these signals so that they come out of the cable 

looking more like "saw-teeth," rather than square waves. The lower the 

capacitance of the cable, the better it performs with high speed data.

3.14 Transducer Quality Factor “Q” 

The quality factor “Q” of tuned circuit, search units or individual transducer 

element is a performance measurement of their frequency selectivity. It is thru 

ration of search unit fundamental (resonance ) frequency f o to the band width 

(f 2 -f 1 ) at 3dB down point at both sides.

Quality Factor “Q”

Quality Factor “Q” 

High quality Q-factor has a narrow frequency range (bandwidth) (i.e. little 

damping) and a correspond long spatial pulse length, where as a Low quality 

Q-factor transducer has a wide frequency range (bandwidth) and a shorter 

spatial pulse length. 

As discussed previously highly damped transducer, gives a wider frequency 

range provide better spatial resolution. Thus a Low quality Q-factor does not 

mean poor choice of transducer. 

Continuous-wave ultrasound testing usually employed High qiality Q-factor 

transducer. 

http://www.slideshare.net/vsrbhupal/echo-meet-final?related=2&utm_campaign=related&utm_medium=1&utm_source=6


Ultrasonic data can be collected and displayed in a number of different 

formats. The three most common formats are know in the NDT world as: 

A-scan, 

B-scan 

C-scan presentations 

D-scan presentations. 

Shadow Methods (modified A-Scan ?) 

Each presentation mode provides a different way of looking at and evaluating 

the region of material being inspected. Modern computerized ultrasonic 

scanning systems can display data in all three presentation forms 

simultaneously.

Data Presentation: A, B and C-scan recording and principle of scanning

Data Presentation:

3.15.1 A-Scan Presentation 

The A-scan presentation displays the amount of 

received ultrasonic energy as a function of time. 

The relative amount of received energy is 

plotted along the vertical axis and the elapsed 

time (which may be related to the sound energy 

travel time within the material) is displayed 

along the horizontal axis. Most instruments with 

an A-scan display allow the signal to be 

displayed in its: 

natural radio frequency form (RF), 

as a fully rectified RF signal, or 

as either the positive or negative half of the RF 

signal. 

In the A-scan presentation, relative discontinuity size can be estimated by 

comparing the signal amplitude obtained from an unknown reflector to that 

from a known reflector. Reflector depth can be determined by the position of 

the signal on the horizontal sweep.

In the A-scan presentation, relative discontinuity size can be estimated by 

comparing the signal amplitude obtained from an unknown reflector to that 

from a known reflector. Reflector depth can be determined by the position of 

the signal on the horizontal sweep. 

Reflector depth 

Relative discontinuity size

A-Scan

A-Scan 

http://static3.olympus-ims.com/data/Flash/HTML5/a_scan/A-scan.html?rev=F2E2

In the illustration of the A-scan presentation to the right, the initial pulse 

generated by the transducer is represented by the signal IP, which is near 

time zero, the transducer is scanned along the surface of the part, four other 

signals are likely to appear at different times on the screen. When the 

transducer is in its far left position, only the IP signal and signal A, the sound 

energy reflecting from surface A, will be seen on the trace. As the transducer 

is scanned to the right, a signal from the backwall BW will appear later in time, 

showing that the sound has traveled farther to reach this surface. When the 

transducer is over flaw B, signal B will appear at a point on the time scale that 

is approximately halfway between the IP signal and the BW signal. Since the 

IP signal corresponds to the front surface of the material, this indicates that 

flaw B is about halfway between the front and back surfaces of the sample. 

When the transducer is moved over flaw C, signal C will appear earlier in time 

since the sound travel path is shorter and signal B will disappear since sound 

will no longer be reflecting from it.

3.15.2 B-Scan 

http://static2.olympus-ims.com/data/Flash/HTML5/B_Scan/B-scan.html?rev=5E4D

B-Scan

B-Scan 


B-Scan Presentation 

The B-scan presentations is a profile (cross-sectional) view of the test 

specimen. In the B-scan, the time-of-flight (travel time) of the sound energy is 

displayed along the vertical axis and the linear position of the transducer is 

displayed along the horizontal axis. From the B-scan, the depth of the 

reflector and its approximate linear dimensions in the scan direction can be 

determined. The B-scan is typically produced by establishing a trigger gate on 

the A-scan. Whenever the signal intensity is great enough to trigger the gate, 

a point is produced on the B-scan. The gate is triggered by the sound 

reflecting from the backwall of the specimen and by smaller reflectors within 

the material. In the B-scan image above, line A is produced as the transducer 

is scanned over the reduced thickness portion of the specimen. When the 

transducer moves to the right of this section, the backwall line BW is 

produced. When the transducer is over flaws B and C, lines that are similar to 

the length of the flaws and at similar depths within the material are drawn on 

the B-scan. It should be noted that a limitation to this display technique is that 

reflectors may be masked by larger reflectors near the surface.

It should be noted that a limitation to this display technique is that reflectors 

may be masked by larger reflectors near the surface. 

Masked by “C” above

B-Scan

Q: In a B-scan display, the length of a screen indication from a discontinuity is 

related to: 

A. A discontinuity’s thickness as measured parallel to the ultrasonic beam 

B. The discontinuity’s length in the direction of the transducer travel 

C. Both A and B 

D. None of the above

3.15.3 C-Scan Presentation 

The C-scan presentation provides a plan-type view of the location and size of 

test specimen features. The plane of the image is parallel to the scan pattern 

of the transducer. C-scan presentations are produced with an automated data 

acquisition system, such as a computer controlled immersion scanning 

system. Typically, a data collection gate is established on the A-scan and the 

amplitude or the time-of-flight of the signal is recorded at regular intervals as 

the transducer is scanned over the test piece. The relative signal amplitude or 

the time-of-flight is displayed as a shade of gray or a color for each of the 

positions where data was recorded. The C-scan presentation provides an 

image of the features that reflect and scatter the sound within and on the 

surfaces of the test piece.

C-Scan 

The (1) relative signal 

amplitude or (2) the timeof-flight 

is displayed as a 

shade of gray or a color 

for each of the positions 

where data was recorded. 

http://www.ndt.net/article/pohl/pohl_e.htm

C-Scan

C-Scan / A-Scan

High resolution scans can produce very detailed images. Below are two 

ultrasonic C-scan images of a US quarter. Both images were produced using 

a pulse-echo technique with the transducer scanned over the head side in an 

immersion scanning system. For the C-scan image on the left, the gate was 

setup to capture the amplitude of the sound reflecting from the front surface of 

the quarter. Light areas in the image indicate areas that reflected a greater 

amount of energy back to the transducer. In the C-scan image on the right, 

the gate was moved to record the intensity of the sound reflecting from the 

back surface of the coin. The details on the back surface are clearly visible 

but front surface features are also still visible since the sound energy is 

affected by these features as it travels through the front surface of the coin.

C-Scan

C-Scan Recording

C-Scan Recording

3.15.4 The D scan- The D scan gives a side view of the defect seen from a 

viewpoint normal to 

the B scan. It is usually automated, and shows the length, depth and 

through thickness of a defect. The D scan should not be confused with the 

delta technique.

The D scan- The D scan gives a side view of the defect seen from a 

viewpoint normal to 

the B scan. It is usually automated, and shows the length, depth and 

through thickness of a defect. The D scan should not be confused with the 

delta technique.

AUT Displays

3.15.5 The Through Transmission Shadow Method 

This method is also called the intensity-measurement or through-transmission 

method and is explained in Fig. 12.1. The shadow of an in-homogeneity, 

which is illuminated by an ultrasonic wave, reduces under certain conditions 

the intensity of the wave received by a second probe. The name throughtransmission 

method arises obviously from the fact that two probes are often 

positioned face to face on opposite sides of the specimen but that may not 

always be the case. Figure 12.2 shows an alternative arrangement of the 

shadow method where the beam is reflected before being influenced by the 

defect, and equally is could also be reflected afterwards. 

The transmission method, which may include either reflection or through 

transmission, involves only the measurement of signal attenuation. This 

method is also used in flaw detection. 


In the pulse-echo method, it is necessary that an internal flaw reflect at least 

part of the sound energy onto a receiving transducer. However, echoes from 

flaws are not essential to their detection. Merely the fact that the amplitude of 

the back reflection from a test piece is lower than that from an identical 

workpiece known to be free of flaws implies that the test piece contains one 

or more flaws. The technique of detecting the presence of flaws by sound 

attenuation is used in transmission methods as well as in the pulse-echo 

method. The main disadvantage of attenuation methods is that flaw depth 

cannot be measured.

Fig. 12.1 Principle of the shadow method

Fig. 12.2 Shadow method with reflection 

Fig. 12.3 Shadow method with guidance of the sound

3.15.6 Other Presentations

3.16 Testing Techniques 

3.16.1 Pulse Echo Method 

1. The advantages of pulse echo method is that the deflector could be locate 

and assess accurately from one side of specimen. 

2. The disadvantage ids that the sound path has to travel twice the distance, 

thus more attenuations. 

3. The presentation is an A-Scan Dispaly

3.16.2 Through Transmission Techniques 

Two probes are used, positioned on opposite sides. The present of reflector is 

indicated by reduction or loss of receiving signal amplitude. 

1. The advantages is that the sound has to travel a single path, thus material 

with higher attenuation could be checked, thicker material could be 

checked and higher frequency with improved sensitivity and resolution 

could be realized. 

2. The disadvantages is that there is no indication of depth, access to both 

sides of specimen is required and 

change in coupling condition may 

be mistaken as defect. More 

elaborate set-up 

3. The presentation is a Shadow Method

Through Transmission Techniques

The Through Transmission Shadow Method 


3.16.3 The Tandem Techniques 

The tandem method employed 2 probe on the same side , with each other 

spaced at a predetermined length. One transmitting signal the other set to 

received signal if reflected from a defect,\. The distance between the probe 

depends on the probe angle, material thickness and the depth of expected 

defects. The techniques are used to find for defects at predetermined depth 

such as in the root of double V weld. The presentation could be a A-Scan 

display.

The Tandem Techniques 

Phased array: Complete coverage 

with two probes 

Conventional UT: Complete 

coverage with > 24 probes 

Illustration showing the inspection of one 

zone. Phased array technology allows the 

simultaneous inspection of all zones with 

the same probe. Phased array offers 

complete coverage of the weld with one 

probe on either side of the weld. 

Illustration showing the inspection of 

one zone. With conventional UT 

technology several probes are needed 

to cover all zones. 

http://www.olympus-ims.com/en/pipewizard/

3.16.4 Immersion Methods 

In immersion method, compressional probe is mounted on a bridge immersed 

in water. The probe could be normal to the test piece as compressional probe 

or the bridge could be tilted to generate shear wave of various shear angle. 

Probe frequency of 25MHz is not uncommon for immersion method unlike the 

contact methods where the thin crustal may be too fragile to handle. The 

display could be a A, B, C Scans or through transmission shadow display.

During the set-up of immersion methods, the water path between the probe 

and the material surface is delay off the screen, so that the Zero starting point 

at the screen represent the front surface of the test material. 

It is important to note that the longitudinal velocity in steel is 4 times of that of 

water, so the testing of steel the water gap should be greater than one quarter 

( ¼ ) of steel thickness 

Gap water > ¼ Steel Thickness, < 

(e.g. for 100mm steel the water gap shall be >25mm)

¼ T 

T

3.17 UT Equipment Circuitry & Controls 

As with computers, the technology concerning ultrasonic equipment and 

systems is becoming somewhat transitory. Ultrasonic systems are either 

battery operated portable units, multi-component laboratory ultrasonic 

systems, or something in between. Whether they are based on modern digital 

technology or the fast disappearing analog original, systems (often defined as 

instrument plus transducer and cable) basically comprise the following 

components circuitry and controls.

3.17.1 Instrument Circuitry: 

Although the electronic equipment used for ultrasonic inspection can vary 

greatly in detail among equipment manufacturers, all general-purpose units 

consist of a power supply, a pulser circuit, a search unit, a receiver-amplifier 

circuit, an oscilloscope, and an electronic clock. Many systems also include 

electronic equipment for signal conditioning, gating, automatic interpretation, 

and integration with a mechanical or electronic scanning system. Moreover, 

advances in microprocessor technology have extended the data acquisition 

and signal-processing capabilities of ultrasonic inspection systems.

Instrument Circuitry: Time base 

The function of the time base, also called "sweep generator" in analog-display 

instruments, is to establish a display of sound travel time on the horizontal 

scale of the display. The horizontal scale can then be used for distance 

readout. The range (coarse range, test range) control adjusts the scale for the 

range of distance to be displayed.

Instrument Circuitry: Screen picture of a specimen with back echo R and a 

group of defect indications F, with normal sweep at I-m range (a) and with 

scale expansion to 250 mm (b)

Instrument Circuitry: Power Supply. 

Circuits that supply current for all functions of the instrument constitute the 

power supply, which is usually energized by conventional 115-V or 230-V 

alternating current. There are, however, many types and sizes of portable 

instruments for which the power is supplied by batteries contained in the unit. 

Instrument Circuitry: Pulser Circuit. 

When electronically triggered, the pulser circuit generates a burst of 

alternating voltage. The principal frequency of this burst, its duration, the 

profile of the envelope of the burst, and the burst repetition rate may be either 

fixed or adjustable, depending on the flexibility of the unit.

Instrument Circuitry: Receiver-amplifier circuits 

electronically amplify return signals from the receiving transducer and often 

demodulate or otherwise modify the signals into a form suitable for display. 

The output from the receiver-amplifier circuit is a signal directly related to the 

intensity of the ultrasonic wave impinging on the receiving transducer. This 

output is fed into an oscilloscope or other display device. 

Instrument Circuitry: Oscilloscope. 

Data received are usually displayed on an oscilloscope in either video mode 

or radio frequency mode. In video mode display, only peak intensities are 

visible on the trace; in the RF mode, it is possible to observe the waveform of 

signal voltages. Some instruments have a selector switch so that the operator 

can choose the display mode, but others are designed for single-mode 

operation only

Instrument Circuitry: Signal-conditioning and gating 

circuits are included in many commercial ultrasonic instruments. One 

common example of a signal-conditioning feature is a circuit that 

electronically compensates for the signal-amplitude loss caused by 

attenuation of the ultrasonic pulse in the test piece. Electronic gates, which 

monitor returning signals for pulses of selected amplitudes that occur within 

selected time-delay ranges, provide automatic interpretation. The set point of 

a gate corresponds to a flaw of a certain size that is located within a 

prescribed depth range. Gates are often used to trigger alarms or to operate 

automatic systems that sort test pieces or identify rejectable pieces.

Instrument Circuitry: Image- and Data-Processing Equipment. 

As a result of the development of microprocessors and modern 

electronics, many ultrasonic inspection systems possess substantially 

improved capabilities in terms of signal processing and data acquisition. This 

development allows better flaw detection and evaluation (especially in 

composites) by improving the acquisition of transient ultrasonic waveforms 

and by enhancing the display and analysis of ultrasonic data. The 

development of microprocessor technology has also been useful in portable 

C-scan systems with hand-held transducers (see the section "Scanning 

Equipment" in this article).

Instrument Circuitry: Clock 

The clock circuit initiates a chain of events that results in one complete cycle 

of a UT examination. The clock sends a trigger signal, at a regular interval, to 

both the (1) time base and to the (2) pulser. As the name “clock”' implies, this 

trigger signal is repeated at a given frequency, called the pulse repetition rate 

(PRR). On some instruments pulse repetition rate is adjustable by the 

examiner; other instruments do it automatically. The electronic clock, or timer, 

serves as a source of logic pulses, reference voltage, and reference 

waveform. The clock coordinates operation of the entire electronic system.

Instrument Circuitry: Pulse Repetition Rate PRR 

The pulse repetition rate establishes the number of times per second that a 

complete test cycle will occur. In instruments with adjustable pulse repetition 

rate, adjustment is made by a pulse repetition rate control, sometimes labeled 

REP RATE.

Instrument Circuitry: Pulser-Receiver 

The pulser emits the electrical signal that activates the transducer. This 

signal, known as the initial pulse, is quite brief, usually lasting only several 

nanoseconds (10 -9 , billionths of a second). The output of the initial pulse is in 

the order of hundreds of volts; the brief duration provides a fast rise time to 

the full voltage. The pulser is connected via output connectors on the 

instrument front panel to the transducer cable. The pulser is also connected, 

internally, through the receiver circuit, to the display, thus making available 

(depending upon the delay setting) a displayed initial pulse signal. This signal 

is, of course, present whether or not a transducer is connected to the 

instrument. When a transducer is connected, it is in the signal path between 

the pulser and the receiver and its output is displayed.

3.17.2 Instrument Control: 

Even though the nomenclature used by different instrument manufacturers 

may vary, certain controls are required for the basic functions of any 

ultrasonic instrument. These functions include power supply, clock, pulser, 

receiver-amplifier, and display. In most cases, the entire electronic assembly, 

including the controls, is contained in one instrument.

Instrument Control: REJECT Control 

It is intended for preventing the display of undesired low amplitude signals, 

called grass or hash, caused by metal noise such as echoes from material 

grain boundaries or inherent fine porosity. There are two types of REJECT 

controls installed on UT instruments: nonlinear REJECT and the more 

recently linear REJECT controls. Linear REJECT controls offer the advantage 

in that they do not affect vertical linearity of the display.

Instrument Control: DELAY and RANGE Controls 

The controls are used to adjust the instruments time base for proper display 

of distances. The delay control shifts the horizontal signals to the left and right 

without altering the spacing between them. The RANGE control expands or 

contracts the spacing between horizontal signals, corresponding to the Range 

of the sound travel to be displayed.

Instrument Control: GAIN Control 

The sound amplitudes of individual reflectors returning to the transducer 

determine the relative heights of the corresponding vertical signals on the 

CRT. By adjusting the Gain Control, vertical display sensitivity and therefore 

determines the actual amplitude at which signals are displayed. 

Gain controls for the receiver-amplifier circuit usually consist of fine- and 

coarse-sensitivity selectors or one control marked "sensitivity." For a clean 

video display, with low-level electronic noise eliminated, a reject control can 

be provided.

Instrument Control: Display Control 

The display (oscilloscope) controls are usually screwdriver-adjusted, with the 

exception of the scale illumination and power on/off. After initial setup and 

calibration, the screwdriver-adjusted controls seldom require additional 

adjustment. The controls and their functions for the display unit usually 

consist of the following: 

• Controls for vertical position of the display on the oscilloscope screen. 

• Controls for horizontal position of display on the oscilloscope screen. 

• Controls for brightness of display. 

• Control for adjusting focus of trace on the oscilloscope screen. 

• Controls to correct for distortion or astigmatism that may be introduced as 

the electron beam sweeps across the oscilloscope screen. 

An optical system with astigmatism is one where rays that propagate in two 

perpendicular planes have different foci. If an optical system with astigmatism 

is used to form an image of a cross, the vertical and horizontal lines will be in 

sharp focus at two different distances.

• A control that varies the level of illumination for a measuring grid usually 

incorporated in the transparent faceplate covering the oscilloscope screen. 

• Timing controls, which usually consist of sweep-delay and sweep-rate 

controls, to provide coarse and fine adjustments to suit the material and 

thickness of the test piece. The sweep-delay control is also used to 

position the sound entry point on the left side of the display screen, with a 

back reflection or multiples of back reflections visible on the right side of 

the screen. 

• On/off switch.

Instrument Controls: 

• A marker circuit, which provides regularly spaced secondary indications 

(often in the form of a square wave) on or below the sweep line to serve 

the same purpose as scribe marks on a ruler. This circuit is activated or 

left out of the display by a marker switch for on/off selection. Usually there 

will also be a marker-calibration or marker-adjustment control to permit 

selection of marker-circuit frequency. The higher the frequency, the closer 

the spacing of square waves, and the more accurate the measurements. 

Marker circuits are controlled by timing signals triggered by the electronic 

clock. Most modern ultrasonic instruments do not have marker circuits 

• A Gain circuit to electronically compensate for a drop in the amplitude of 

signals reflected from flaws located deep in the test piece. This circuit may 

be known as distance-amplitude correction, sensitivity-time control, timecorrected 

gain, or time-varied gain 

• Damping controls that can be used to shorten the pulse duration and 

thus adjust the length of the wave packet emanating from the transducer. 

Resolution is improved by higher values of damping

• High-voltage or low-voltage driving current, which is selected for the 

transducer with a transducer voltage switch. 

• Gated alarm units, which enable the use of automatic alarms when flaws 

are detected. This is accomplished by setting up controllable time spans 

on the display that correspond to specific zones within the test piece. 

Signals appearing within the gates may automatically operate visual or 

audible alarms. These signals may also be passed on to display devices 

or strip-chart recorders or to external control devices. Gated alarm units 

usually have three controls: the gate-start or delay control, which adjusts 

the location of the leading edge of the gate on the oscilloscope trace; the 

gate-length control, which adjusts the length of the gate or the location of 

the gate trailing edge; and the alarm-level or sensitivity control, which 

establishes the minimum echo height necessary to activate an alarm 

circuit. A positive/negative logic switch determines whether the alarm is 

triggered above or below the threshold level.

Instrument Control: Gates 

Most UT equipment is equipped with “gates” that can be superimposed on the 

time base so that a rapid response from a particular reflector can be obtained 

when they reach a certain predetermined amplitude. This can be adapted as 

a “go/no-go” monitoring device for some examinations. Gates can be set for 

an alarm to be triggered at a pre-determined amplitude (positive) with an 

increasing signal or (negative) with a decreasing signal amplitude. Gates are 

essential for some types of recording systems where they also serve to 

provide information to the recording devices or storage systems.

3.17.3 Pulse-Echo Instrumentation (A-Scan) 

The UT system includes: the instrument, transducers, calibration standards, 

and the object being examined. These elements function together to form a 

chain of events during a typical UT that can be summarized as follows: 

1. The instrument’s pulser electrically activates the transducer, causing it to 

send sound pulses into the test object. 

2. The activation signal, called the initial pulse, is displayed as a vertical 

signal on the CRT. 

3. As sound travels through the test object, it reflects from boundaries as well 

as from discontinuities within the material. 

4. The instrument's time base initiates readout of time/distance information 

on the horizontal scale of the display. 

5. A reflection from the surface opposite the entry surface is called a back 

reflection. These reflections reach the transducer, which converts them 

into electrical signals that are displayed on the CRT.

Figure above Block diagram circuitries are: 

1. Transducer 

2. Pulser (clock) 

3. Receiver/amplifier 

4. Display (screen) 

To understand how a typical ultrasonic system operates, it is necessary to 

view one cycle of events, or one pulse. The sequence is as follows. 

1. The clock signals the pulser to provide a short, high-voltage pulse to the 

transducer while simultaneously supplying a voltage to the time-base trigger 

module. 

2. The time-base trigger starts the spot in the CRT on its journey across the 

screen.

3. The voltage pulse reaches the transducer and is converted into mechanical 

vibrations (see piezoelectricity ), which enter the test piece. These vibrations 

(energy) now travel along their sound path through the test piece. All this 

time, the spot is moving horizontally across the CRT. 

4. The energy in the test piece now reflects off the interface (back wall) back 

toward the transducer, where it is reconverted into a voltage. (The 

reconverted voltage is a fraction of its original value.) 

5. This voltage is now received and amplified by the receiver/amplifier

Pulse Repetition Rate 

- Gain 

- Frequency 

-Reject 

Sweep & Range

Typical block diagram of an analog A-scan setup, including video-mode 

display, for basic pulse-echo ultrasonic inspection

A basic instrument contains several circuits: 

• power supply, clock (also called synchronizer or timer), 

• time base (called sweep generator), 

• pulser (also called transmitter), 

• receiver (also called receiver-amplifier), and 

• display.

3.17.4 B Scan Block diagram: 

B-scan display is a plot of time versus distance, in which 

• one orthogonal axis on the display corresponds to elapsed time (depth), 

• while the other axis represents the position of the transducer along a line 

on the surface of the test piece relative to the position of the transducer at 

the start of the inspection. 

Echo intensity is not measured directly as it is in A-scan inspection, but is 

often indicated semi quantitatively by the relative brightness of echo 

indications on an oscilloscope screen. A B-scan display can be likened to an 

imaginary cross section through the test piece where both front and back 

surfaces are shown in profile. Indications from reflecting interfaces within the 

test piece are also shown in profile, and the position, orientation, and depth of 

such interfaces along the imaginary cutting plane are revealed.

Applications. 

The chief value of B-scan presentations is their ability to reveal the 

distribution of flaws in a part on a cross section of that part. Although B-scan 

techniques have been more widely used in medical applications than in 

industrial applications, B-scans can be used for the rapid screening of parts 

and for the selection of certain parts, or portions of certain parts, for more 

thorough inspection with A-scan techniques. Optimum results from B-scan 

techniques are generally obtained with small transducers and high 

frequencies.

Typical B-scan setup, including video-mode display, for basic pulse-echo 

ultrasonic inspection

• First, the display is generated on an oscilloscope screen that is composed 

of a long-persistence phosphor, that is, a phosphor that continues to 

fluoresce long after the means of excitation ceases to fall on the 

fluorescing area of the screen. This characteristic of the oscilloscope in a 

B-scan system allows the imaginary cross section to be viewed as a whole 

without having to resort to permanent imaging methods, such as 

photographs. (Photographic equipment, facsimile recorders, or x-y plotters 

can be used to record B-scan data, especially when a permanent record is 

desired for later reference.) 

• Second, the oscilloscope input for one axis of the display is provided by an 

electromechanical device that generates an electrical voltage or digital 

signals proportional to the position of the transducer relative to a reference 

point on the surface of the test piece. Most B-scans are generated by 

scanning the search unit in a straight line across the surface of the test 

piece at a uniform rate. One axis of the display, usually the horizontal axis, 

represents the distance traveled along this line.

• Third, echoes are indicated by bright spots on the screen rather than by 

deflections of the time trace. The position of a bright spot along the axis 

orthogonal to the search-unit position axis, usually measured top to bottom 

on the screen, indicates the depth of the echo within the test piece. Finally, 

to ensure that echoes are recorded as bright spots, the echo-intensity 

signal from the receiver-amplifier is connected to the trace-brightness 

control on the oscilloscope. In some systems, the brightness 

corresponding to different values of echo intensity may exhibit enough 

contrast to enable semi quantitative appraisal of echo intensity, which is 

related to flaw size and shape.

Signal Display. 

The oscilloscope screen in Fig. 11 above illustrates the type of video-mode 

display that is generated by B-Scan equipment. On this screen, the internal 

flaw in the test piece shown at left in Fig. 11 above is shown only as a profile 

view of its top reflecting surface. Portions of the test piece that are behind this 

large reflecting surface are in shadow. The flaw length in the direction of 

search-unit travel is recorded, but the width (in a direction mutually 

perpendicular to the sound beam and the direction of search-unit travel) is not 

recorded except as it affects echo intensity and therefore echo-image 

brightness. Because the sound beam is slightly conical rather than truly 

cylindrical, flaws near the back surface of the test piece appear longer than 

those near the front surface.

3.17.5 C-scan display 

C-scan display records echoes from the internal portions of test pieces as a 

function of the position of each reflecting interface within an area. Flaws are 

shown on a readout, superimposed on a plan view of the test piece, and both 

flaw size (flaw area) and position within the plan view are recorded. Flaw 

depth normally is not recorded, although it can be measured semi 

quantitatively by restricting the range of depths within the test piece that is 

covered in a given scan. With an increasing number of C-scan systems 

designed with on-board computers, other options in image processing and 

enhancement have become widely used in the presentation of flaw depth and 

the characterization of flaws. An example of a computer-processed C-scan 

image is shown in Fig. 11, in which a graphite-epoxy sample with impact 

damage was examined using time-of-flight data. The depth of damage is 

displayed with a color scale in the original photograph.

Typical C-scan setup, including display, for basic pulse-echo ultrasonic 

immersion inspection

System Setup. 

In a basic C-scan system, shown schematically in Fig. 12 above, the search 

unit is moved over the surface of the test piece in a search pattern. The 

search pattern may take many forms; for example, a series of closely spaced 

parallel lines, a fine raster pattern, or a spiral pattern (polar scan). Mechanical 

linkage connects the search unit to x-axis and y-axis position indicators, 

which in turn feed position data to the x-y plotter or facsimile device. Echo 

recording systems vary; some produce a shaded-line scan with echo intensity 

recorded as a variation in line shading, while others indicate flaws by an 

absence of shading so that each flaw shows up as a blank space on the 

display (Fig. 12) above.

Gating. (Depth Gate) 

An electronic depth gate is another essential element in C-scan systems. A 

depth gate is an electronic circuit that allows only those echo signals that are 

received within a limited range of delay times following the initial pulse or 

interface echo to be admitted to the receiver-amplifier circuit. Usually, the 

depth gate is set so that front reflections and back reflections are just barely 

excluded from the display. Thus, only echoes from within the test piece are 

recorded, except for echoes from thin layers adjacent to both surfaces of the 

test piece. Depth gates are adjustable. By setting a depth gate for a narrow 

range of delay times, echo signals from a thin slice of the test piece parallel to 

the scanned surface can be recorded, with signals from other portions being 

excluded from the display. 

Some C-scan systems, particularly automatic units, incorporate additional 

electronic gating circuits for marking, alarming, or charting. These gates can 

record or indicate information such as flaw depth or loss of back reflection, 

while the main display records an overall picture of flaw distribution.

Q79: In the pulse echo instrument, the synchronizer, clock, or timer circuit 

determine the: 

a) Pulse length 

b) Gain 

c) Pulse repetition rate 

d) Sweep range

Q1: In an ultrasonic test system where signal amplitudes are displayed, an 

advantage of a frequency independent attenuator over a continuously 

variable gain control is that: 

A. The pulse shape is less distorted 

B. The signal amplitude measured using the attenuator is independent 

of frequency 

C. The dynamic range of the system id decreased 

D. The effect of amplification threshold is avoided. 

Definition: Switch that controls the output power of the HV generator is 

the attenuator.

Q1: The rate generator in B-scan equipment will invariably be directly 

connected to the: 

A. The display intensity circuit 

B. The pulser circuit 

C. The RF amplifier circuit 

D. The horizontal sweep circuit

Q30: The time from the start of the ultrasonic pulse to the reverberations 

complete decay limit the maximum usable: 

A. Pulse time-flaw rate 

B. Pulse/receiver rate 


D. Modified pulse-time rate 

Hint: A/B/D could not be the correct answers as they were not even the standard terms used.

Q129: An A-scan display, which shows a signal both above and below the 

sweep line is called: 

A. A video display 

B. A RF display 

C. An audio display 

D. Frequency modulated display

Q166: In a basic pulse echo instrument, the sunchronizer, clock or timer 

circuit determines the: 

A. Pulse length 

B. Gain 


D. Sweep length

Q32: On many ultrasonic testing instruments, an operator conducting an 

immersion test can remove that portion of the screen presentation that 

represents water distance by adjusting a: 

A. Pulse length control. 

B. Reject control. 

C. Sweep delay control. 

D. Sweep length control.

121. In an ultrasonic instrument, the number of pulses produced by an 

instrument in a given period of time is known as the: 

A. Pulse length of the instrument 

B. Pulse recovery time 

C. Frequency 

D. Pulse repetition rate 

122. In a basic pulse echo ultrasonic instrument, the component that 

coordinates the action and timing of other components is called a: 

A. Display unit 

B. Receiver 

C. Marker circuit or range marker circuit 

D. Synchronizer, clock, or timer


produces the voltage that activates the transducer is called: 

A. An amplifier 

B. A receiver 

C. A pulser 

D. A synchronizer 

124. In basic pulse echo ultrasonic instrument, the component that produces 

the time base line is called a: 

A. Sweep circuit 

B. Receiver 

C. Pulser 

D. Synchronizer


produces visible signals on the CRT which are used to measure distance is 

called a: 


B. Marker circuit 

C. Receiver circuit 

D. Synchronizer 

126. Most basic pulse echo ultrasonic instruments use: 

A. Automatic read-out equipment 

B. An A-scan presentation 

C. A B-scan presentation 

D. A C-scan presentation

3.18 Further Reading on Sub-Section 3 

3.18.1 What is reflection, refraction, diffraction, and interference? 

What exactly is reflection, refraction, diffraction, and interference? 

Reflection occurs when a wave hits something and then bounces it off it. 

Refraction is the bending of a wave caused by a change in its speed as it 

moves from one medium to another. 

Diffraction occurs when an object causes a wave to change direction and 

bend around it. Interference is when two or more waves overlap and combine 

to make a new wave of lesser or more amplitude. 

This picture shows how reflection of light works 

and the names of the beams in a reflection. 

http://light-and-soundproject.wikispaces.com/3.+What+is+reflection,+refraction,+diffraction,+ 

and+interference%3F

3.18.2 Reflection 

西塘

In this picture there is two different beams, and those beams create angles. 

The beams are referred to as the reflected beam and the incident beam. The 

dotted line is the line that is perpendicular to the mirror, and it splits the large 

angle into the two different angles. The first angle is the angle of reflection, 

and it is formed by the reflected beam and the perpendicular line. The other 

angle is the angle of incidence which is formed by the incident beam and the 

perpendicular line. These two angles are always the same measure, although 

it sometimes might be a larger or smaller angle.

How do reflection, refraction, and diffraction relate to light? 

Reflection happens when a light is turned on, and it is in an enclosed area. If 

someone is in a enclosed area, and a light is turned on they are going to be 

able to see it. Then the light will continue, hit a wall, and it would reflect back 

to the human eye.

This picture shows how water waves will diffract around an island. This 

picture also shows constructive and destructive interference. 

The diffraction happens in this picture when the water waves pass between 

the two rocks. When the waves get onto the other side of the two rocks the 

waves are shaped as an arc (a U shape). The constructive and destructive 

interference happens by the rock in the middle of the picture to the left. The 

waves that are passing between the two rocks meet up with the waves 

passing around the one rock to the left, and the waves combine. Some waves 

will cancel each other out, and some will add to each other and make a 

bigger amplitude.

3.18.3 Refraction happens 

when light is shown through 

another material, and it changes 

the way it is being shown. An 

example is when you fill a cup 

with water, and then you place 

a pencil in the water. When you 

look at the pencil from the side 

it looks as though the pencil is 

broken where the pencil enters 

the water. This is due to 

refraction, and the bending of 

the waves before it enters your 

eyes. This picture shows the 

broken pencil experiment.

3.18.4 Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction

Diffraction happen when light tries to go through an opening. If you are in a 

dark hallway, and a room has a light on, you will be able to see he light, but it 

will only light up a section of the hallway, and you won't be in the light until 

you are almost directly in front of the room.

This diagram shows an interference. In this diagram it happens to be 

constructive interference, but this is not the only type of interference.

3.18.5 Interference

Interference

Interference

3.19 Questions & Answers

Q11: When maximum sensitivity is required from a transducer: 

A. Straight beam transducer should be used 

B. Large diameter crystals are required 

C. The piezoelectric element should be driven at its fundamental 

frequency 

D. The bandwidth of the transducer should be as large as possible.

Q12: The 1 MHz transducer that should normally have the best time of 

distance resolution is a: 

A. Quartz crystal with air backing 

B. Quartz crystal with phenolic backing 

C. Barium titanate transducer with phenolic backing 

D. Lithium Sulphate transducer with epoxy backing 

Hint: 1 MHz as Lithium Sulphate is not easily cut to very thin thickness, best 

distance resolution due to the fact the Lithium Sulphate is the best receiver of 

ultrasound energy.

Q3: The ultrasonic instrument used for examination of welding shall be 

capable of generating frequencies: 

A. more than 5 MHz 

B. more than 10 MHz 

C. less than 1 MHz 

D. 1 MHz to 5 MHz 

Q4. Calibration of ultrasonic equipment shall be done 

A. at beginning of examination 

B. both at beginning and end of the examination 

C. both at beginning and also at every two hours interval 

D. at beginning end, every two hours interval and whenever a change 

operator

Q4. Calibration of ultrasonic equipment shall be done 

• at beginning of examination 

• both at beginning and end of the examination 

• both at beginning and also at every two hours interval 

• at beginning end, every two hours interval and whenever a change 

operator

Q15: Entry surface resolution is a characteristic of an ultrasonic testing 

system which defines its ability to: 

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam. 

B. Detect discontinuities located in the center of a forging containing a fine 

metallurgic structure. 

C. Detect minute surface scratches. 

D. Detect discontinuities located just beneath the entry surface in the 

part being tested.

Discussion Topic: Factors affecting the Entry Surface Resolution 

Q15: Entry surface resolution is a characteristic of an ultrasonic testing system which defines its ability to: 

A. Detect discontinuities oriented in a direction parallel to the ultrasonic beam. 

B. Detect discontinuities located in the center of a forging containing a fine metallurgic structure. 

C. Detect minute surface scratches. 

D. Detect discontinuities located just beneath the entry surface in the part being tested. 

List of factors:

Expert at Works-Salute!

Experts at Work-Salute!

Section 4: Calibration Methods

Content: Section 4: Calibration Methods 




4.2.2: Finding the probe index 

4.2.3: Checking the probe angle 

4.2.4: Calibration of shear waves for range V1 Block 



4.2.8: Linearity Checks (Time Base/ Equipment Gain/ Vertical Gain) 

4.2.9: TCG-Time Correction Gain 



4.5: Questions & Answers 

4.6: Video Time


Calibration refers to the act of evaluating and adjusting the precision and 

accuracy of measurement equipment. In ultrasonic testing, several forms of 

calibration must occur. First, the electronics of the equipment must be 

calibrated to ensure that they are performing as designed. This operation is 

usually performed by the equipment manufacturer and will not be discussed 

further in this material. It is also usually necessary for the operator to perform 

a "user calibration" of the equipment. This user calibration is necessary 

because most ultrasonic equipment can be reconfigured for use in a large 

variety of applications. The user must "calibrate" the system, which includes 

the equipment settings, the transducer, and the test setup, to validate that the 

desired level of (1) precision and (2) accuracy are achieved. The term 

calibration standard is usually only used when an absolute value is measured 

and in many cases, the standards are traceable back to standards at the 

National Institute for Standards and Technology.

Calibrations

In ultrasonic testing, there is also a need for reference standards. Reference 

standards are used to establish a general level of consistency in 

measurements and to help interpret and quantify the information contained in 

the received signal. Reference standards are used to validate that the 

equipment and the setup provide similar results from one day to the next and 

that similar results are produced by different systems. Reference standards 

also help the inspector to estimate the size of flaws. In a pulse-echo type 

setup, signal strength depends on both the size of the flaw and the distance 

between the flaw and the transducer. The inspector can use a reference 

standard with an artificially induced flaw of known size and at approximately 

the same distance away for the transducer to produce a signal. By comparing 

the signal from the reference standard to that received from the actual flaw, 

the inspector can estimate the flaw size.

This section will discuss some of the more common calibration and reference 

specimen that are used in ultrasonic inspection. Some of these specimens 

are shown in the figure above. Be aware that there are other standards 

available and that specially designed standards may be required for many 

applications. The information provided here is intended to serve a general 

introduction to the standards and not to be instruction on the proper use of the 

standards.

Introduction to the Common Standards 

Calibration and reference standards for ultrasonic testing come in many 

shapes and sizes. The type of standard used is dependent on the NDE 

application and the form and shape of the object being evaluated. The 

material of the reference standard should be the same as the material being 

inspected and the artificially induced flaw should closely resemble that of the 

actual flaw. This second requirement is a major limitation of most standard 

reference samples. Most use drilled holes and notches that do not closely 

represent real flaws. In most cases the artificially induced defects in reference 

standards are better reflectors of sound energy (due to their flatter and 

smoother surfaces) and produce indications that are larger than those that a 

similar sized flaw would produce. Producing more "realistic" defects is cost 

prohibitive in most cases and, therefore, the inspector can only make an 

estimate of the flaw size. Computer programs that allow the inspector to 

create computer simulated models of the part and flaw may one day lessen 

this limitation.

The IIW Type Calibration Block

The IIW Type Calibration Block

The IIW Type 2 Calibration Block

The IIW Type I Calibration Block

EN12223:1999 Calibration Block

The IIW Phase Array Calibration Block

The IIW Calibration Block 

1 st Check Index / Check Range


2 nd Check Angle


2 nd Check Angle

Find probe angle 

Find Index/Range/Resolution

The IIW Phase Array Calibration Block 

3 rd Check Resolution

V2 Calibration Block

The IIW 2 Calibration Block 

Check focal point 

Check probe angle 

Check range 

Can not Check resolution

Calibration Blocks

Calibration Blocks- Area Amplitude Block

The standard shown in the above figure is commonly known in the US as an 

IIW type reference block. IIW is an acronym for the International Institute of 

Welding. It is referred to as an IIW "type" reference block because it was 

patterned after the "true" IIW block but does not conform to IIW requirements 

in IIS/IIW-23-59. "True" IIW blocks are only made out of steel (to be precise, 

killed, open hearth or electric furnace, low-carbon steel in the normalized 

condition with a grain size of McQuaid-Ehn #8) where IIW "type" blocks can 

be commercially obtained in a selection of materials. The dimensions of "true" 

IIW blocks are in metric units while IIW "type" blocks usually have English 

units. IIW "type" blocks may also include additional calibration and references 

features such as notches, circular groves, and scales that are not specified by 

IIW. There are two full-sized and a mini versions of the IIW type blocks. The 

Mini version is about one-half the size of the full-sized block and weighs only 

about one-fourth as much. The IIW type US-1 block was derived the basic 

"true" IIW block and is shown below in the figure on the left. The IIW type US- 

2 block was developed for US Air Force application and is shown below in the 

center. The Mini version is shown on the right.

IIW Blocks- US-1 

IIW Type US-1

IIW Blocks- IIW Type US-2

IIW Blocks- IIW Type Mini

V1/5, A2 Block

IIW type blocks are used to calibrate instruments for both angle beam and 

normal incident inspections. Some of their uses include setting metal-distance 

and sensitivity settings, determining the sound exit point and refracted angle 

of angle beam transducers, and evaluating depth resolution of normal beam 

inspection setups. Instructions on using the IIW type blocks can be found in 

the annex of American Society for Testing and Materials Standard E164, 

Standard Practice for Ultrasonic Contact Examination of Weldments. 

The Miniature Angle-Beam or ROMPAS Calibration Block

DSC Block, Mini block, Rompas Block are all mini blocks. 

ROMPAS Calibration Block 

AWS Shear Wave 

Distance/Sensitivity 

Calibration (DSC) Block

A block that closely resembles the miniature angle-beam block and is used in 

a similar way is the DSC AWS Block. This block is used to determine the 

beam exit point and refracted angle of angle-beam transducers and to 

calibrate distance and set the sensitivity for both normal and angle beam 

inspection setups. Instructions on using the DSC block can be found in the 

annex of American Society for Testing and Materials Standard E164, 

Standard Practice for Ultrasonic Contact Examination of Weldments.

A block that closely resembles the miniature angle-beam block and is used in 

a similar way is the DSC AWS Block. This block is used to determine the 

beam exit point and refracted angle of angle-beam transducers and to 

calibrate distance and set the sensitivity for both normal and angle beam 

inspection setups. Instructions on using the DSC block can be found in the 



DSC AWS Block

Calibration Range Using DSC AWS Block 

www.youtube.com/embed/TEQ8Qrz4D-A

AWS Shear Wave Distance Calibration (DC) Block

AWS Shear Wave Distance Calibration (DC) Block

The DC AWS Block is a metal path distance and beam exit point calibration 

standard that conforms to the requirements of the American Welding Society 

(AWS) and the American Association of State Highway and Transportation 

Officials (AASHTO). Instructions on using the DC block can be found in the 



AWS Resolution Calibration (RC) Block 

The RC Block is used to determine the resolution of angle beam transducers 

per the requirements of AWS and AASHTO. Engraved Index markers are 

provided for 45, 60, and 70 degree refracted angle beams.

The RC Block is used to determine the resolution of angle beam transducers 

per the requirements of AWS and AASHTO. Engraved Index markers are 

provided for 45, 60, and 70 degree refracted angle beams.

30 FBH Resolution Reference Block 

The 30 FBH resolution reference block is used to evaluate the near-surface 

resolution and flaw size/depth sensitivity of a normal-beam setup. The block 

contains number 3 (3/64"), 5 (5/64"), and 8 (8/64") ASTM flat bottom holes at 

ten metal-distances ranging from 0.050 inch (1.27 mm) to 1.250 inch (31.75 

mm).

Miniature Resolution Block 

The miniature resolution block is used to evaluate the near-surface resolution 

and sensitivity of a normal-beam setup It can be used to calibrate highresolution 

thickness gages over the range of 0.015 inches (0.381 mm) to 

0.125 inches (3.175 mm).

Step and Tapered Calibration Wedges 

Step and tapered calibration wedges come in a large variety of sizes and 

configurations. Step wedges are typically manufactured with four or five steps 

but custom wedge can be obtained with any number of steps. Tapered 

wedges have a constant taper over the desired thickness range.

Distance/Sensitivity (DS) Block 

The DS test block is a calibration standard used to check the horizontal 

linearity and the dB accuracy per requirements of AWS and AASHTO.

Area Amplitude Blocks provide standards for discontinuities of different size 

at the same depth 

Distance Amplitude Blocks provide standards for discontinuities of same size 

at the different depth

The ASTM basic set of Area/Distance Amplitude Blocks consists of ten, two 

inches diameter blocks

The ASTM basic set of Area/Distance Amplitude Blocks consisits of ten, two 

inches diameter blocks

Distance/Area-Amplitude Blocks 

Distance/area amplitude correction blocks typically are purchased as a tenblock 

set, as shown above. Aluminum sets are manufactured per the 

requirements of ASTM E127 and steel sets per ASTM E428. Sets can also be 

purchased in titanium. Each block contains a single flat-bottomed, plugged 

hole. The hole sizes and metal path distances are as follows: 

• 3/64" at 3" 

• 5/64" at 1/8", 1/4", 1/2", 3/4", 11/2", 3", and 6" 

• 8/64" at 3" and 6" 

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and 

Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the 

requirements of ASTM E127, Standard Practice for Fabricating and Checking 

Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are 

fabricated per the requirements of ASTM E428, Standard Practice for 

Fabrication and Control of Steel Reference Blocks Used in Ultrasonic 

Inspection.

ASTM E 127

Area-Amplitude Blocks 

Area-amplitude blocks are also usually purchased in an eight-block set and 

look very similar to Distance/Area-Amplitude Blocks. However, areaamplitude 

blocks have a constant 3-inch metal path distance and the hole 

sizes are varied from 1/64" to 8/64" in 1/64" steps. The blocks are used to 

determine the relationship between flaw size and signal amplitude by 

comparing signal responses for the different sized holes. Sets are commonly 

sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and Type 304 Corrosion 

Resistant Steel. Aluminum blocks are fabricated per the requirements of 

ASTM E127, Standard Practice for Fabricating and Checking Aluminum Alloy 

Ultrasonic Standard Reference Blocks. Steel blocks are fabricated per the 

requirements of ASTM E428, Standard Practice for Fabrication and Control of 

Steel Reference Blocks Used in Ultrasonic Inspection.

Distance-Amplitude #3, #5, #8 FBH Blocks 

Distance-amplitude blocks also very similar to the distance/area-amplitude 

blocks pictured above. Nineteen block sets with flat-bottom holes of a single 

size and varying metal path distances are also commercially available. Sets 

have either a #3 (3/64") FBH, a #5 (5/64") FBH, or a #8 (8/64") FBH. The 

metal path distances are 1/16", 1/8", 1/4", 3/8", 1/2", 5/8", 3/4", 7/8", 1", 1-1/4", 

1-3/4", 2-1/4", 2-3/4", 3-14", 3-3/4", 4-1/4", 4-3/4", 5-1/4", and 5-3/4". The 

relationship between the metal path distance and the signal amplitude is 

determined by comparing signals from same size flaws at different depth. 

Sets are commonly sold in 4340 Vacuum melt Steel, 7075-T6 Aluminum, and 

Type 304 Corrosion Resistant Steel. Aluminum blocks are fabricated per the 

requirements of ASTM E127, Standard Practice for Fabricating and Checking 

Aluminum Alloy Ultrasonic Standard Reference Blocks. Steel blocks are 

fabricated per the requirements of ASTM E428, Standard Practice for 

Fabrication and Control of Steel Reference Blocks Used in Ultrasonic 

Inspection.

Key Words: 

Distance Amplitude Blocks 

DSC 

DC 

SC 

AWS RC 

Distance sensitivity calibration 

Distance calibration 

Sensitivity calibration 

AWS Resolution Calibration.

Q56: On the area-amplitude ultrasonic standard test blocks, the flat-bottomed 

holes in the blocks are: 

A. All of the same diameter 

B. Different in diameter, increasing by 1/64 inch increments from the 

No. 1 block to the No. 8 block 

C. Largest in the No. 1 block and smallest in the No. 8 block 

D. Drilled to different depths from the front surface of the test block

Q: A primary purpose of a reference standard is: 

A. To provide a guide for adjusting instrument controls to reveal 

discontinuities that are considered harmful to the end use of the 

product. 

B. To give the technician a tool for determining exact discontinuity size 

C. To provide assurance that all discontinuities smaller than a certain 

specified reference reflector are capable of being directed by the test. 

D. To provide a standard reflector which exactly simulates natural 

discontinuities of a critical size.



Distance Amplitude Correction (DAC): Acoustic signals from the same 

reflecting surface will have different amplitudes at different distances from the 

transducer. Distance amplitude correction (DAC) provides a means of 

establishing a graphic ‘reference level sensitivity’ as a function of sweep 

distance on the A-scan display. The use of DAC allows signals reflected from 

similar discontinuities to be evaluated where signal attenuation as a function 

of depth has been correlated. Most often DAC will allow for loss in amplitude 

over material depth (time), graphically on the A-scan display but can also be 

done electronically by certain instruments. Because near field length and 

beam spread vary according to transducer size and frequency, and materials 

vary in attenuation and velocity, a DAC curve must be established for each 

different situation. DAC may be employed in both longitudinal and shear 

modes of operation as well as either contact or immersion inspection 

techniques.

DAC Curve

http://www.huatecgroup.com/china-digital_portable_dac_avg_curves_ultrasonic_flaw_detector_ut_flaw_detector_fd350-632512.html

DAC- Distance Amplitude Correction

DAC- Distance Amplitude Correction 

DGS- Distance Gain Size

A distance amplitude correction curve is constructed from the peak amplitude 

responses from reflectors of equal area at different distances in the same 

material. A-scan echoes are displayed at their non-electronically 

compensated height and the peak amplitude of each signal is marked on the 

flaw detector screen or, preferably, on a transparent plastic sheet attached to 

the screen. Reference standards which incorporate side drilled holes (SDH), 

flat bottom holes (FBH), or notches whereby the reflectors are located at 

varying depths are commonly used. It is important to recognize that 

regardless of the type of reflector used, the size and shape of the reflector 

must be constant. Commercially available reference standards for 

constructing DAC include ASTM Distance/Area Amplitude and ASTM E1158 

Distance Amplitude blocks, NAVSHIPS Test block, and ASME Basic 

Calibration Blocks.

The following applet shows a test block with a side drilled hole. The 

transducer was chosen so that the signal in the shortest pulse-echo path is in 

the far-field. The transducer may be moved finding signals at depth ratios of 1, 

3, 5, and 7. Red points are "drawn" at the peaks of the signals and are used 

to form the distance amplitude correction curve drawn in blue. Start by 

pressing the green "Test now!" button. After determining the amplitudes for 

various path lengths (4), press "Draw DAC" and then press the green "Test 

now!" button.

DAC Java 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/applet2/applet2.htm

Developing a Distance Amplitude Correction (DAC) Curve 

Distance Amplitude Correction (DAC) provides a means of establishing a 

graphic ‘reference level sensitivity’ as a function of sweep distance on the A- 

scan display. The use of DAC allows signals reflected from similar 

discontinuities to be evaluated where signal attenuation as a function of depth 

may be correlated. In establishing the DAC curve, all A-scan echoes are 

displayed at their non-electronically compensated height. 

Construction of a DAC involves the use of reference standards which 

incorporate side drilled holes (SDH), flat bottom holes (FBH), or notches 

whereby the reflectors are located at varying depths. It is important to 

recognize regardless of the type of reflector that is used in constructing the 

DAC, the size and shape of the reflector must be constant over the sound 

path distance. Commercially available reference standards for constructing 

DAC include ASTM Distance/Area Amplitude and ASTM E1158 Distance 

Amplitude blocks, NAVSHIPS Test block, and ASME Basic Calibration 

Blocks.

Sequence for constructing a DAC curve when performing a straight 

beam contact inspection on 1 ¾” thick material. 

1.) Using a suitable reference standard, calibrate the sweep for a distance 

appropriate for the material to be inspected, i.e.. using a 1” thick standard, 

calibrate the sweep for 2” of material travel.

Back Wall Echo

Back Wall Echo 

Sweep 2” / Distance 1”

2.) This example represents the use a 1 3/4” thick reference standard with 

1/8” side drilled holes located at 1/4 T and 3/4 T respectively. ‘T’ being equal 

to the block thickness.

3.) Position the transducer over the 1/4T hole and peak the signal to 

approximately 80% FSH (Full screen height), mark the peak of the echo on 

the display using a suitable marker, and record the gain setting.

4.) With no further adjustments to the gain control, position the transducer 

over the 3/4T hole and peak the signal, mark the peak of the echo on the 

display.

5.) To complete the DAC curve connect the dots with a smooth line. The 

completed curve represents the ‘reference level sensitivity’ for this application.

Plotting DAC Curve

DAC Curve

DAC Curve

Gain Control for FSH: It should be remember that the dB is a means of 

comparing signals. All UT sets are different and a FSH with a gain controls of 

36dB in one UT set and be at FSH at another UT set with a gain control 

reading of 26dB. 

The gain controls allow us to set sensitivity and form the basis of Ultrasonic 

Sizing Techniques.

Birring NDT Series, Ultrasonic Distance Amplitude Correction - DAC 

www.youtube.com/embed/qUqaF0PnLGA?list=UUZncq6JFram3pfQDlzGggwA

Alta Vista UT Calibration DAC Curve 

www.youtube.com/embed/VNgMKlp43I8

4.2.2: Finding the probe index

Exit Point 

A2 Block

Exit Point- A5 Block

Q16: Notches are frequently used as a reference reflector for: 

A. Distance amplitude calibration for shear wave 

B. Area amplitude calibration 

C. Thickness calibration for plate 

D. Determining the near-surface resolution

5.2.3: Checking the probe angle

Probe Angles- A2 Block

Probe Angles- A5 Block

4.2.4: Calibration of shear waves for range V1 Block

Calibration of shear waves for range V1 Block

1 st Echo from circular Section

Echo from 100mm circular Section

Calibration of shear waves for range V1 Block 

Test block 1 for calibrating the 

time base (depth scale) of a flaw 

detector for vertical probes 

(longitudinal waves) for angle 

probes (transverse waves), for 

determining the probe index and 

beam angle of angle probes, and 

for checking the short term 

consistency of the sensitivity of 

vertical probes


25 mm radius from V2 Block

50 mm radius from V2 Block

100 mm radius from K2 Block


Shear Wave Distance Calibration IIW Block & DSC Blocks 

www.youtube.com/embed/RmtHmtOozic

Exit Point /Range/Probe Angle calibration using IIW Block (Repeat-Code1) 

www.youtube.com/embed/Qr0dGNuq9yY


Determine the dead zone by finding the hole echo which is easily 

identifiable from the probe noise at the shortest range

Dead Zone 

Determine the dead zone by 

finding the hole echo which is 

easily identifiable from the probe 

noise at the shortest range

4.2.6: 20 dB Profile- A5 Block

20 dB Profile 

Probe Beam Line of Symmetry

20 dB Profile 

Probe Beam Sound Pressure


Methods of compensating for transfer and attenuation loss differences for 

0attenuation 000compression probes and for shear wave compression 

probes. These are based on obtaining similar echo responses on both the 

calibration block and on the component. 

For 0degree probes backwall echoes are used to probes establish transfer 

and attenuation correction. 

For shear wave probes two identical probes are used in “pitch-catch” in 

order to obtain what are effectively backwall echoes. 

either method cannot be used if the either component does not have a 

convenient parallel section.

Example: 

0 degree Probe Calibration 

40mm thick block – Gain to achieve FSH

Example: 

0 degree Probe Calibration 

30mm thick block – Gain to achieve FSH

TRANSFER & ATTENUATION CORRECTION: 

0 degree Probes 

If the results are plotted on 

log -linear paper they will 

form straight parallel lines 

provided that there is no 

attenuation difference if an 

attenuation difference 

occurs then the resultant 

lines will no longer be 

parallel.

Transfer and Attenuation Correction: Shear Probe 

The principle for obtaining transfer correction for shear wave probes is the 

same as it was for compression probes except that backwall echoes are 

replaced by pitch --catch responses.

4.2.8: Linearity Checks (Time Base / Equipment Gain / Vertical Gain)

4.2.8.1: Linearity of time base 

General 

This check may be carried out using a standard calibration block eg A2, 

and a compressional wave probe. The linearity should be checked over a 

range at least equal to that which is to be used in subsequent testing. 

Method 

a) Place the probe on the 25mm thickness of the A2 block and adjust the 

controls to display ten BWEs. 

b) Adjust the controls so that the first and last BWEs coincide with the scale 

marks at 1 and 10. 

c) Increase the gain to bring successive backwall echoes to 80% FSH. The 

leading edge of each echo should line up with the appropriate reticules 

line. 

d) Record any deviations at approximately half screen height. Deviations 

should be expressed as a percentage of the range between the first and 

last echoes displayed (ie 225mm).

Tolerance 

Unless otherwise specified by the testing standard, a tolerance of ±2% is 

considered acceptable. 

Frequency of checking 

This check shall be carried out at least once per week.

Ultrasonic Testing - Horizontal Linearity (Calibration) 

www.youtube.com/embed/NuS6j0SmjKQ

4.2.8.2: Linearity of Equipment Gains 

General 

This is a check on both the linearity of the amplifier within the set and the 

calibrated gain control. It can be carried out on any calibration block 

containing a side-drilled hole and should be the probe to be used in 

subsequent testing. Reject/suppression controls shall be switched off. 

Method 

• Position the probe on a calibration block to obtain a reflected signal from a 

small reflector eg 1.5mm hole in the A2 block. 

• Adjust the gain to set this signal to 80% FSH and note the gain setting (dB). 

- Increase the gain by 2dB and record the amplitude of the signal. 

- Remove the 2dB and return the signal to 80% FSH. 

- Reduce the gain by 6dB and record signal amplitude. 

- Reduce the gain by a further 12dB (18 intotal) and record signal amplitude. 

- Reduce the gain by a further 6dB (24 in total) and record signal amplitude.

Tolerance 

Frequency of checking 

The check shall be carried out at least once per week.

5.2.8.3-1: Linearity of vertical display to EN12668-1 

Procedure: Test the ultrasonic instrument screen linearity by altering the 

amplitude of a reference input using an external calibrated attenuator and 

observing the change in the signal height on the ultrasonic instrument 

screen. Report the gain setting at the beginning of the test. Check the 

linearity at prescribed intervals from 0 dB to - 26 dB of full screen height. 

Repeat the test for centre frequencies for of each filter as measured in 9.5.2. 

Using the same set-up shown in Figure 6 set the external calibrated attenuator 

to 2 dB and adjust the input signal and the gain of the ultrasonic instrument 

so the signal is 80 % of full screen height. Without changing the gain of the 

ultrasonic instrument switch the external calibrated attenuator to the values 

given in the Table 4. For each setting measure the amplitude of the signal on 

the ultrasonic instrument screen. 

Extract from: BS EN 12668-1:2010 Non-destructive testing- Characterization and verification of ultrasonic examination equipment 

Part 1: Instruments

Figure 6 — General purpose set-up for equipment

4.2.8.3-2: Linearity of vertical display to ASTM E317-01 

Vertical Limit and Linearity: 

Significance—Vertical limit and linearity have significance when echo signal 

amplitudes are to be determined from the display screen or corresponding 

output signals, and are to be used for evaluation of discontinuities or 

acceptance criteria. A specified minimum trace deflection and linearity limit 

may 

be required to achieve the desired amplitude accuracy. For other situations they 

may not be important, for example, go/no-go examinations with flaw alarms 

or evaluation by comparison with a reference level using calibrated gain 

controls. 

This practice describes both the two-signal ratio technique (Method A) and the 

input/output attenuator technique (Method B). 

Extract from: ASTM E317-01 Standard Practice for Evaluating Performance Characteristics of Ultrasonic Pulse-Echo Examination 

Instruments and Systems without the Use of Electronic Measurement Instruments 

Note: Method A: two-signal ratio technique collecting 2 signal from the 

reflectors of same size at different depth.

Method A: 

6.3.2.1 Apparatus—A test block is required that produces two non interfering 

signals having an amplitude ratio of 2 to 1. These are compared over the 

usable screen height as the instrument gain is changed. The two amplitudes 

will be referred to as HA and HB (HA > HB). The two signals may occur in 

either screen order and do not have to be successive if part of a multipleecho 

pattern. Unless otherwise specified in the requesting document, any 

test block that will produce such signals at the nominal test settings specified 

can be used. For many commonly used search units and test conditions, the 

test block shown in Fig. 1 will usually be satisfactory when the beam is 

directed along the H dimension toward the two holes. The method is 

applicable to either contact or immersion tests; however, if a choice exists, 

the latter may be preferable for ease of set-up and coupling 

stability……(more…)

4.2.9: Time Correction Gain (TCG) 

Please read: 

http://aqualified.com/tcg-dac-ndt-ultrasound/

Q61: The vertical linear range of a test instrument may be determined by 

obtaining ultrasonic responses from: 

A. a set of distance amplitude blocks 

B. steel ball located at several different water path distances 

C. a set of area amplitude blocks 

D. all of the above

Q29: Test sensitivity correction for a metal distance and discontinuity area 

responses are accomplished by using: 

A. An area amplitude set of blocks 

B. An area amplitude and a distance amplitude set of blocks 

C. A distance amplitude set of blocks 

D. Steel balls of varying diameters.


Curvature in the surface of a component will 

have an effect on the shape of the ultrasonic 

beam. The image to the right shows the beam 

from a focused immersion probe being 

projected on to the surface of a 

component. Lighter colors represent areas of 

greater beam intensity. It can be seen that 

concave surfaces work to focus the beam and 

convex surfaces work to defocus the 

beam. Similar effects are also seen with 

contact transducers. When using the 

amplitude of the ultrasonic signal to size flaws 

or for another purpose, it is necessary to 

correct for surface curvature when it is 

encountered. The "correction" value is the 

change in amplitude needed to bring signals 

from a curved surface measurement to the flat 

surface or DAC value.

Convex surfaces work to defocus the beam 

Diverge if the surface is convex. 

Concave surface contour- 

Focusing effects

Concave surfaces work to focus the beam 

Diverge if the surface is convex. 

Concave surface contour- 

Focusing effects

convex surfaces work to defocus the beam 

Convex surfaces work to defocus the beam

Convex surfaces work to defocus the beam- When sound travels from a 

liquid through a metal, it will converge if the surface is concave or diverge if 

the surface is convex.

Q: In an immersion method, the incident sound path enter the specimen 

interface with convex geometry, the sound path on entry into the specimen, 

the convex surface works to 

a) De-focus the sound 

b) Focus the sound 

c) Has no effect on the focusing or de-focusing the sound 

d) Reflected totally all the incident sound.

Q: In transmitting sound energy into a part shown below in a immersion 

testing, the sound beam will be: 

a) Diverge 

b) Converge 

c) Straight into 

d) Will not enter

A curvature correction curve can be generated experimentally in a manner 

similar to that used to generate a DAC curve, This simply requires a 

component with a representative reflector at various distances below the 

curved surface. Since any change in the radius will change the focus of the 

sound beam, it may be necessary to develop reference standards with a 

range of surface curvatures. 

However, computer modeling can also be used to generate a close 

approximation of the curvature correction value. Work by Ying and Baudry 

(ASME 62-WA175, 1962) and by Birchak and Serabian (Mat. Eval. 36(1), 

1978) derived methods for determining "correction factors" to account for 

change in signal amplitude as a function of the radius of curvature of convex, 

cylindrical components. 

An alternative model for contact and immersion probe inspection was more 

recently by researchers at the Center for NDE at Iowa State University. This 

mathematical model further predicts transducer radiation patterns using the 

Gauss-Hermite model, which has been used extensively for simulation of 

immersion mode inspections.

The resulting model allows computationally efficient prediction of the full 

ultrasonic fields in the component for 

1. any frequency, including broadband measurements. 

2. both circular and rectangular crystal shapes. 

3. general component surface curvature 

4. both normal and oblique incidence (e.g., angle beam wedges) transducers. 

When coupled with analytical models for defect scattering amplitudes, the 

model can be used to predict actual flaw waveforms. The image shown 

above was generated with this model.

The plot to the right shows an example curvature correction curve and DAC 

curve. This curvature correction curve was generated for the application of 

detecting a #4 flat bottom hole under a curved surface as shown in the 

sketch and photograph. An immersion techniques was used generate a 

shear wave since the reflective surface of the target flaw was not parallel with 

the surface. The DAC curve drops monotonically since the water path 

ensures that the near field of the sound beam is always outside the part. The 

correction factor starts out negative because of the focusing effect of the 

curved surface. At greater depths, the correction factor is positive due to the 

increased beam spread beyond the focal zone caused by the surface 

curvature.

Curvature Corrections

A table of correction values and the DAC and curvature correction curves for 

different size radiuses can be found at the following link. 

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/CalibrationMeth/table/table.htm

Curvature Correction

Curvature Correction


What are standards? 

Standards are documented agreements containing technical specifications or 

other precise criteria to be used consistently as rules, guidelines, or 

definitions of characteristics, in order to ensure that materials, products, 

processes, and services are fit for their purpose. 

For example, the format of the credit cards, phone cards, and "smart" cards 

that have become commonplace is derived from an ISO International 

Standard. Adhering to the standard, which defines such features as an 

optimal thickness (0.76 mm), means that the cards can be used worldwide.

An important source of practice codes, standards, and recommendations for 

NDT is given in the 

Annual Book of the American Society of Testing and Materials, 

ASTM. Volume 03.03, Nondestructive Testing 

is revised annually, covering acoustic emission, eddy current, leak testing, 

liquid penetrant, magnetic particle, radiography, thermography, and 

ultrasonics. 

There are many efforts on the part of the National Institute of Standards and 

Technology (NIST) and other standards organizations, both national and 

international, to work through technical issues and harmonize national and 

international standards.

Reference Reflectors: 

are used as a basis for establishing system performance and sensitivity.

Spherical reflectors are often used in immersion techniques for assessing 

sound fields. 

1. Omni direction 

2. Sphere directivity patterns reduce reflectance as compare with plane 

reflector 

3. Sphere of any materials could be used, however steel balls are often 

preferred.

Reference Reflectors are used as a basis for establishing system 

performance and sensitivity.


Exercises

Q80: The 50 mm diameter hole in an IIW block is used to: 

(a) Determine the beam index point 

(b) Check resolution 

(c) Calibrate angle beam distance 

(d) Check beam angle 

Q81: The 100 mm radius in an IIW block is used to: 

(a) Calibrate sensitivity level 



(d) Check beam angle

Q6: The Notches are frequently used for reference reflectors for: 

A. Distance amplitude calibration for shear wave 

B. Area amplitude calibration 

C. Thickness calibration of plate 

D. Determine of near surface resolution 

Q17: Notches provide good reference discontinuities when UT examination is 

conducted to primarily detect defects such as: 

A. Porosity in rolled plate 

B. Inadequate penetration at the root of weld 

C. Weld porosity 

D. Internal inclusion

4.6: Video Time 

http://v.pps.tv/play_315ARS.html

Birring NDT Series, UT of Welds Part 1 of 2 - CALIBRATION 

https://www.youtube.com/embed/SRJktrHUlM4

Birring NDT Series, Ultrasonic Testing # 4, Angle Beam Shear Wave UT as 

per AWS D1.1 

www.youtube.com/embed/vXcAI-Zci30

Section 5: Measurement Techniques

Content: Section 5: Measurement Techniques 


5.2: Angle Beams 








5.10: Scanning Patterns 


5.12: Interferences & Non Relevant Indications 


5.14: The Concept of Effective Distance 

5.15: Exercises

Expert at works


Pulse-echo ultrasonic measurements can determine the location of a 

discontinuity in a part or structure by accurately measuring the time required 

for a short ultrasonic pulse generated by a transducer to travel through a 

thickness of material, reflect from the back or the surface of a discontinuity, 

and be returned to the transducer. In most applications, this time interval is a 

few microseconds or less. The two-way transit time measured is divided by 

two to account for the down-and-back travel path and multiplied by the 

velocity of sound in the test material. The result is expressed in the wellknown 

relationship: 

where d is the distance from the surface to the discontinuity in the test piece, 

v is the velocity of sound waves in the material, and t is the measured 

round-trip transit time. 

d = vt/2 or v = 2d/t

d 1 = v½t 

d 2 = v½t 

= d 1 +d 2 

2vt 

2vt

A-Scan

A Scan 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/MeasurementTech/applet_4_1/applet_4_1.htm

Precision ultrasonic thickness gages usually operate at frequencies between 

500 kHz and 100 MHz, by means of piezoelectric transducers that generate 

bursts of sound waves when excited by electrical pulses. A wide variety of 

transducers with various acoustic characteristics have been developed to 

meet the needs of industrial applications. Typically, 

1. lower frequencies are used to optimize penetration when measuring thick, 

highly attenuating or highly scattering materials, 

2. while higher frequencies will be recommended to optimize resolution in 

thinner, non-attenuating, non-scattering materials. 

0.5 MHz ~ 100 MHz

In thickness gauging, ultrasonic techniques permit quick and reliable 

measurement of thickness without requiring access to both sides of a part. 

Accuracy's as high as ±1 micron or ±0.0001 inch can be achieved in some 

applications. It is possible to measure most engineering materials 

ultrasonically, including metals, plastic, ceramics, composites, epoxies, and 

glass as well as liquid levels and the thickness of certain biological specimens. 

On-line or in-process measurement of extruded plastics or rolled metal often 

is possible, as is measurements of single layers or coatings in multilayer 

materials. Modern handheld gages are simple to use and very reliable.

5.2: Angle Beams I 

Angle Beam Transducers and wedges are typically used to introduce a 

refracted shear wave into the test material. An angled sound path allows the 

sound beam to come in from the side, thereby improving detectability of flaws 

in and around welded areas. 

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, 

Surface distance = SinƟ x S, Depth= CosƟ x S

A-Scan 


Angle Beam Transducers and wedges are typically used to introduce a 

refracted shear wave into the test material. The geometry of the sample 

below allows the sound beam to be reflected from the back wall to improve 

detectability of flaws in and around welded areas. 

Ɵ = Angle of reflection, T=Material thickness, S= Sound path, 

Skip = 2(T x TanƟ), Leg = T/CosƟ, V Path = 2 x Leg

A-Scan 


Flaw Location and Echo Display






Dead Zone

Near Surface Detectability with Angle Beam Transducer

Flaw Location

Flaw Location with Angle Beam Transducer




Why angle beam assemblies are used 

Cracks or other discontinuities perpendicular to the surface of a test piece, or 

tilted with respect to that surface, are usually invisible with straight beam test 

techniques because of their orientation with respect to the sound beam. 

Perpendicular cracks do not reflect any significant amount of sound energy 

from a straight beam because the beam is looking at a thin edge that is much 

smaller than the wavelength, and tilted cracks may not reflect any energy 

back in the direction of the transducer. This situation can occur in many types 

of welds, in structural metal parts, and in many other critical components. An 

angle beam assembly directs sound energy into the test piece at a selected 

angle. A perpendicular crack will reflect angled sound energy along a path 

that is commonly referred to as a corner trap, as seen in the illustration below. 

http://www.olympus-ims.com/en/applications/angle-beam-transducers/

The angled sound beam is highly sensitive to cracks perpendicular to the far 

surface of the test piece (first leg test) or, after bouncing off the far side, to 

cracks perpendicular to the coupling surface (second leg test). A variety of 

specific beam angles and probe positions are used to accommodate different 

part geometries and flaw types. In the case of angled discontinuities, a 

properly selected angle beam assembly can direct sound at a favorable angle 

for reflection back to the transducer.

http://www.olympus-ims.com/en/applications/angle-beam-transducers/

How they work -- Snell's Law 

A sound beam that hits a surface at perpendicular incidence will reflect 

straight back. A sound beam that hits a surface at an angle will reflect forward 

at the same angle.

Sound energy that is transmitted from one material to another bends in 

accordance with Snell's Law of refraction. Refraction is the bending of a 

sound beam (or any other wave) when it passes through a boundary between 

two materials of different velocities. A beam that is traveling straight will 

continue in a straight direction, but a beam that strikes a boundary at an angle 

will be bent according to the formula: 

Typical angle beam assemblies make use of mode conversion and Snell's 

Law to generate a shear wave at a selected angle (most commonly 30, 45, 

60, or 70 degrees) in the test piece. As the angle of an incident 

longitudinal wave with respect to a surface increases, an increasing 

portion of the sound energy is converted to a shear wave in the second 

material, and if the angle is high enough, all of the energy in the second 

material will be in the form of shear waves.

There are two advantages to designing common angle beams to take 

advantage of this mode conversion phenomenon: 

(1) First, energy transfer is more efficient at the incident angles that 

generate shear waves in steel and similar materials. 

(2) Second, minimum flaw size resolution is improved through the use of 

shear waves, since at a given frequency, the wavelength of a shear 

wave is approximately 60% the wavelength of a comparable longitudinal 

wave, and minimum flaw size resolution increases as the wavelength of 

a sound beam gets smaller.

Selecting the right angle beam assembly 

The parameters that affect angle beam performance include not only the 

(1) beam angle generated by the wedge, but also (2) transducer frequency 

and (3) element size. The optimum beam angle will generally be governed 

by the geometry of the test piece and the orientation of the discontinuities 

that the test is intended to find. Transducer frequency affects penetration 

and flaw resolution: 

1. As frequency increases, the distance the sound wave will travel in a given 

material decreases, but resolution of small discontinuities improves. 

2. As frequency decreases, the distance the sound wave will travel increases 

but the minimum detectable flaw size will become larger. 

3. Similarly, larger element sizes may decrease inspection time by increasing 

coverage area, but the reflected echo amplitude from small discontinuities 

will decrease. Smaller element sizes will increase reflection amplitude from 

small discontinuities, but the inspection may take longer because the 

smaller beam covers less area. 

These conflicting factors must be balanced in any given application, based on 

specific test requirements.

Contoured wedges

The IIW recommends the use of a contoured wedge whenever the gap 

between the wedge and the test surface exceeds 0.5 mm (approximately 

0.020 in.). Under this guideline, a contoured wedge should be used whenever 

part radius is less than the square of a wedge dimension (length or width) 

divided by four: 

where 

R = radius of test surface 

W = width of wedge if testing in axial orientation, length of wedge if testing in 

circumferential orientation 

Of course switching to a small wedge, if possible within the parameters of 

inspection requirements, will improve coupling on curved surfaces. As a 

practical matter, contouring should be considered whenever signal strength 

diminishes or couplant noise increases to a point where the reliability of an 

inspection is impaired.

Focused dual element angle beams 

The vast majority of angle beam assemblies use single element, unfocused 

transducers. However, in some tests involving highly attenuating or scattering 

materials such as coarse grain cast stainless steel, focused dual element 

angle beams are useful. Because they have separate transmitting and 

receiving elements, dual element transducers can typically be driven at higher 

excitation energies without noise problems associated with ringdown or 

wedge noise. Focusing permits a higher concentration of sound energy at a 

selected depth within the test piece, increasing sensitivity to discontinuities in 

that region.

High temperature wedges 

Standard angle beam assemblies are designed for use at normal 

environmental temperatures only. For situations where metal must be 

inspeced at elevated temperature, special high temperature wedges are 

available. Some of these wedges will tolerate brief contact with surfaces as 

hot as 480° C or 900° F. However, it is important to note that high 

temperature wedges require special attention with regard to the sound path 

they generate. With any high temperature wedge, sound velocity in the wedge 

material will decrease as it heats up, and thus the refracted angle in metals 

will increase as the wedge heats up. If this is of concern in a given test, 

refracted angle should be verified at actual operating temperature. As a 

practical matter, thermal variations during testing will often make precise 

determination of the actual refracted angle difficult. 

Surfaces as hot as 480°C / 900°F

threaded 

snap-in 

steel with a shear wave velocity of approximately 3,250 M/S or 0.1280 in/uS.


There are many sizing methods, these include: 

5.3.1 Crack Tip Diffraction 

When the geometry of the part is relatively uncomplicated and the orientation 

of a flaw is well known, the length (a) of a crack can be determined by a 

technique known as tip diffraction. One common application of the tip 

diffraction technique is to determine the length of a crack originating from on 

the backside of a flat plate as shown below. In this case, when an angle beam 

transducer is scanned over the area of the flaw, the principle echo comes 

from the base of the crack to locate the position of the flaw (Image 1). A 

second, much weaker echo comes from the tip of the crack and since the 

distance traveled by the ultrasound is less, the second signal appears earlier 

in time on the scope (Image 2).

Crack Tip Diffraction Methods 

No animation.

Crack height (a) is a function of the ultrasound velocity (v) in the material, the 

incident angle (Q2) and the difference in arrival times between the two signal 

(dt). Since the incident angle and the thickness of the material is the same in 

both measurements, two similar right triangle are formed such that one can 

be overlayed on the other. A third similar right triangle is made, which is 

comprised on the crack, the length dt and the angle Q2. The variable dt is 

really the difference in time but can easily be converted to a distance by 

dividing the time in half (to get the one-way travel time) and multiplying this 

value by the velocity of the sound in the material. Using trigonometry an 

equation for estimating crack height from these variables can be derived as 

shown below.

Crack Tip Diffraction Method 

The equation is complete once 

distance dt is calculated by dividing 

the difference in time between the 

two signals (dt) by two and 

multiplying this value by the sound 

velocity.

5.3.2 6 dB Drop Sizing- 

For Large Reflector (greater than beam width), i.e. there is no BWE.

6 dB Drop Method

6 dB Drop Method

6 dB Drop Method 

www.youtube.com/embed/hsR17WA3nHg

6 dB Drop Method

5.3.3 The 20 dB drop sizing method 

We can use a beam plot to find the edge of a defect by using the edge of 

the sound beam. 

If we know the width of a beam at a certain distance from the crystal, we 

can mark the distance across a defect from where the extreme edges of 

the beam touch each end of the defect and then subtract the beam width to 

get the defect size. 

When the signal from the defect drops by 20dB from its peak, we judge 

that the edge of the beam is just touching the end of the defect. We can 

find the width of the sound beam at that range by consulting the beam plot 

that we have made 

Note: The peak of the defect is normally taken as being the last peak on 

the screen before the probe goes off the end of the defect, not necessarily 

the maximum signal from a defect.

20 dB Drop Method

20 dB Drop Sizing- For Small Reflector (smaller than beam width). 

To use this method the transducer beam width need to be first determined.

Construction of a beam edge plot -20dB – Normal Beam 

Find the hole at a depth of 13mm on an IOW block with a 0 degree probe and 

maximise the signal. Move the probe until you get the highest signal you 

can from the hole, then turn the signal to FSH using gain. Mark the position 

of the middle of the probe on the side of the block. 

Move the probe to one side until the signal drops to 10%FSH (-20dB) and 

mark the centre of the probe on the side of the block.

Move the probe to the other side of the hole until the signal drops to 

10%FSH (-20dB) and mark the centre of the probe on the block. 

Use the distances between the marks on the block to plot the beam on a 

piece of graph paper. Measure 13mm depth on the paper then mark the 

distances of the probe centre at -20dB from the beam centre at 100%FSH 

on either side.

Now find the 25mm hole and maximise the signal, turning it to 100%FSH. 

Move the probe to either side of the hole marking the centre of the probe 

on the side of the block where the signal drops by 20dB. 

Measure 25mm on the paper and use the distances on the block to plot the 

beam dimensions at 25mm. 

Repeat using the 32mm hole. Join up the points marking the probe centre 

at 20dB to obtain a beam plot.

Note that we have only drawn the beam width in one plane, so the probe 

must be marked accordingly and used to measure defects in this plane. 

We use knowledge of the beam spread to size defects, find the edges and 

hence their width, length and sometimes orientation.

Construction of a beam edge plot -20dB – Angle Beam

5.3.4 Equalization Back Wall Sizing- The probe moving off the edges of 

the reflector until the amplitude is equal to the rising BWE

5.3.5 Maximum Amplitude Techniques 

The technique is used for small reflector. The probe moving off the edges of 

the reflector until the amplitude is maximum and the line joining the boundary 

is the size of reflector cluster.

5.3.6 The DGS Method 

Distance Gain Size Method. The technique is used to find the equivalent 

reflector size by comparing the gain between the flaw and the known size 

reflector.


Ultrasonic scanning systems are used for automated data acquisition and 

imaging. They typically integrate a ultrasonic instrumentation, a scanning 

bridge, and computer controls. The signal strength and/or the time-of-flight of 

the signal is measured for every point in the scan plan. The value of the data 

is plotted using colors or shades of gray to produce detailed images of the 

surface or internal features of a component. Systems are usually capable of 

displaying the data in A-, B- and C-scan modes simultaneously. With any 

ultrasonic scanning system there are two factors to consider: 

■ 

■ 

how to generate and receive the ultrasound. 

how to scan the transducer(s) with respect to the part being inspected.

Automatic Scanning

The most common ultrasonic scanning systems involve the use of an 

immersion tank as shown in the image above. The ultrasonic transducer and 

the part are placed under water so that consistent coupling is maintained by 

the water path as the transducer or part is moved within the tank. However, 

scanning systems come in a large variety of configurations to meet specific 

inspection needs. In the image to the right, an engineer aligns the heads of a 

squirter system that uses a through-transmission technique to inspect aircraft 

composite structures. In this system, the ultrasound travels through columns 

of forced water which are scanned about the part with a robotic system. A 

variation of the squirter system is the "Dripless Bubbler" scanning system, 

which is discussed below.

Dripless Bubbler

It is often desirable to eliminate the need for the water coupling and a number 

of state-of-the-art UT scanning systems have done this. Laser ultrasonic 

systems use laser beams to generate the ultrasound and collect the resulting 

signals in an noncontact mode. Advances in transducer technology has lead 

to the development of an inspection technique known as air-coupled 

ultrasonic inspection. These systems are capable of sending ultrasonic 

energy through air and getting enough energy into the part to have a useable 

signal. These system typically use a through-transmission technique since 

reflected energy from discontinuities are too weak to detect.

The second major consideration is how to scan the transducer(s) with respect 

to the part being inspected. When the sample being inspected has a flat 

surface, a simple raster-scan can be performed. If the sample is cylindrical, a 

turntable can be used to turn the sample while the transducer is held 

stationary or scanned in the axial direction of the cylinder. When the sample 

is irregular shaped, scanning becomes more difficult. As illustrated in the 

beam modeling animation, curved surface can steer, focus and defocus the 

ultrasonic beam. For inspection applications involving parts having complex 

curvatures, scanning systems capable of performing contour following are 

usually necessary. 

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/AppleScan/Apple2.swf


Changes in ultrasonic wave propagation speed, along with energy losses, 

from interactions with a materials microstructures are often used to 

nondestructively gain information about a material's properties. 

Measurements of sound velocity and ultrasonic wave attenuation can be 

related to the elastic properties that can be used to characterize the texture of 

polycrystalline metals. These measurements enable industry to replace 

destructive microscopic inspections with nondestructive methods. 

Of interest in velocity measurements are longitudinal wave, which propagate 

in gases, liquids, and solids. In solids, also of interest are transverse (shear) 

waves. The longitudinal velocity is independent of sample geometry when the 

dimensions at right angles to the beam are large compared to the beam area 

and wavelength. The transverse velocity is affected little by the physical 

dimensions of the sample.

Pulse-Echo and Pulse-Echo-Overlap Methods 

Rough ultrasonic velocity measurements are as simple as measuring the time 

it takes for a pulse of ultrasound to travel from one transducer to another 

(pitch-catch) or return to the same transducer (pulse-echo). Another method 

is to compare the phase of the detected sound wave with a reference signal: 

slight changes in the transducer separation are seen as slight phase changes, 

from which the sound velocity can be calculated. These methods are suitable 

for estimating acoustic velocity to about 1 part in 100. Standard practice for 

measuring velocity in materials is detailed in ASTM E494. 

ASTM E494 - 10 

Measuring Ultrasonic Velocity in Materials 

Active Standard ASTM E494 | Developed by Subcommittee: E07.06 

Book of Standards Volume: 03.03

Precision Velocity Measurements (using EMATs) 

Electromagnetic-acoustic transducers (EMAT) generate ultrasound in the 

material being investigated. When a wire or coil is placed near to the surface 

of an electrically conducting object and is driven by a current at the desired 

ultrasonic frequency, eddy currents will be induced in a near surface region. If 

a static magnetic field is also present, these currents will experience Lorentz 

forces of the form 

F = J x B 

where F is a body force per unit volume, J is the induced dynamic current 

density, and B is the static magnetic induction.

EMATs 

http://www.resonic.com/error%20scan.swf 

http://www.resonic.com/scan2.swf 

http://www.resonic.com/emar_how_it_works.html

The most important application of EMATs has been in nondestructive 

evaluation (NDE) applications such as flaw detection or material property 

characterization. Couplant free transduction allows operation without contact 

at elevated temperatures and in remote locations. The coil and magnet 

structure can also be designed to excite complex wave patterns and 

polarizations that would be difficult to realize with fluid coupled piezoelectric 

probes. In the inference of material properties from precise velocity or 

attenuation measurements, use of EMATs can eliminate errors associated 

with couplant variation, particularly in contact measurements. 

Differential velocity is measured using a T1-T2---R fixed array of EMAT 

transducer at 0, 45°, 90° or 0°, 90° relative rotational directions depending on 

device configuration:

EMAT Driver Frequency: 450-600 KHz (nominal) 

Sampling Period: 100 ns 

Time Measurement Accuracy: 

-- Resolution 0.1 ns 

-- Accuracy required for less than 2 KSI Stress Measurements: 

Variance 2.47 ns 

-- Accuracy required for texture: Variance 10.0 Ns 

------ W440 < 3.72E-5 

------ W420 < 1.47E-4 

------ W400 < 2.38E-4

Time Measurement Technique 

Fourier Transform-Phase-Slope determination of delta time between received 

RF bursts (T2-R) - (T1-R), where T2 and T1 EMATs are driven in series to 

eliminate differential phase shift due to probe liftoff. 

Slope of the phase is determined by linear regression of weighted data points 

within the signal bandwidth and a weighted y-intercept. The accuracy obtained 

with this method can exceed one part in one hundred thousand (1:100,000).


Ultrasonic wave propagation is influenced by the microstructure of the 

material through which it propagates. The velocity of the ultrasonic waves is 

influenced by the elastic moduli and the density of the material, which in turn 

are mainly governed by the amount of various phases present and the 

damage in the material. Ultrasonic attenuation, which is the sum of the 

(1)absorption and the (2)scattering, is mainly dependent upon the damping 

capacity and scattering from the grain boundary in the material. However, to 

fully characterize the attenuation required knowledge of a large number of 

thermo-physical parameters that in practice are hard to quantify. 

A o 

U t 

A

Relative measurements such as the change of attenuation and simple 

qualitative tests are easier to make than absolute measure. Relative 

attenuation measurements can be made by examining the exponential decay 

of multiple back surface reflections. However, significant variations in 

microstructural characteristics and mechanical properties often produce only 

a relatively small change in wave velocity and attenuation. Absolute 

measurements of attenuation are very difficult to obtain because the echo 

amplitude depends on factors in addition to amplitude.

The most common method used to get quantitative results is to use an 

ultrasonic source and detector transducer separated by a known distance. 

By varying the separation distance, the attenuation can be measured from the 

changes in the amplitude. To get accurate results, the influence of coupling 

conditions must be carefully addressed. To overcome the problems related to 

conventional ultrasonic attenuation measurements, ultrasonic spectral 

parameters for frequency-dependent attenuation measurements, which are 

independent from coupling conditions are also used. For example, the ratio of 

the amplitudes of higher frequency peak to the lower frequency peak, has 

been used for microstructural characterization of some materials.

Attenuation: 

A o 

U t 

A

Attenuation:


Spread spectrum ultrasonics makes use of the correlation of continuous 

signals rather than pulse-echo or pitch-catch techniques. 

Spread spectrum ultrasonics is a patented new broad band spread-spectrum 

ultrasonic nondestructive evaluation method. In conventional ultrasonics, a 

pulse or tone burst is transmitted, then received echoes or throughtransmission 

signals are received and analyzed. 

In spread spectrum ultrasonics, encoded sound is continuously transmitted 

into the part or structure being tested. Instead of receiving echoes, spread 

spectrum ultrasonics generates an acoustic correlation signature having a 

one-to-one correspondence with the acoustic state of the part or structure (in 

its environment) at the instant of the measurement. In its simplest 

embodiment, the acoustic correlation signature is generated by cross 

correlating an encoding sequence, with suitable cross and auto correlation 

properties, transmitted into a part (structure) with received signals returning 

from the part (structure).

Section of bi-phase modulated spread spectrum ultrasonic waveform 

Multiple probes may be used to ensure that acoustic energy is propagated 

through all critical volumes of the structure. Triangulation may be incorporated 

with multiple probes to locate regions of detected distress. Spread spectrum 

ultrasonics can achieve very high sensitivity to acoustic propagation changes 

with a low level of energy.

Two significant applications of Spread Spectrum Ultrasonics are: 

1. Large Structures that allow ultrasonic transducers to be "permanently" 

affixed to the structures, eliminating variations in transducer registration 

and couplant. Comparisons with subsequent acoustic correlation 

signatures can be used to monitor critical structures such as fracture 

critical bridge girders. In environments where structures experience a 

great many variables such as temperature, load, vibration, or 

environmental coupling, it is necessary to filter out these effects to obtain 

the correct measurements of defects. 

In the example below, simulated defects were created by setting a couple of 

steel blocks on the top of the bridge girder.

Spread Spectrum UT

2. Piece-part assembly line environments where transducers and couplant 

may be precisely controlled, eliminating significant variations in transducer 

registration and couplant. Acoustic correlation signatures may be statistically 

compared to an ensemble of known "good" parts for sorting or 

accepting/rejecting criteria in a piece-part assembly line environment. 

Impurities in the incoming steel used to forge piece parts may result in sulfite 

stringer inclusions. In this next example simulated defects were created by 

placing a magnetized steel wire on the surface of a small steel cylindrical 

piston used in hydraulic transmissions.

Two discrimination technique are tested here, which are SUF-1 and SUF-2, 

with the latter giving the best discrimination between defect conditions. The 

important point being that spread spectrum ultrasonics can be extremely 

sensitive to the acoustic state of a part or structure being tested, and 

therefore, is a good ultrasonic candidate for testing and monitoring, especially 

where scanning is economic unfeasible.

EMATs with Spread Spectrum Ultrasonic 

http://www.resonic.com/error%20scan.swf 

http://www.resonic.com/scan2.swf 

http://www.resonic.com/emar_how_it_works.html


Signal processing involves techniques that improve our understanding of 

information contained in received ultrasonic data. Normally, when a signal is 

measured with an oscilloscope, it is viewed in the time domain (vertical axis is 

amplitude or voltage and the horizontal axis is time). For many signals, this is 

the most logical and intuitive 直观的 way to view them. Simple signal 

processing often involves the use of gates to isolate the signal of interest or 

frequency filters to smooth or reject unwanted frequencies. 

When the frequency content of the signal is of interest, it makes sense to view 

the signal graph in the frequency domain. In the frequency domain, the 

vertical axis is still voltage but the horizontal axis is frequency.

Display 

Time/Magnitude 

domain 

Frequency 

/Magnitude domain

The frequency domain display shows how much of the signal's energy is 

present as a function of frequency. For a simple signal such as a sine wave, 

the frequency domain representation does not usually show us much 

additional information. However, with more complex signals, such as the 

response of a broad bandwidth transducer, the frequency domain gives a 

more useful view of the signal. 

Fourier theory says that any complex periodic waveform can be decomposed 

into a set of sinusoids with different amplitudes, frequencies and phases. The 

process of doing this is called Fourier Analysis, and the result is a set of 

amplitudes, phases, and frequencies for each of the sinusoids that makes up 

the complex waveform. Adding these sinusoids together again will reproduce 

exactly the original waveform. A plot of the frequency or phase of a sinusoid 

against amplitude is called a spectrum.

Fourier Analysis

Fourier Analysis

Fourier Analysis

The following Fourier Java applet, adapted with permission of Stanford 

University, allows the user to manipulate discrete time domain or frequency 

domain components and see the relationships between signals in time and 

frequency domains. 

The top row (light blue color) represents the real and imaginary parts of the 

time domain. Normally the imaginary part of the time domain signal is 

identically zero. 

The middle row (peach color) represents the the real and imaginary parts of 

the frequency domain. 

The bottom row (light green color) represents the magnitude (amplitude) and 

phase of the frequency domain signal. Magnitude is the square root of the 

sum of the squares of the real and imaginary components. Phase is the 

angular relationship of the real and imaginary components. Ultrasonic 

transducer manufactures often provide plots of both time domain and 

frequency domain (magnitude) signals characteristic of each transducer. Use 

this applet to explore the relationship between time and frequency domains.

Fourier Analysis


Direct contact, 

single element probe 

Direct contact, 

dual element probe 

Fixed delay 

Through transmission 

Immersion testing

5.9.1 Pulse Echo Method

Pulse Echo Method: Sound pressure on axis (schematic) for the incident 

wave (top) and the wave reflected from a reflector in form a circular disc 

(bottom).

Pulse Echo Method

Pulse Echo Method

Pulse Echo Method- Schematic screen pictures obtained by the pulse-echo 

method. a Small flaw in sound beam; b two small flaws in sound beam; c 

large flaw in sound beam, smaller second flaw and back wall masked; d large, 

obliquely orientated flaw, back wall masked; e small flaw but no back wall 

echo because the axis of the beam is not incident at right angles on back wall; 

f strong attenuation of sound beam due to scattering, no echo from flaw or 

back wall, only "grass"

Pulse Echo Method

Pulse Echo Method- Multiple echoes in a plate. a schematic; b actual screen 

picture without time or thickness scale; steel plate 50 mm thick, frequency 4 

MHz

Amplitude loss: Inverse Square Law

Influence of Shadow on axial defects

Influence of reflector orientation on signal

Influence of reflector size on signal

Pulse Echo Method 

IP 

BE 

F 

plate 

delamination 

0 2 4 6 8 10 

IP = Initial pulse 

F = Flaw 

BE = Backwall echo

Pulse Echo Method 

s 

Probe 

Sound travel path 

Flaw 

Work piece

5.9.2 Pitch-Catch Methods 

Advantage: 

Sensitive to near surface defect 

Capable of penetrating thicker material due to pitch-catch mode. 

Disadvantage: 

It measures only sound energy loss at the receiver, without giving details 

information of location.

5.9.2.1 Pitch-Catch Methods- Through Transmission 

Through transmission testing uses two search units; one unit is used as a 

transmitter and the other unit is used as a receiver, as shown in Figure below. 

With this technique, the ultrasonic beam passes through the test piece or is 

attenuated by one or more discontinuities. Total or partial attenuation of the 

signal is possible depending on the severity of the discontinuity. Both 

transducers must be properly coupled with a liquid coupling agent to obtain 

reliable results. As with other techniques using two search units, greater 

efficiency may be obtained by using a ceramic element in the transmitting 

search unit and a lithium sulfate element in the receiving unit.

Pitch-Catch Methods- Through Transmission

Pitch-Catch Methods- Through Transmission 

Through transmission signal 

1 

T 

R 

1 

2 

T 

R 

2 

0 2 4 6 8 10 

Flaw 

Back wall Echo

5.9.2.2 Pitch-Catch Methods- Tandem 

The tandem method, the examination is normally carried out using two similar 

45° angle probes, one probe operating as the transmitter and the other probe 

as receiver. For wall thicknesses greater than approximately 160 mm, probes 

with different transducer sizes are preferred in order to ensure approximately 

the same beam diameters in the examination zone. 

The use of probe angles other than 45° may be necessary to comply with 

particular geometrical conditions. Probe angles that give rise to mode 

conversions shall be avoided. The probes are located in a line with their 

acoustic axis in the same direction. In this way the sound beam from the rear 

probe will, after reflection from the opposite surface, intersect the sound beam 

from the front probe at the centre of the examination zone. 

Extract from: EN 583-4 Non-destructive testing - Ultrasonic examination - Part 4: Examination for discontinuities 

perpendicular to the surface

Figure 1 shows the relationship between the spacing of the probes (y) and the 

examination depth of the cross point (t m ) and the height of the examination 

zone (t z ). When examining objects with plane parallel surfaces the distance 

between the probes can be defined using the following equation: 

y = 2 tan α (d – t m ) or 2 tan α (bottom depth)

Distance Between Transmitter / Receiver Probes 

y 

Depth t m 

α 

x 

α 

Plate Thickness d 

L 

tan α = L / (d + d - t m ) , L = 2d- t m tan α , 

tan α = x / t m , x = t m tan α 

y = L - x = (2d - t m tan α) – (t m tan α) 

y = tan α (2d-t m -t m ) = 2 tan α (d - t m )

Video on Through Transmission Methods 

www.youtube.com/embed/bRgCLb2cDU4?list=UUSOUDD4-FPV4tzqvUnquwXQ

5.9.3 Immersion Methods 

Many of the same techniques used in contact testing can be used in 

immersion testing. One advantage of immersion testing is that water makes a 

very effective coupling agent. A wetting agent is often used with the water to 

reduce surface tension and minimize air bubble formation on probes and test 

parts. 

The main advantages of immersion testing are: 

■ 

■ 

■ 

■ 

speed of inspection, 

immersion medium provides excellent coupling, 

ability to direct the sound at any desired angle, and 

the ease of incorporating automatic scanning techniques. 

With immersion testing, the time to send the beam through the water is 

usually greater than the time to send the beam through the test piece. All 

immersion search units are basically straight beam units that are directed to 

produce either longitudinal or shear waves in the test material.

Immersion Methods 

For immersion testing of steel and aluminum in water, the water path shall be 

at least 1” for every 4” thickness of the specimen (or ¼ of specimen thickness 

minimum). If the transducer is too close, the 2 nd front reflection will appeared 

between the 1 st front reflection and the 1 st backwall echo and this may be 

wrong interpreted as discontinuity.

Immersion Methods- Since sound waves travel about four times faster in 

steel and aluminum than they do in water, a general rule of thumb is that the 

water distance should be 1/4 T s the part thickness plus 1/4 in (6mm). When 

immersion testing is used for tapered plates, there should be a uniform water 

path above the test surface. With immersion testing, false indications from 

contoured surfaces will result in broad-based noise echoes. 

T s Minimum + [¼” 6mm (?)]

Immersion Methods- The water path shall be ¼ of specimen thickness 

minimum. (plus 6mm) 

Minimum + [¼ “(?)]

Modified Immersion Methods- Bubbler Chamber

Modified Immersion Methods – Irrigation Dam

Angle Beam Immersion Methods 

Note the small front surface reflection. This due to the inclined incident angle 

reflected away from the transducer.

Straight Beam Immersion Methods 

1 2 

surface = sound entry 

backwall 

water delay 

flaw 

IP 

IE 

IP 

1 2 

IE 

BE 

F 

BE 

0 2 4 6 8 10 0 2 4 6 8 10

Angle Beam Immersion Methods- Pipe & Tubing Testing 

.

Angle Beam Immersion Methods- Weld Testing

Immersion Testing Set-up

Immersion Testing Set-up

Manipulators 

The manipulator is primarily intended to provide a means of scanning the test 

specimen with an immersed transducer. 

• The manipulator is mounted on a traversing mechanism, which allows 

movement of the manipulator from side to side. 

• The traversing mechanism is an integral component of the bridge 

assembly. 

A search tube is usually held rigid at right angles to the surface of the test 

specimen. Locking knobs are provided on the manipulator to allow positioning 

of the search tube in two planes for angle-beam testing.

Manipulators 

Bridge 

Bridge 

Manipulator

Bridges 

When the manipulator is automated, electric motors are added to power the 

bridge carriage, the traversing mechanism, and the up and down movement 

of the search tube. The pulse-echo unit and the recording unit are also 

mounted on the bridge, with all power cords secured overhead to allow 

movement of the bridge along the full length of the tank.

Wands / Support Tubes 

The support tube for the immersion probe is sometimes called a wand. Its 

vertical height can be adjusted to vary water path distance and the adjuster 

which can manipulate probe angle of incidence at the tip of the wand.

Immersion Testing Set-up 

Manipulator 

Bridge 

Wand / Tube


Manipulator 

Bridge 

Wand / Tube


Manipulator 

Bridge 

Wand / Tube

Other Reading (Olympus)- Angle Beam Immersion Methods 

Immersion transducers offer three major advantages over contact transducers: 

1. Uniform coupling reduces sensitivity variations. 

2. Reduction in scan time due to automated scanning. 

3. Focusing of immersion transducers increases sensitivity to small reflectors. 

Focusing Configurations 

Immersion transducers are available in three different configurations: 

• unfocused (“flat”), 

• spherically (“spot”) focused, and 

• cylindrically (“line”) focused. 

Focusing is accomplished by either the addition of a lens or by 

curving the element itself. The addition of a lens is the most 

common way to focus a transducer.

An unfocused transducer may be used in general applications or for 

penetration of thick materials. A spherically focused transducer is commonly 

used to improve sensitivity to small flaws and a cylindrical focus is typically 

used in the inspection of tubing or bar stock. Examples of spherical and 

cylindrical focusing are shown in Figure (17) below. 

Cylindrical 

Spherical

Unfocused transducer 

By definition, the focal length of a transducer is the distance from the face 

of the transducer to the point in the sound field where the signal with the 

maximum amplitude is located. In an unfocused transducer, this occurs at a 

distance from the face of the transducer which is approximately equivalent 

to the transducer’s near field length. Because the last signal maximum occurs 

at a distance equivalent to the near field, a transducer, by definition, can not 

be acoustically focused at a distance greater than its near field.

Focus may be designated in three ways: 

FPF (Flat Plate Focus) - For an FPF focus, the lens is designed to produce 

a maximum pulse/echo response from a flat plate target at the distance 

indicated by the focal length 

PTF (Point Target Focus) - For a PTF focus, the lens is designed to produce 

a maximum pulse/echo response from a small ball target at the distance 

indicated by the focal length 

OLF (Optical Limit Focus) - The OLF designation indicates that the lens is 

designed according to the lens maker’s formula from physical 

optics and without reference to any operational definition of 

focal length. The OLF designation describes the lens and 

ignores diffraction effects.

Video on Immersion Testing 

www.youtube.com/embed/W07-Z9at=UUSOUDD4-FPV4tzqvUnquwXQ

Q: In immersion testing, to remove the second water reflection (2nd entry 

surface signal) from between the entry surface signal and the first back 

reflection, you should: 

A. Increase repetition rate 

B. Decrease frequency 

C. Decrease sweep length 

D. Increase water depth 

Q110: In addition to other functions, a transducer manipulator in a mechanical 

immersion-scanning unit permits: 

A. Use of the through transmission techniques 

B. Use of high scanning speed 

C. Detection of obliquely oriented discontinuities 

D. Utilization of skill operators

Q1: Which of the following scanning methods could be classified as an 

immersion type test? 

A. Tank in which the transducer and test piece are immersed 

B. Squirter bubbler method in which the sound is transmitted in a column of 

flowing water 

C. Scanning with a wheel-type transducer with the transducer inside a liquid 

filled tire 

D. All of the above

Q2: In an immersion test of a piece of steel or aluminum, the water distance 

appears on the display as a fairly wide space between the initial pulse and 

the front surface reflection because of: 

A. Reduced velocity of sound in water as compared to test specimen 

B. Increased velocity of sound in water as compared to test specimen 

C. Temperature of the water 

D. All of the above

Q2: Using the immersion method, a distance amplitude curve (DAC) for a 19 

mm diameter, 5 MHz transducer shows the high point of the DAC at the 

B/51 mm block. One day later, the high point of the DAC for the same 

transducer is at the J/102 mm block. Assuming the calibration has not 

change, this would indicate that the transducer: 

A. Is improving in resolution 

B. Is becoming defective 

C. Has the beam of smaller transducer 

D. Both A & B 

Hint: B leads to C, thus D is the standard answer. 

http://www.ndt-instrument.com/UltrasonicThicknessGauge.asp?sort=Ultrasonic+Flaw+Detector

Q176: To evaluate and accurately locate discontinuities after scanning a part 

with a paintbrush transducer, it is generally necessary to use a: 


B. Scrubber 

C. Grid map 

D. Crystal collimator

38. The component in a conventional immersion system which spans the 

width of the immersion tank is called: 

A. An articulator. 

B. A bridge. 

C. A manipulator. 

D. A search tube.

5.10: Scanning Patterns

Scanning Patterns


The energy of the generated sound depend on the pulse repetition rate, the 

higher the repetition rate the higher the energy and the sound able to 

penetrate thicker material. However if the PRR is excessive, ghost signal may 

formed, this is due to the fact that the next sequence of pulse is generated 

before the expected returning signal reaching the receiver. 

1. The pulse repetition frequency or pulse repetition rate PRR: 

is the number of pulse of ultrasonic energy that leave the probe in a given 

time (per second). Each pulse of energy that leave the probe must return 

before the next pulse leave, otherwise they will collide causing ghost 

echoes. 

2. Transit time: The time taken for the pulse to travel from the probe and 

return 

3. Clock interval: The time between pulse leaving the probe. 

The transit time must be shorter than the Clock interval else, ghost signal may 

formed. Typically the Clock interval should be 5 time the transit time.

PRR- Pulse Repetitive Frequency/Rate and Maximum Testable Thickness 

Clock interval = 1/PRR 

When Transit time = Clock interval 

For pulse echo method: 

Maximum testable length = ½ x Velocity x Clock interval 

Typically the Clock interval should be 5 time the transit time, i.e. the sound 

path should travel 5 times the maximum testable length. (1st BWE, 2nd BWE, 

3rd BWE, 4th BWE to 5th BWE.) 

Note: The Clock interval has neglected the time occupied by each pulse.

Pulse Repetition Rate and Penetration

Pulse Repetition Rate and Penetration

Pulse-Length and Near Surface Sensitivity

Q186: The maximum scanning speed possible is primarily determined by: 

A. The frequency of transducer 

B. Viscous drag problem 

C. The pulse repetition rate of test instrument 

D. The persistency of the ultrasonic instrument display 

Q200: When setting up an ultrasonic inspection, the repetition frequency for 

the ultrasonic instrument should be set: 

A. So that its period is at least as long as the operating time 

B. The same as the transducer resonance frequency 

C. As low as possible to avoid over-pulsing and distortion 

D. According to the instrument manual 

E. None of the above

5.12: Interferences & Non-Relevant Indications 

Following are signal interferences that may produce non-relevant UT 

indications: 

1. Electrical interference 

2. Transducer interference 

3. Test specimen geometric interference 

4. Test specimen surface interferences 

5. Test material structure interferences 

6. Test material internal mode conversion interference 

7. UT techniques induced interferences (In correct PRR/ Band width/ 

Frequency selection/ Excessive Beam Spread/ etc.)

Transducer Interference- Transducer internal reflections & Mode conversion 

may cause interference

Specimen Surface Interference 

Excessive surface roughness, 

air bubbles on the surface (on the transducer front, specimen front and back 

for immersion techniques. 

Surface wave for testing near the edges

Specimen Surface Interference 

? 

?

Specimen Surface Interference- You can determined whether the signal is 

from the surface wave or the refracted wave simply by touching the surface 

ahead of the wave (assuming the velocity of surface wave at 0.9 of the shear 

wave)

Mode Conversion Interference 

The mode conversion interference during testing of long cylindrical specimen 

with longitudinal wave often appeared after the first back wall echo. The 

signal can be easily distinguished and ignore.

Material Geometric Interference 

False signals may generated due to the test specimen structural 

configurations resulting in spurious signals.

Non Relevant Indications 

Transducer with Excessive Beam Spread may generate signal, usually after 

the 1 st BWE. The example below the convex surface defocused the beam 

and lead to excessive beam spread, using a proper contoured probe may 

eliminate the problem. However excessive contour may results in generation 

of surface wave.

Non Relevant Indication 

Large grain size especially casting may cause excessive hash or grass signal. 

Properly selecting probe with lower frequency may relieve the problem. 

However this can only de accomplished with reduction in sensitivity.

Non Relevant Indication 

Large grain size at heat affected zone HAZ (CGHAZ) may cause localized 

signal due to large grain size. The signal may be wrongly assessed as a 

defect.

Non Relevant Indications 

The geometric abnormalities at root penetration and weld surface (crown) 

may reflect the sound path, returning to the receiver as signals. To 

distinguished the non relevant indications, finger touching will damped the 

signals. Further testing may be necessary to ensure the signals were not from 

the surface defects like surface crack. Any near surface indication that are 

unusually consistent could be a non relevant indication.


Entry surface variables include: 

1. surface roughness 

2. surface coatings 

3. couplant condition. 

5.13.1 Surface Roughness 

Surface roughness will have several possible effects on the inspection of a 

test piece. In contact testing roughness on a gross scale results from: weld 

spatter, plate scale, dirt (sand) and rough cast surfaces from sand casting. 

These irregularities will cause some points of contact to push away the 

couplant and force it into the lower areas around the probe. If the couplant is 

not sufficiently viscous it will drain away quickly and fail to couple the probe to 

the test piece. See Figure 8-3. 

http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm

Entry surface variables: Surface roughness 

Air Gap 

Low Viscosity 

Couplant 

High Points of Rough Surface

In addition to reduced coupling, which will reduce signal amplitudes, the 

rough surface increases the rate of wear on the probe. On an otherwise 

smooth surface isolated protrusions such as weld spatter can hinder or stop 

probe motion or in the case of mechanized systems there may be sufficient 

force to move the probe past the obstruction but this could result in damaging 

the probe by either tearing it from its mounting or severely scoring the plastic 

wedge. When the dirt on the test piece is very fine (similar to a flour texture) 

coupling can be prevented due to surface tension preventing the liquid 

couplant penetrating to the metal. Unless a transfer value has been 

established between test piece and calibration piece, this could go 

undetected. 

In addition to affecting coupling, surface roughness tends to reduce signal 

amplitude by scattering and focusing the beam. This applies to both contact 

and immersion testing.

Whether uniform or irregular, a rough surface has the potential to present a 

scattering effect at an interface where a beam impinges. The degree of 

scattering is based on the ratio of roughness to wavelength. 

When roughness is less than about 1/10 a wavelength, scatter will be 

negligible. 

To reduce signal losses due to scattering an operator can select a lower 

frequency probe. With a wavelength of 0.37mm in water for a 4MHz probe, 

signal loss due to scatter can occur for irregularities as small as about 

0.04mm. 

In addition to signal reduction another effect of surface irregularities is to 

redirect and mode convert some energy which when returned to the probe 

can be the source of spurious signals. In contact testing false indications from 

standing waves resulting from scatter on rough surfaces will normally have 

short sound paths. They can be eliminated as true flaws by failing to locate 

any trace of indication from the full skip or from the opposite side.

Unless done properly, removal of surface roughness by mechanical means 

can result in further scattering problems. Small curved gouges left by a 

grinding wheel used to remove spatter or machining grooves can form small 

lenses. The affect of grinding can be unpredictable. Some of the lensing may 

concentrate the beam thereby increasing signal amplitude, or, the lens effect 

may be a de-focusing of the beam, again resulting in lower than expected 

signal amplitudes. 

Uniform surface preparation by sand or shot blasting usually provides a good 

surface for ultrasonic testing. Removal of excess metal by a hand held 

grinding wheel is commonly used on weld caps and roots. When a pipe weld 

has had its root ground flush and inspection can only be performed from the 

outside diameter, quality of grinding can result in unnecessary repair calls if 

grinding has been along the weld axis. The small grooves made by the 

grinding wheel run parallel to the root edge and are easily confused with lack 

of fusion, missed edge or undercut defects.

Keywords on Rough Surface: 

1. The degree of scattering is based on the ratio of roughness to 

wavelength. When roughness is less than about 1/10 a wavelength, 

scatter will be negligible. 

2. Consequences of Surface Roughness: 

• Signal reduction 

• Redirect and mode convert some energy which when returned to the 

probe can be the source of spurious signals. 

3. The False Indications: In contact testing false indications from standing 

waves resulting from scatter on rough surfaces will normally have short 

sound paths. They can be eliminated as true flaws by failing to locate any 

trace of indication from the full skip or from the opposite side

5.13.2 Surface Coatings 

Surface coatings are added to protect a surface from corrosion or to enhance 

its appearance. Thin films, such as oxide layers, anodizing layers or 

electroplated finishes, and the slightly thicker coatings of paint or lacquer are 

usually well bonded to the surface. Quality of bond may be compared to the 

uncoated reference block by a simple transfer value. Even a slight loss due to 

the coating may be preferable to removing the coating and trying to inspect 

on the rough surface it hides. 

When thickness testing is done on a painted surface the paint thickness can 

add error to the reading. For example: 

A nominal 25mm steel plate has a cellulose paint coating of 0.5mm. V steel = 

5980m/s, V paint = 2600m/s. 

If a digital thickness meter is calibrated on a 25mm thick piece of the steel 

plate without the paint coating and then placed on the painted surface an 

error will occur.

The coating is sufficiently thin that its interface with the metal will occur in the 

dead zone but the duration of time spent in the paint is added to the travel 

time to the opposite wall of the plate. If the true plate thickness at the point of 

measurement is 25.16mm and the paint coating is 0.5mm thick, the time in 

the paint is 0.5/ 2.6 x 10 6 = 0.19µs. 0.19 microseconds is equivalent to 

1.15mm in steel. The reading on the digital meter would combine the two 

thickness as though all travel was in steel. This results in 25.16 + 1.15 = 

26.31mm as the indicated thickness. 

This problem can be overcome by using an A-scan display and measuring 

the interval between the first and second echo instead of the main bang and 

first echo. This is shown in Figure 8-4.

Thickness Measurement with Surface Coating 

Figure 8-4.

5.13.3 Couplant Condition 

Both contact and immersion methods utilize intervening media to transfer 

sound from the probe into the test piece and back to the receiver. With 

immersion methods it is accomplished by a single fluid medium. In contact 

testing there are nearly always at least two intervening media; the delayline or 

protective face and the thin film of coupling fluid or grease. Attenuation and 

acoustic velocity are the two main properties that dictate the performance of a 

couplant. Attenuation affects amplitude of the signal and velocity will 

determine both transit time and refracted angles. But attenuation and velocity 

of couplants are not independent properties. Each is a function of other 

parameters. Unless these parameters are controlled or in some way 

compensated for, gross variations from the reference value or calibration 

conditions can result.

Attenuation of couplants varies with material composition as would be 

expected. Published attenuation values are available for many materials as 

indicated in the table below. Attenuation coefficients are often quoted in 

Neper which allow for frequency dependence. 1 Np = 8.686 dB. 

Attenuation per unit length= Attenuation Coefficient x f 2 x 8.686 dB/cm. 

Table 8-1 indicates Attenuation Coefficient of some common liquids. 

-15

In more practical terms, for water with longitudinal wave of 500KHz this would 

mean an attenuation of about 5 dB per meter. 

Example: 

Attenuation factor for water = 25.3x 10 -15 Neper 

Frequency= 0.5MHz 

The attenuation = Attenuation Coefficient x f 2 

The attenuation = 25.3 x 10 -15 x (0.5x10 6 ) 2 Neper/ cm 

The attenuation = 25.3 x 10 -15 x (0.5x10 6 ) 2 x 8.686 = 0.055 dB/cm or 5.5 dB/m 

Since such long water path lengths are not normally used the 0.005 dB/mm 

attenuation is considered negligible. But for the heavier oils attenuations 200 

to 500 times greater can have significant effects on signal amplitude and 

frequency content. 

For the fixed delay-lines or wedge materials used in contact testing 

attenuation variations can be far more pronounced and variation between 

manufacturers can cause considerable response differences.

Table 8.2 

For example the plastics listed in table 8-2 are typical wedge materials 

selected by manufacturers and based on velocity for refraction purposes, but 

attenuation differences would result in noticeable amplitude response 

variation and frequency content of transmitted waveforms. Since the operator 

rarely knows what wedge material a manufacturer has used, little can be 

done to correct for potential variations in periodic inspections where results of 

tests taken with one or more years separation are compared.

For example the plastics listed in table 8-2 are typical wedge materials 

selected by manufacturers and based on velocity for refraction purposes, but 

attenuation differences would result in noticeable amplitude response 

variation and frequency content of transmitted waveforms. Since the operator 

rarely knows what wedge material a manufacturer has used, little can be 

done to correct for potential variations in periodic inspections where results of 

tests taken with one or more years separation are compared.

5.13.4 More Reading: What is Neper ? 

The Neper (unit symbol Np) is a logarithmic unit for ratios of measurements of 

physical field and power quantities, such as gain and loss of electronic 

signals. The unit's name is derived from the name of John Napier, the 

inventor of logarithms. As is the case for the decibel and Bel, the Neper is unit 

of the International System of Quantities (ISQ), but not part of the 

International System of Units (SI), but it is accepted for use alongside the SI. 

The Neper is a natural linear unit of relative difference, meaning in Neper 

(logarithmic units), relative differences add, rather than multiply. This property 

is shared with logarithmic units in other bases, such as the Bel. 

http://en.wikipedia.org/wiki/Neper

Like the decibel, the Neper is a unit in a logarithmic scale. While the Bel uses 

the decadic (base-10) logarithm to compute ratios, the Neper uses the natural 

logarithm, based on Euler's number (e ≈ 2.71828). 

The value of a ratio in Neper is given by 

where x1 and x2 are the values of interest, and ln is the natural logarithm. 

In the ISQ, the Neper is defined as 1 Np = 1. 

1 Np = ln (2.718) when the ration of = 2.718 

The neper is defined in terms of ratios of field quantities (for example, voltage 

or current amplitudes in electrical circuits, or pressure in acoustics), whereas 

the decibel was originally defined in terms of power ratios. A power ratio 10 

log r dB is equivalent to a field-quantity ratio 20 log r dB, since power is 

proportional to the square (Joule's laws) of the amplitude.

Hence the Neper and dB are related via: The decibel and the Neper have a 

fixed ratio to each other. Example: For a ratio of (x 1 /x 2 ) The (voltage) level 

ratio is:

Hence:

Q31: Rough surfaces can cause undesirable effects which are noticeable 

when parts are tested ultrasonically, include: 

A. Annular maximum rings 

B. An increase in width of front face echo and consequent loss of 

resolving power 

C. Acoustic mismatch 

D. Asymmetrical modes 

Q32: Rough surfaces cause echo amplitude from discontinuities within the 

part to: 

A. Increase 

B. Decrease 

C. Not change 

D. Change frequency

5.14 The Concept of Effective Distance 

Effective distance in ultrasonic testing is a distance which take into account of 

the required sound beam overlap and hits. The effective distance always 

smaller in the calculation. 

Keywords: 

Loop or full path = Thickness x 2 

Effective Distance D eff = Beam Diameter / overlap, or 

Effective Distance D eff = Beam Diameter / No. of hits 

Effective Distance D eff = Loop x hits 

T = Time (of interest) 

T = D eff / Velocity 

PRR = 1/T 

RPM = Revolution per minute 

Maximum linear traversing speed V for effective inspection = RPM x D eff

Scanning Speed: 

Scanner speed = (PRR × Effective diameter of probe / Number of hits) 

Speed of test part = (PRR × Effective diameter of probe / Number of hits) 

Where: 

Effective dia. of probe = Dia. of probe – 2 [ (Dia. of probe) × (Percent of 

overlap between scan / 100) ] 

Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60)

Q&A on The Concept of Effective Distance 

Distance

Q1: A tubular product is tested by AUT (or UT). The tube is rotated at 500 

RPM If beam diameter is 10 mm and overlap between scan is 50%. 

Calculate maximum length of the tube that can be tested. 

Answer: 

The effective distance covered by each revolution (D eff ) is 5mm. 

For 500 RPM the total distance covered: 500 x 5 mm = 2500 mm 

The inspection rate = 2500/ minute# 

Effective coverage 

50% overlap = 5mm 

Beam diameter (coverage) =10mm

Q2: A steel bar with 200 mm thick is scanned by UT. Minimum number of hits 

required is 10. What is the maximum PRR to avoid the ghost echo. (VL, 

steel = 0.59 cm/μs ) 

Answer: 

The total distance to travel D eff = Thickness x [2 x 10 hits] = 400cm = 4m 

The time taken to complete the 10 loops (T) = [effective distance / Velocity] 

T= 4/ 5900 =1/1475 

The maximum pulse repetition rate PRR = 1/T = 1475 

Distance travelled by 

10hits= 10x 200x2=4m 

200mm 

10 hits

Q3: Minimum number of hits required is 2. What is the maximum allowable 

axial speed for a probe with effective diameter of 102 mm and PRR of 800 

Hz. 

Answer: 

The diameter of the beam = 102mm 

The effective distance by one pulse, D eff = Beam Diameter / No of hits 

D eff = 102/2 = 51mm 

The total distance covered 800 pulse = 51x800 =40.8m 

The axial speed is 40.8m/s

Q4: Assuming that Minimum 3 pulses are required to trigger the alarm in 

AUT (or UT) . What would be (maximum) scanning speed to detect 0.5 

mm size defect while using 4 MHz probe, 10 mm beam diameter with PRR 

of 0.5 kHz. 

Answer: 

The distance Covered by scan in 1 second 

= PRR x Diameter = 500 x 10 = 5m/s 

The effective distance covered = D/ hits = 5/3 = 1.667m/s 

Hint: the size of the defect and mode frequency has no implication of the 

calculation

Q5: What is the maximum PRR is needed for contact test of steel material 

with 100 mm thick using longitudinal wave. Minimum number of hits 

required is 10. (VL, steel = 0.59 cm/μs ) 

Answer: 

Distance = 100 x 2 =200mm for single loop. 

Effective distance D eff = 200 x10 =2000 mm 

Time = D eff / Velocity = 1/2950 

PRR = 1/T = 2950

Q6: What is the maximum PRR is needed for immersion testing of aluminum 

with 80 mm thick using longitudinal wave. The water path is 10 mm. 

Minimum number of hits required is 20. (VL, aluminum = 6.32 × 10 3 m/s, 

VL, water = = 1.48 × 10 3 m/s) 

Answer: 

Single Water path =10mm, Single Al path= 80mm 

Loop H2O = 0.02 m, loop Al = 0.16 m 

Time for traversing single hit = [0.02/1480] + [0.16/6320] = 3.883 x 10 -5 s 

Time for traversing 20 hits T’ = 20 x T = 3.883s 

PRR = 1/T’ = 1287

Q7: A steel plate size 6.2 m ×1.8 m ×0.1m is scanned using 25 mm 

diameter normal probe and overlap between scan is 20%. Minimum 

number of hits required is 15. Calculate the inspection time if scanning 

speed is 500 mm/s. 

Answer: (Standard Answer) 

Effective area of probe = 25 x0.8 x 500 = 10000 mm 2 

Area of plate = 6200 x 1800 mm 2 

T = (6200 x 1800)/ 10000 = 1116 s 

20% overlap 

25mm 

Effective area = 25 x 500 x 

0.8 = mm 2 

Scanning speed 500 mm/s

Q7: A steel plate size 6.2 m ×1.8 m ×0.1m is scanned using 25 mm 

diameter normal probe and overlap between scan is 20%. Minimum 

number of hits required is 15. Calculate the inspection time if scanning 

speed is 500 mm/s. 

Answer: In-correct answer. 

Area covered by probe in 1 second = 25 x 500 = 12500 mm 2 

Effective plate area = 6200 x 1800 x 1.2 mm 2 

Time to scan the plate = [6200 x1800 x 1.2 / 12500] = 1071.36s 

(maintaining the probe area, increase the surface area by the overlap) 

Hint: the plate thickness is of no concern.

Q8: Assume that the minimum PRR is needed for contact test of a given 

steel plate is 240 Hz. How much volume would be covered under test if the 

PRR is set at 120 Hz. 

Answer: 

Volume inspected by ultrasound per unit time 

= PRR x Pulse length x Cross sectional area of beam at point of interest. 

= PRR x constant (k) 

Where k = Pulse length x Cross sectional area of beam at point of interest 

The ration of volume covered = 120k / 250k x 100 = 50%


Exercises

The 6 dB Method 

For Large Reflector (greater than beam width), i.e. there is no BWE.

Compared 6 dB Drop Sizing with Equalization Technique 

The Equalization Back Wall Sizing- The probe moving off the edges of the 

reflector until the amplitude is equal to the rising BWE

Q1 What is the correct water path between the transducer and the steel front 

surface to focused a transducer for a area of interest at ½ below a steel 

surface? 

Given that: 

Focal length of transducer in water = 6” 

Velocity of sound in water= 1484 m/s 

Velocity of sound in steel = 5920 m/s 

Equivalent depth in water for ½ steel depth = 4x ½ = 2” 

The water path= 6”- 2” = 4”

Q5: Ultrasonic inspection is being for a circumferential weld of a pressure 

vessel. Equipment is calibrated at the beginning of the examination and 

scanning started at 9.00 AM. In between calibration is also done and at 

12.00 noon the equipment is not functioning properly. Still 30 % of weld 

is to be examined. As per procedure and as level II one can: 

A. go ahead with the scanning by doing recalibration 

B. perform the inspection fully from the beginning after recalibration 

C. bring another equipment and proceed scanning from the left out place 

D. check and recalibrate the equipment and continue scanning 

from the portion where scanning started after calibration 

at 11.00 AM

Q5: When dissimilar metal welds is to be tested ultrasonically and scanning 

is to be performed from both sides of the weld the calibration blocks shall 

be made from: 

A. material having higher tensile strength 

B. both the materials 

C. material subjected to heat treatment 

D. calibration block material is aluminum

Break Time 

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/ 

39440/v0001/reflector:20587?BBC- 

UID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f556 

74c540f1856834&SSO2-UID=

Section 6: Selected Applications 

& Techniques

Content: Section 6: Selected Applications & Techniques 

6.1: Defects & Discontinuities 

6.2: Rail Inspection 

6.3: Weldments (Welded Joints) 

6.4: Pipe & Tube 

6.5: Echo Dynamic 

6.6: Technique Sheets 

6.7: Material Properties-Elastic Modulus Measurements 

6.8: High Temperature Ultrasonic Testing 

6.9: Thickness Gauging 

6.10: In-Service Inspection 

Continues next page….

6.11: Casting 

6.12: Inspection of bonded Joints 

6.13: Corrosion Monitoring 

6.14: Crack Monitoring 

6.15: Residual Stress Measurements 

6.16: Bond Testing 

Appendix: (Non-exam) 

6.App-1: TOFD Introduction

6.1: Defects & Discontinuities

6.1.1 Casting Defects & Discontinuities

Casting Defects & Discontinuities

Casting Defects & Discontinuities- A Cold Shut is caused when a molten 

metal is poured over solidified metal without fusing.

Casting Defects & Discontinuities – Hot tear or shrinkage crack forms 

when the molten section of unequal thickness solidified and the shrinkage 

stress tear the partially molten apart.


Micro-shrinkage is usually many small subsurface holes that appear at the 

gate of casting / can also occur when molten metal must flow from a thin 

section into thicker section of casting. 

Blow hole are small hole at the surface of the casting caused by gas which 

comes from the mold itself. (wet sand mould forming steam resulting in blowhole) 

Porosity is caused by entrapped gas. It is usually subsurface or surface 

depending on the mold design.


Casting Defects & Discontinuities- Hot Tear

Casting Defects & Discontinuities- Blister

Casting Defects & Discontinuities- Porosity




Casting Defects & Discontinuities - Mismatch

Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Missrun

Casting Defects & Discontinuities- Misrun

Casting Defects & Discontinuities- Blow Hole

Casting Defects & Discontinuities- Gas Porosity


Casting Defects & Discontinuities- Cold Shut

Casting Defects & Discontinuities- Shrinkage Cavity

Casting Defects & Discontinuities- Assorted

6.1.2 Processing Defects & Discontinuities

Processing Defects & Discontinuities

Salute to the Steel Workers!

Processing Defects & Discontinuities- Lamination formed when the 

casting defects are flatten during rolling, forging, extrusion or other 

mechanical working processes.

Processing Defects & Discontinuities- Stringers formed when the billet is 

rolled into shape the casting non metallic inclusions are squeezed into long 

and thinner inclusions.

Processing Defects & Discontinuities- Forging lap is caused by folding of 

metal on the surface, usually when some of the metal is squuaed ot between 

the two dies.

Processing Defects & Discontinuities- Forging burst is a rupture causes 

by forging at improper temperature. The burst may be internal or external.

Processing Defects & Discontinuities

Q9: The preferred method of ultrasonically inspecting a complex-shape 

forging: 

A. Is an automated immersion test of the finished forging using instrument 

containing a calibrated attenuator in conjunction with a C-scan recorder 

B. Combined thorough inspection of the billet prior to forging with a 

careful inspection of the finished part in all areas where shape permit 

C. Is a manual contact test of the finished part 

D. Is an automated immersion test of the billet prior to forging

6.1.3 Welding Defects & Discontinuities

Welding Defects & Discontinuities






Welding Defects & Discontinuities- Incomplete Penetration

Welding Defects & Discontinuities- Slag Inclusion

Welding Defects & Discontinuities- Cluster Porosity

Welding Defects & Discontinuities- Lack of Sidewall Fusion (with Slag 

entrapped)

Welding Defects & Discontinuities- Wagon Track 

(slag inclusion at hot pass)

Welding Defects & Discontinuities- Burn Thru

Welding Defects & Discontinuities- Offset with LOP

Welding Defects & Discontinuities- Excessive Penetration

Welding Defects & Discontinuities- Internal (Root) Under Cut

Welding Defects & Discontinuities- Transverse Crack

Welding Defects & Discontinuities- Tungsten Inclusion

Welding Defects & Discontinuities- Root Pass Porosity

6.1.4 Service Induced Defects & Discontinuities

Service Induced Defects & Discontinuities 

http://failure-analysis.info/2010/05/analyzing-material-fatigue/

Service Induced Defects & Discontinuities- Fatigue Cracks

Figure 4-24 – In a carbon steel sample, metallographic section through a 

thermal fatigue crack indicates origin at the toe of an attachment weld. Mag. 

50X, etched.

Figure 4-26 – Metallographic cross-section of a superheated steam outlet that 

failed from thermal fatigue. Unetched.

Figure 4-36 – Weld detail used to join a carbon steel elbow (bottom) to a weld 

overlaid pipe section (top) in high pressure wet H2S service. Sulfide stress 

cracking (SSC) occurred along the toe of the weld (arrow), in a narrow zone 

of high hardness.

Figure 4-37 – High magnification photomicrograph of SSC in pipe section 

shown in Figure 4-36.

Figure 4-38 – Failure of DMW joining 1.25Cr-0.5Mo to Alloy 800H in a Hydrodealkylation 

(HAD) Reactor Effluent Exchanger. Crack propagation due to 

stresses driven at high temperature of 875°F (468°C) and a hydrogen 

partial pressure of 280 psig (1.93 MPa).

Figure 4-57 – Vibration induced fatigue of a 1-inch socket weld flange in a 

thermal relief system shortly after startup.

Figure 4-58 – Cross-sectional view of the crack in the socket weld in Figure 4- 

57.

Figure 5-1 – Localized amine corrosion at the weld found in piping from 

reboiler to regenerator tower in an MEA unit. Many other similar cases found, 

some going as deep as half thickness. They were originally found and 

mistaken as cracks with shear wave UT inspection.

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-46 – Overhead interstage knockout drum vapor outlet nozzle.

Figure 5-47 – Carbonate cracking adjacent to a weld (Reference 6).

Figure 5-48 – Metallographic sample showing intergranular carbonate 

cracking developed after 6 months service (Reference 6).lean amine 

(Reference 5)

Figure 5-49 – Most cracks originate in base metal but this weldment 

contained a crack that originated at the root and propagated through the weld 

metal. Other cracks appear to have initiated in the HAZ (Reference 7).

6.2: Rail Inspection

Rail Inspection 

One of the major problems that railroads have faced since the earliest days is 

the prevention of service failures in track. As is the case with all modes of 

high-speed travel, failures of an essential component can have serious 

consequences. The North American railroads have been inspecting their 

most costly infrastructure asset, the rail, since the late 1920's. With increased 

traffic at higher speed, and with heavier axle loads in the 1990's, rail 

inspection is more important today than it has ever been. Although the focus 

of the inspection seems like a fairly well-defined piece of steel, the testing 

variables present are significant and make the inspection process challenging. 

Rail inspections were initially performed solely by visual means. Of course, 

visual inspections will only detect external defects and sometimes the subtle 

signs of large internal problems.

The need for a better inspection method became a high priority because of a 

derailment at Manchester, NY in 1911, in which 29 people were killed and 60 

were seriously injured. In the U.S. Bureau of Safety's (now the National 

Transportation Safety Board) investigation of the accident, a broken rail was 

determined to be the cause of the derailment. The bureau established that the 

rail failure was caused by a defect that was entirely internal and probably 

could not have been detected by visual means. The defect was called a 

transverse fissure (example shown on the bottom). The railroads began 

investigating the prevalence of this defect and found transverse fissures were 

widespread.

Transverse Fissure

Transverse Fissure

Transverse Fissure

One of the methods used to inspect rail is ultrasonic inspection. Both 

normal- and angle-beam techniques are used, as are both pulse-echo and 

pitch-catch techniques. The different transducer arrangements offer different 

inspection capabilities. Manual contact testing is done to evaluate small 

sections of rail but the ultrasonic inspection has been automated to allow 

inspection of large amounts of rail. 

Fluid filled wheels or sleds are often used to couple the transducers to the 

rail. Sperry Rail Services, which is one of the companies that perform rail 

inspection, uses Roller Search Units (RSU's) comprising a combination of 

different transducer angles to achieve the best inspection possible. A 

schematic of an RSU is shown below.

Techniques: Wheel Probe

Techniques: Examples of axles with outside bearings of the Deutsche 

Bundesbahn. (a) Of goods truck; (b) axle with roller bearing, bearing ring not 

removed; c same with additional brake disc

Techniques: (c) same with additional brake disc

6.3: Weldments (Welded Joints)

6.3.1: UT of Weldments (Welded Joints) 

The most commonly occurring defects in welded joints are porosity, slag 

inclusions, lack of side-wall fusion, lack of inter-run fusion, lack of root 

penetration, undercutting, and longitudinal or transverse cracks. 

With the exception of single gas pores all the defects listed are usually well 

detectable by ultrasonics. Most applications are on low-alloy construction 

quality steels, however, welds in aluminum can also be tested. Ultrasonic flaw 

detection has long been the preferred method for nondestructive testing in 

welding applications. This safe, accurate, and simple technique has pushed 

ultrasonics to the forefront of inspection technology. 

Ultrasonic weld inspections are typically performed using a straight beam 

transducer in conjunction with an angle beam transducer and wedge. A 

straight beam transducer, producing a longitudinal wave at normal incidence 

into the test piece, is first used to locate any laminations in or near the heataffected 

zone. This is important because an angle beam transducer may not 

be able to provide a return signal from a laminar flaw.

UT of Weldments (Welded Joints) 

F 

s 

0 20 40 60 80 100 

x 

a = s sinß 

a' = a - x 

d' = s cosß 

d = 2T - t' 

a 

a' 

ß = probe angle 

s = sound path 

a = surface distance 

a‘ = reduced surface distance 

d‘ = virtual depth 

d = actual depth 

T = material thickness 

ß 

Work piece with welding 

s 

Lack of fusion 

d

UT Calculator

Flaw Detection- Depth Determination

The second step in the inspection involves using an angle beam transducer 

to inspect the actual weld. Angle beam transducers use the principles of 

refraction and mode conversion to produce refracted shear or longitudinal 

waves in the test material. [Note: Many AWS inspections are performed using 

refracted shear waves. However, material having a large grain structure, such 

as stainless steel may require refracted longitudinal waves for successful 

inspections.] This inspection may include the root, sidewall, crown, and heataffected 

zones of a weld. The process involves scanning the surface of the 

material around the weldment with the transducer. This refracted sound wave 

will bounce off a reflector (discontinuity) in the path of the sound beam. With 

proper angle beam techniques, echoes returned from the weld zone may 

allow the operator to determine the location and type of discontinuity.

T = Plate Thickness 

ϴ = Shear wave angle 

LEG = T/Cos ϴ, V path= 2 x LEG. 

Skip = 2.T Tan ϴ

https://www.mandinasndt.com/index.php?option=com_content&view=article&id=32%253A 

ut-angle-beam-calculator&catid=12%253Atools&Itemid=18 

https://www.nde-ed.org/GeneralResources/Formula/AngleBeamFormula/AngleBeamTrig.htm

Flaw Detection- Triangulations of reflector 

ϴ = Refracted angle T= Thickness LEG1=LEG2= T/Cos ϴ 

V PATH= 2x LEG= 2T/Cos ϴ 

SKIP= 2.T Tan ϴ 

ϴ

Flaw Detection- Triangulations of reflector 

ϴ = Refracted angle T= Thickness Surface Distance= S.Sin ϴ 

Depth= S.Cos ϴ 

ϴ

To determine the proper scanning area for the weld, the inspector must first 

calculate the location of the sound beam in the test material. Using the 

refracted angle, beam index point and material thickness, the V-path and skip 

distance of the sound beam is found. Once they have been calculated, the 

inspector can identify the transducer locations on the surface of the material 

corresponding to the crown, sidewall, and root of the weld.

6.3.2 Weld Scanning

Expert at works

Typical Scanning Patterns: 

Typically the weld should be inspected in the 1 st or 2 nd leg (1 st Skip).

Typically scanning patterns

Weld Scanning

Weld Scanning

Weld Scanning

Weld Scanning

Echo Dynamic- Position of Defects 

Sometimes it will be possible to differentiate between these 2 defects simply 

by plotting their position within the weld zone:

Echo Dynamic- Position of Defects

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning

Plate Weld Scanning


52. One of the most apparent characteristics of a discontinuity echo, as 

opposed to a non-relevant indication is: 

(a) Lack of repeatability 

(b) Sharp, distinct signal 

(c) Stable position with fixed transducer position 

(d) High noise level 

58. What useful purpose may be served by maintaining grass on the baseline? 

(a) To estimate casting grain size 

(b) To provide a reference for estimating signal to noise ratio 

(c) To verify adequate coupling to the test piece 

(d) All of the above


62. Which of the following conditions would be most likely to cause strong, 

interfering surface waves? 

(a) High frequency transducers 

(b) Testing on a small diameter surface 

(c) Testing on a flat surface 

(d) Testing on a curved surface with a contoured wedge and transducer

6.4: Pipe & Tube

Pipe & Tube

Pipe & Tube

Experts at work

Pipe Scanning

Pipe Scanning

Pipe Scanning 

48.59 o max 

30 o max

Pipe Scanning

Pipe Scanning

Pipe Scanning- thickness/OD ratio

Pipe Scanning- thickness/OD ratio 

When the t/OD ratio = .2 , t=.2OD, ID=OD-2t= OD-.4OD= .6OD 

ϴ max = Sin -1 (ID/OD), ϴ max = Sin -1 (0.6), ϴ max = 37° Max. 

For the sound path to scans the inner face the maximum shear angle shall be 

37° Max. Therefore 45° /60° /70° probe can not scan the pipe inner face.

Pipe Scanning- Contact Methods



Q: Calculate the maximum shear wave angle and the range for 360° 

revolution scanning when the shear wave angle is 45°. 

Given that the OD=6” Thickness=3/4” 

Answer: 

(a) 

The maximum shear wave angle ϴ = Sin -1 (ID/OD) = Sin -1 (2.25/3) 

ϴ = 48.6° Max. 

(b) ?

Answer part B 

c 

a 

b 

a/Sin A = b/Sin B 

2.25/ Sin 45 = b / Sin B, 3.182= b/ Sin B, 

c = a.Sin B, Sin B= c/a 

3.182= b/c x 2.25, b/c= 1.414

Q35: During immersion testing of pipe or tubing the incident longitudinal wave 

angle must be limited to a narrow range. The reason for the upper limit is: 

(a) To avoid complete reflection of ultrasound from the test piece 

(b) To prevent formation of Rayleigh waves 

(c) To prevent formation of shear waves 

(d) To avoid saturating the test piece with ultrasound

Q35: Which of the following may result in a narrow rod if the beam 

divergence results in a reflection from a side of the test piece before the 

sound wave reaches the back surface: 

A. Multiple indications before the first back reflection 

B. Indications from multiple surface reflections 

C. Conversion from longitudinal mode to shear mode 

D. Loss of front surface indications

6.5: Echo Dynamic

Expert at works

6.5.1 Basic echodynamic pattern of reflectors 

Echo Dynamic of Discontinuity- Non-destructive testing of welds - 

Ultrasonic testing - Characterization of indications in welds; German version 

EN 1713:1998 + A1:2002

Basic echodynamic pattern of reflectors 

C.1 Pattern 1 

Point-like reflector response, figure C.1. At any probe position the A-scan 

show a single sharp echo. As the probe is moved this rises in amplitude 

smoothly to a single maximum before falling smoothly to noise level. 

4 

5 

2 

3 

6 

1 

7

C.1 Pattern 1 Point-like reflector

C.1 Pattern 1 Point-like reflector

C.2 Pattern 2 

Extended (elongated) smooth reflector respond, figure C.2. At any probe 

position the A-scan shows a single sharp echo. When the ultrasound beam is 

moved over the reflector the echo rises smoothly to a plateau and is 

maintained with minor variation in magnitude up to 4 dB, until the beam 

moves off the reflector, when the echo fall smoothly to noise level.

C.2 Pattern 2 

Extended (elongated) smooth reflector

C.2 Pattern 2 

Extended (elongated) smooth reflector

C.2 Pattern 2 

Extended (elongated) smooth reflector 

(figure modified to depict obliquely oriented planar face) 

Extended (elongated) 

smooth reflector-planar 

face obliquely oriented

C.3 Pattern 3 

Extended (elongated) rough reflector response. There are two variants of this 

pattern, depending upon the angle of incident of the probe beam on the 

reflector.

C.3 Pattern 3a 

Extended (elongated) rough reflector response. Near normal incidence, figure 

C.3a At any probe position the A-scan shows a single but rugged echo. As 

the probe moved this may undergo large (>+/- 6dB) random fluctuation in 

amplitude. The fluctuation are caused by reflection from the different facets of 

the reflector and by interference of waves scattered from the groups of facets.


Extended (elongated) rough reflector response.


Extended (elongated) rough reflector response.

C.3 Pattern 3b 

Oblique incidence, travelling echo pattern, figure C.3 b At any probe position, 

the A-scan shows an extended train of signals (subsidiary peaks) within a 

bell-shaped pulse envelope. As the probe is moved each subsidiary peak 

travels through the pulse envelop, rising to its own maximum toward the 

center envelop and then falling. The overall signal may shown large (>+/-6dB) 

random fluctuation in amplitude.


Oblique incidence, travelling echo pattern


Oblique incidence, travelling echo pattern

C.4 Pattern 4 

Multiple reflector respond, figure C.4. At any probe position the A-scan shows 

a cluster of signal which may or may not be well resolved in range. As the 

probe is moved the signals rise and fall at random but the signal from each 

separate reflector element ,if resolved, shows pattern 1 respond.

C.4 Pattern 4 

Multiple reflector respond

C.4 Pattern 4 

Multiple reflector respond

Echodynamic- Change of echo height and echo shape when the direction of 

irradiation is changed. (a) On flat or linear flaw; (b) on rounded flaw

Echodynamic- Differences between the indications of inclusions and cracks, 

drawn schematically and exaggerated for greater clarity. a Inclusions; b flake 

cracks. The echoes of the more distantflaws, because of divergence and 

attenuation of the sound beam, are rather weak

Break Time

Echo Dynamic of Discontinuity- Flaw detection

Echo Dynamic of Discontinuity- Flaw Detection

Echo Dynamic of Discontinuity- Flaw detections

Echo Dynamic of Discontinuity- Improper flaw orientation


Echo Dynamic of Discontinuity- Reflection angle

Echo Dynamic of Discontinuity- Angles of reflection


Echo Dynamic of Discontinuity- Perfect flaw orientation


Echo Dynamic of Discontinuity- Vertical near surface flaw

Echo Dynamic of Discontinuity- Tandem Techniques



Echo Dynamic

Echo Dynamic- Root Concavity

Echo Dynamic

Echo Dynamic

Echo Dynamic

Echo Dynamic

Echo Dynamic 

Crack

Echo Dynamic- Broad indication with low amplitude

Echo Dynamic- Shaper indication and higher amplitude than porosity

Echo Dynamic

Echo Dynamic 

Threadlike defects, point defects and flat planar defects orientated nearnormal 

to the beam axis all produce an echo response which has a single 

peak

Echo Dynamic 

The echo response from a large slag inclusion or a rough crack is likely to 

have multiple peaks:

Echo Dynamic 

In case “a” it will be difficult to determine whether the defect is slag or a crack. 

“Rotational- Swivel” or “orbital” probe movements may help:

Echo Dynamic 

Typical Echo Dynamic Patterns

Echo Dynamic 


Echo Dynamic 


Q. A smooth flat discontinuity whose major plane is not perpendicular to the 

direction of sound propagation may be indicated by: 

A. An echo amplitude comparable in magnitude to the back surface reflection 

B. A complete loss of back surface reflection 

C. An echo amplitude larger in magnitude than the back surface reflection 

D. All of the above

Q183. In immersion testing, irrelevant or false indications caused by 

contoured surfaces are likely to result in a: 

A. Broad base indication 

B. Peaked indication 

C. Hashy signal 

D. Narrow based indication

Q24. During inspection of a parallel sided machined forging using straight 

beam immersion techniques, a diminishing back reflection in a localized 

area in the absence of a defect indication would least likely represent: 

A. A course grain structures 

B. A small non-metallic stringer 

C. A defect oriented at a severe angle to the entry surface 

D. A large inclusion.

Q46. Which best describes a typical display of a crack whose major surface is 

perpendicular to the ultrasound beam? 

A. A broad indication 

B. A sharp indication 

C. A indication will not show due to improper orientation 

D. A broad indication with high amplitude

Q46. A smooth flat discontinuities whose major plane is not perpendicular to 

the direction of sound propagation may be indicated by: 




D. All of the above

6.6: Technique Sheets

Expert at works

Hanger Pin Testing using Shear Wave 

http://www.fhwa.dot.gov/publications/research/infrastructure/structures/04042/index.cfm#toc

Physical Dimension

Physical Dimension

Physical Dimension

Physical Dimension

Reporting: Basic Pin Information

Reporting: Scanning Report – Top of Pin

Reporting: Scanning Report – Bottom of Pin

Mock-Up

Mock-Up

Mock-Up

Mock-Up

Mock-Up

Reporting: Basic Pin Information

Hanger Pin Testing using Shear Wave

Pitch and Catch Methods- Echo Dynamic

Pitch and Catch Methods- Set-up

Pitch and Catch Methods- Echo Dynamic

6.7: Material Properties- 

Elastic Modulus Measurements

6.7.1 Determination of Microstructural Differences 

Ultrasonic methods can be used to determine microstructural differences in 

metals. For this, contact testing with the pulse-echo technique is used. The 

testing can be either the measurement of (1) ultrasonic attenuation or the (2) 

measurement of bulk sound velocity.

6.7.2 The attenuation method 

The attenuation method is based on the decay of multiple echoes from test 

piece surfaces. Once a standard is established, other test pieces can be 

compared to it by comparing the decay of these echoes to an exponential 

curve. This test is especially suited for the microstructural control of 

production parts, in which all that is necessary is to determine whether or not 

the parts conform to a standard. An example of the use of ultrasonic 

attenuation in the determination of differences in microstructure is the control 

of graphite-flake size in gray iron castings, which in turn controls tensile 

strength. In one application, a water-column search unit that produced a 

pulsed beam with a frequency of 2.25 MHz was used to test each casting 

across an area of the casting wall having uniform thickness and parallel front 

and back surfaces.

A test program had been first carried out to determine the maximum size of 

graphite flakes that could be permitted in the casting and still maintain a 

minimum tensile strength of 200 MPa (30 ksi). Then, ultrasonic tests were 

made on sample castings to determine to what intensity level the second 

back reflection was lowered by the attenuation effects of graphite flakes larger 

than permitted. Next, a gate was set on the ultrasonic instrument in the region 

of the second back reflection, and an alarm was set to signal whenever the 

intensity of this reflection was below the allowable level. The testing 

equipment was then integrated into an automatic loading conveyor, where the 

castings were 100% inspected and passed or rejected before any machining 

operation.

6.7.3 Velocity Measurements 

Velocity Measurements When considering the compressional and shear 

wave velocities given in Table 1, there may be small deviations for crystalline 

materials because of elastic anisotropy. This is important and particularly 

evident in copper, brass, and austenitic steels. The following example 

illustrates the variation of sound velocity with changes in the microstructure of 

leaded free-cutting brass.

6.7.4 Elastic Modulus Measurement 

Application: 

Measurement on Young's Modulus and Shear Modulus of Elasticity, and 

Poisson's ratio, in non-dispersive isotropic engineering materials. 

Background: 

1. Young's Modulus of Elasticity is defined as the ratio of stress (force per 

unit area) to corresponding strain (deformation) in a material under tension 

or compression. 

2. Shear Modulus of Elasticity is similar to the ratio of stress to strain in a 

material subjected to shear stress. 

3. Poisson's Ratio is the ratio of transverse strain to corresponding axial 

strain on a material stressed along one axis. 

http://www.olympus-ims.com/en/applications/elastic-modulus-measurement/ 

http://www.olympus-ims.com/en/applications/?347[search][sCategoryId][1166017122]=1166017163&347[search][submit]=Search

Elastic Modulus Measurement – Young’s Modulus & Shear Modulus 

http://en.wikipedia.org/wiki/Shear_modulus

Elastic Modulus Measurement- Poisson Ratio

These basic material properties, which are of interest in many manufacturing 

and research applications, can be determined through computations based 

on measured sound velocities and material density. 

Sound velocity can be easily measured using ultrasonic pulse-echo 

techniques with appropriate equipment. The general procedure outlined 

below is valid for any (1) homogeneous, (2) isotropic, (3) non-dispersive 

material (velocity does not change with frequency). 

This includes most common metals, industrial ceramics, and glasses as long 

as cross sectional dimensions are not close to the test frequency wavelength. 

Rigid plastics such as polystyrene and acrylic can also be measured, 

although they are more challenging due to higher sound attenuation. 

Keyword: 

non-dispersive material (velocity does not change with frequency).

Rubber cannot be characterized ultrasonically because of its high dispersion 

and nonlinear elastic properties. Soft plastics similarly exhibit very high 

attenuation in shear mode and as a practical matter usually cannot be tested. 

In the case of anisotropic materials, elastic properties vary with direction, and 

so do longitudinal and/or shear wave sound velocity. Generation of a full 

matrix of elastic moduli in anisotropic specimens typically requires six 

different sets of ultrasonic measurements. 

Porosity or coarse granularity in a material can affect the accuracy of 

ultrasonic modulus measurement since these conditions can cause variations 

in sound velocity based on grain size and orientation or porosity size and 

distribution, independent of material elasticity. 

Keyword: 

anisotropic materials, elastic properties vary with direction

Equipment: 

The velocity measurements for modulus calculation are most commonly 

made with precision thickness gages such as models 38DL PLUS and 45MG 

with Single Element software, or a flaw detector with velocity measurement 

capability such as the EPOCH series instruments. Pulser/receivers such as 

the Model 5072PR or 5077PR can also be used with an oscilloscope or 

waveform digitizer for transit time measurements. 

This test also requires two transducers appropriate to the material being 

tested, for pulse-echo sound velocity measurement in longitudinal and shear 

modes. Commonly used transducers include an M112 or V112 broadband 

longitudinal wave transducer (10 MHz) and a V156 normal incidence shear 

wave transducer (5 MHz). These work well for many common metal and fired 

ceramic samples. Different transducers will be required for very thick, very 

thin, or highly attenuating samples. Some cases may also require use of 

through transmission techniques, with pairs of transducers positioned on 

opposite sides of the part. It is recommended that in all cases the user consult 

Olympus for specific transducer recommendations and assistance with 

instrument setup.

The test sample may be of any geometry that permits clean pulse/echo 

measurement of sound transit time through a section on thickness. Ideally 

this would be a sample at least 0.5 in. (12.5 mm) thick, with smooth 

parallel surfaces and a width or diameter greater than the diameter of the 

transducer being used. Caution must be used when testing narrow 

specimens due to possible edge effects that can affect measured pulse 

transit time. Resolution will be limited when very thin samples are used 

due to the small changes in pulse transit time across short sound paths. 

For that reason we recommend that samples should be at least 0.2 in. (5 

mm) thick, preferably thicker. In all cases the thickness of the test sample 

must be precisely known. 

Keywords: 

1. Caution must be used when testing narrow specimens due to possible 

edge effects that can affect measured pulse transit time. 

2. Resolution will be limited when very thin samples are used due to the 

small changes in pulse transit time across short sound paths.

Testing Procedure: Equipment Used. 

Measure the (1) longitudinal and (2) shear wave sound velocity of the test 

piece using the appropriate transducers and instrument setup. 

The shear wave measurement will require use of a specialized high viscosity 

couplant such as our SWC. A Model 38DL PLUS a 45MG thickness gage 

can provide a direct readout of material velocity based on an entered sample 

thickness, and an EPOCH series flaw detector can measure velocity through 

a velocity calibration procedure. In either case, follow the recommended 

procedure for velocity measurement as described in the instrument's 

operating manual. If using a pulser/receiver, simply record the round-trip 

transit time through an area of known thickness with both longitudinal and 

shear wave transducers, and compute: 

Question: For measurement of shear wave velocity is normal incident 

transverse wave used? (hint by the used of highly viscous couplant 

requirement)

Testing Procedure: Velocity Measurements & Calculations 

Velocity= Distance / ( ½ Round trip traverse time) 

Convert units as necessary to obtain velocities expressed as inches per 

second or centimeters per second. (Time will usually have been measured in 

microseconds, so multiply in/uS or cm/uS by 10 6 to obtain in/S or cm/S.) The 

velocities thus obtained may be inserted into the following equations. 

Poisson Ratio (v) = 

Young’s Modulus = 

Shear Modulus =

Velocity & Equations 

Poisson Ratio (v) = 

Young’s Modulus (E) = 

Shear Modulus (G) = , 

V L , V S 

v 

p 

= Longitudinal and Shear Velocity 


= Material density

Note on units: If sound velocity is expressed in cm/S and density in g/cm 3 , 

then Young's modulus will be expressed in units of dynes/cm 2 . If English units 

of in/S and lbs/in 3 are used to compute modulus in pounds per square inch 

(PSI), remember the distinction between "pound" as a unit of force versus a 

unit of mass. Since modulus is expressed as a force per unit area, when 

calculating in English units it is necessary to multiply the solution of the above 

equation by a mass/force conversion constant of (1 / Acceleration of Gravity) 

to obtain modulus in PSI. Alternately, if the initial calculation is done in metric 

units, use the conversion factor 1 psi = 6.89 x 104 dynes/cm 2 . Another 

alternative is to enter velocity in in/S, density in g/cm 3, and divide by a 

conversion constant of 1.07 x 104 to obtain modulus in PSI.

6.8: High Temperature Ultrasonic Testing

Experts at work

1.0 Background: 

Although most ultrasonic flaw detection and thickness gauging is performed 

at normal environmental temperatures, there are many situations where it is 

necessary to test a material that is hot. This most commonly happens in 

process industries, where hot metal pipes or tanks must be tested without 

shutting them down for cooling, but also includes manufacturing situations 

involving hot materials, such as extruded plastic pipe or thermally molded 

plastic immediately after fabrication, or testing of metal ingots or castings 

before they have fully cooled. Conventional ultrasonic transducers will 

tolerate temperatures up to approximately 50° C or 125° F. At higher 

temperatures, they will eventually suffer permanent damage due to internal 

disbonding caused by thermal expansion. If the material being tested is hotter 

than approximately 50° C or 125° F, then high temperature transducers and 

special test techniques should be employed. 

http://www.olympus-ims.com/en/applications/high-temperature-ultrasonic-testing/

This application note contains quick reference information regarding selection 

of high temperature transducers and couplants, and important factors 

regarding their use. It covers conventional ultrasonic testing of materials at 

temperatures up to approximately 500°C or 1000°F. In research applications 

involving temperatures higher than that, highly specialized waveguide 

techniques are used. They fall outside the scope of this note. 

Testing Methods used: 

Methods used to increase the useful range for high temperature application 

are: 

■ 

■ 

■ 

Delay Line 

High temperature Couplants 

Testing Techniques & Equipment Requirements

Temperature Limitation: 

Conventional ultrasonic 

transducers 50°C



transducers 50°C



transducers 50°C 

http://amazingunseentravel.blogspot.com/2011_08_28_archive.html



transducers 50°C




http://www.wisdompetals.com/index.php/photos/138-wonder-of-the-world-crescent-lake-in-gopi-deser




http://www.wisdompetals.com/index.php/photos/138-wonder-of-the-world-crescent-lake-in-gopi-deser

敦煌大漠美食 -50 度火锅双塔鱼 

http://www.cc6uu.com/science/article/raiders/2407

High Temperature Conventional UT- 

Good Till & No-More.

2.0 Methods used for H.Temperature Scanning 

2.1 Transducers- H.Temperature Delay Line Material 

Panametrics-NDT high temperature transducers fall into two categories, 

■ 

■ 

dual element transducers and 

delay line transducers. 

In both cases, the delay line material (which is internal in the case of duals) 

serves as thermal insulation between the active transducer element and the 

hot test surface. 

For design reasons, there are no high temperature contact or immersion 

transducers in the standard product line. High temperature duals and delay 

line transducers are available for both thickness gaging and flaw detection 

applications. As with all ultrasonic tests, the best transducer for a given 

application will be determined by specific test requirements, including the 

material, the thickness range, the temperature, and in the case of flaw 

detection, the type and size of the relevant flaws.

(1a) Thickness gaging 

The most common application for high temperature thickness gaging is 

corrosion survey work, the measurement of remaining metal thickness of hot 

pipes and tanks with corrosion gages such as Models 38DL PLUS and 45MG. 

Most of the transducers that are designed for use with Olympus corrosion 

gages are suitable for high temperature use. The commonly used D790 

series transducers can be used on surfaces as hot as 500° C or 930° F. For a 

complete list of available corrosion gauging duals that includes temperature 

specifications, see this link: Corrosion Gage Duals.

For precision thickness gauging applications using the Models 38DL PLUS or 

Model 45MG with Single Element software ,such as hot plastics, any of the 

standard Micro-scan delay line transducers in the M200 series (including 

gage default transducers M202, M206, M207, and M208) can be equipped 

with high temperature delay lines. DLHT-1, -2, and -3 delay lines may be 

used on surfaces up to 260° C or 500° F. DLHT-101, -201, and -301 delay 

lines may be used on surfaces up to 175° C or 350° F. These delay lines are 

listed in the Delay Line Option Chart.

In challenging applications requiring low frequency transducers for increased 

penetration, the Videoscan Replaceable Face Transducers and appropriate 

high temperature delay lines can also be used with 38DL PLUS and 45MG 

thickness gages incorporating the HP (high penetration) software option. 

Custom transducer setups will be required. Standard delay lines for this 

family of transducers can be used in contact with surfaces as hot as 480° C 

or 900° F. For a full list of transducers and delay lines, see this link: 

Replaceable Face Transducers.

(1b) Flaw detection 

As in high temperature thickness gaging applications, high temperature flaw 

detection most commonly uses dual element or delay line transducers. All 

standard Panametrics-NDT flaw detection duals offer high temperature 

capability. Fingertip, Flush Case, and Extended Range duals whose 

frequency is 5 MHz or below may be used up to approximately 425° C or 

800° F, and higher frequency duals (7.5 and 10 MHz) may be used up to 

approximately 175° C or 350° F. For a full list of transducers in this category, 

see this link: Flaw Detection Duals. 

All of the Videoscan Replaceable Face Transducers can be used with 

appropriate high temperature delay lines in flaw detection applications. The 

available delay lines for this family of transducers can be used in contact with 

surfaces as hot as 480° C or 900° F. For a full list of transducers and delay 

lines suitable for various maximum temperatures, see this link: Replaceable 

Face Transducers.

Applications involving thin materials are often best handled by the delay line 

transducers in the V200 series (most commonly the V202, V206, V207, and 

V208), any of which can be equipped with high temperature delay lines. 

DLHT-1, -2, and -3 delay lines may be used on surfaces up to 260° C or 500° 

F. DLHT-101, -201, and -301 delay lines may be used on surfaces up to 175° 

C or 350° F. These transducers and delay lines are listed on the Delay Line 

Transducer List. 

We also offers special high temperature wedges for use with angle beam 

transducers, the ABWHT series for use up to 260° C or 500° F and the 

ABWVHT series for use up to 480° C or 900° F. Detailed information on 

available sizes is available from the Sales Department.

2.2 High Temperature Couplants 

Most common ultrasonic couplants such as propylene glycol, glycerin, and 

ultrasonic gels will quickly vaporize if used on surfaces hotter than 

approximately 100° C or 200° F. Thus, ultrasonic testing at high temperatures 

requires specially formulated couplants that will remain in a stable liquid or 

paste form without boiling off, burning, or releasing toxic fumes. It is important 

to be aware of the specified temperature range for their use, and use them 

only within that range. Poor acoustic performance and/or safety hazards may 

result from using high temperature couplants beyond their intended range. 

At very high temperatures, even specialized high temperature couplants must 

be used quickly since they will tend to dry out or solidify and no longer 

transmit ultrasonic energy. Dried couplant residue should be removed from 

the test surface and the transducer before the next measurement.

Note that normal incidence shear wave coupling is generally not possible at 

elevated temperatures because commercial shear wave couplants will liquify 

and lose the very high viscosity that is necessary for transmission of shear 

waves. 

We offer two types of high temperature couplant: 

■ Couplant E - Ultratherm Recommended for use between 500° and 

970° F (260° to 520° C) 

■ Couplant G - Medium Temperature Couplant Recommended for use at 

temperatures up to 600° F (315° C). 

For a complete list of couplants available from Olympus, along with further 

notes on each, please refer to the application note on Ultrasonic Couplants.

Keyword: 

Note that normal incidence shear wave coupling is generally not possible at 

elevated temperatures because commercial shear wave couplants will liquify 

and lose the very high viscosity that is necessary for transmission of shear 

waves. 

http://www.olympus-ims.com/en/applications/normal-incidence-shear-wave-transducers/ 

http://static5.olympus-ims.com/data/Flash/shear_wave.swf?rev=3970 

http://www.olympus-ims.com/en/ultrasonic-transducers/shear-wave/

2.3 Test Techniques 

The following factors should always be taken into consideration in 

establishing a test procedure for any high temperature application: 

Transducer Time of Contacts 

All standard high temperature transducers are designed with a duty cycle in 

mind. Although the delay line insulates the interior of the transducer, lengthy 

contact with very hot surfaces will cause significant heat buildup, and 

eventually permanent damage to the transducer if the interior temperature 

becomes hot enough. For most dual element and delay line transducers, the 

recommended duty cycle for surface temperatures between approximately 

90° C and 425° C (200° F to 800° F) is no more than ten seconds of contact 

with the hot surface (five seconds is recomended), followed by a minimum of 

one minute of air cooling. Note that this is guideline only; the ratio of contact 

time to cooling time becomes more critical at the upper end of a given 

transducer's specified temperature range.

As a general rule, if the outer case of the transducer becomes too hot to 

comfortably hold with bare fingers, then the interior temperature of the 

transducer is reaching a potentially damaging temperature and the transducer 

must be allowed to cool down before testing continues. 

Some users have employed water cooling to accelerate the cooling process, 

however Olympus publishes no official guidelines for water cooling and its 

appropriateness must be determined by the individual user 

Keyword: 

■ 

■ 

10 second contact follows by 60 second air cooling 

Water cooling is not guarantee by Olympus NDT

Coupling Technique: The combination of transducer duty cycle 

requirements and the tendency of couplants to solidify or boil off at the upper 

end of their usable thickness range requires quick work on the part of the 

operator. Many users have found the best technique to be to apply a drop of 

couplant to the face of the transducer and then press the transducer firmly to 

the test surface, without twisting or grinding it (which can cause transducer 

wear). Any dried couplant residue should be removed from the transducer tip 

between measurements.

2.4 Equipment Functions 

Freeze Function 

Olympus Epoch series flaw detectors and all thickness gages have freeze 

functions that can be used to freeze the displayed waveform and reading. The 

freeze function is very useful in high temperature measurements because it 

allows the operator to capture a reading and quickly remove the transducer 

from the hot surface. With gages, the fast screen update mode should be 

used to help minimize contact time. 

High Gain Boost 

Gain Boost: The 38DL PLUS and 45MG gages have user adjustable gain 

boost functions, as do all Epoch series flaw detectors. Because of the higher 

attenuation levels associated with high temperature measurements, it is often 

useful to increase gain before making measurements.

3.0 High Temperature Testing and Variability 

3.1 Velocity Variation: 

Sound velocity in all materials changes with temperature, slowing down as 

the material heats up. Accurate thickness gaging of hot materials always 

requires velocity recalibration. In steel, this velocity change is approximately 

1% per 55°C or 100°F change in temperature. (The exact value varies 

depending on the alloy.) In plastics and other polymers, this change is much 

greater, and can approach 50% per 55°C or 100°F change in temperature up 

to the melting point. If a temperature/velocity plot for the material is not 

available, then a velocity calibration should be performed on a sample of the 

test material at the actual test temperature. The temperature compensation 

software function in the 38DL PLUS gage can be used to automatically adjust 

velocity for known elevated temperatures based on a programmed 

temperature/velocity constant. 

Keyword: 

■ Velocity change of -1% (minus) per 55°C or 100°F change in temperature 

■ Temperature versus velocity plot

Keyword: 

■ 

■ 

Velocity change of -1% (minus) per 55°C or 100°F change in temperature 

Temperature versus velocity plot

3.2 Zero Recalibration: 

When performing thickness gaging with dual element transducers, remember 

that the zero offset value for a given transducer will change as it heats up due 

to changes in transit time through the delay line. Thus, periodic re-zeroing is 

necessary to maintain measurement accuracy. With Olympus corrosion 

gages this can be quickly and easily done through the gage's auto-zero 

function; simply press the 2nd Function > DO ZERO keys.

3.3 Increased Attenuation: 

Sound attenuation in all materials increases with temperature, and the effect 

is much more pronounced in plastics than in metals or ceramics. In typical 

fine grain carbon steel alloys, attenuation at 5 MHz at room temperature is 

approximately 2 dB per 100 mm one-way sound path (equivalent to a round 

trip path of 50 mm each way). At 500°C or 930°C, attenuation increases to 

approximately 15 dB per 100 mm of sound path. This effect can require use 

of significantly increased instrument gain when testing over long sound paths 

at high temperature, and can also require adjustment to distance/amplitude 

correction (DAC) curves or TVG (Time Varied Gain) programs that were 

established at room temperature. 

Temperature/attenuation effects in polymers are highly material dependent, 

but will be typically be several times greater than the above numbers for steel. 

In particular, long high temperature delay lines that have heated up may 

represent a significant source of total attenuation in a test.

Keyword: 

• In typical fine grain carbon steel alloys, attenuation at 5 MHz at room 

temperature is approximately 2 dB per 100 mm one-way sound path 

(equivalent to a round trip path of 50 mm each way). 

• At 500°C or 930°C, attenuation increases to approximately 15 dB per 100 

mm of sound path.

3.4 Angular Variation in Wedges: 

With any high temperature wedge, sound velocity in the wedge material will 

decrease as it heats up, and thus the refracted angle in metals will increase 

as the wedge heats up. If this is of concern in a given test, refracted angle 

should be verified at actual operating temperature. As a practical matter, 

thermal variations during testing will often make precise determination of the 

actual refracted angle difficult. 

Keyword: 

As a practical matter, thermal variations during testing will often make precise 

determination of the actual refracted angle difficult.

Discussion: An offshore installation of Topside to Jacket Legs, hot 

conventional Ultrasonic Testing at elevated temperature below 500 C was 

proposed. What are the critical information to be reviewed? 

Hints: 

High temperature testing methods used & limitations 

Variability due to high temperature & concerns

6.9: Dimension-Measurement Applications

6.9.1 Dimension-Measurement Applications 

Ultrasonic inspection methods can be used for measurement of metal 

thickness. These same methods can also be used to monitor the deterioration 

of a surface and subsequent thinning of a part due to wear or corrosion and to 

determine the position of a solid object or liquid material in a closed metallic 

cavity.

6.9.2 Thickness measurements 

are made using pulse-echo techniques. Resonance techniques were also 

used in the past, but have become obsolete. The results can be read on an 

oscilloscope screen or on a meter, or they can be printed out. Also, the same 

data signals can be fed through gates to operate sorting or marking devices 

or to sound alarms. Resonance thickness testing was most often applied to 

process control inspection where opposite sides of the test pieces are smooth 

and parallel, such as in the inspection of hollow extrusions, drawn tubes, tube 

bends, flat sheet and plate, or electroplated parts. 

The maximum frequency that can be used for the test determines the 

minimum thickness that can be measured. The maximum thickness that can 

be measured depends on such test conditions as couplant characteristics, 

test frequency, and instrument design and on material type, metallurgical 

condition, and surface roughness.

Pulse-echo thickness gages with a digital readout are widely used for 

thickness measurement. Pulse-echo testing can measure such great 

thickness that it can determine the length of a steel reinforcing rod in a 

concrete structure, provided one end of the rod is accessible for contact by 

the search unit. Although pulse-echo testing is capable of measuring 

considerable thicknesses, near-field effects make the use of pulse-echo 

testing ineffective on very thin materials.

6.9.3 Position measurements 

Position measurements of solid parts or liquid materials in closed metallic 

cavities are usually made with pulse echo type equipment. One technique is 

to look for changes in back reflection intensity as the position of the search 

unit is changed. In one variation of this technique, the oil level in differential 

housings was checked to see if the automated equipment used to put the oil 

in the housing on an-assembly line had malfunctioned. The test developed for 

this application utilized a dual-gated pulse-echo system that employed a 1.6- 

MHz immersion-type search unit with a thin, oil filled rubber gland over its 

face. The search unit was automatically placed against the outside surface of 

the housing just below the proper oil level, as shown in Fig. 60(a).

With oil at the correct level, sufficient beam energy was transmitted across 

the boundary between the housing wall and the oil to attenuate the reflected 

beam so that multiple back reflections were all contained in the first gate (Fig. 

60b). The lack of oil at the correct level allowed the multiple back reflections 

to spill over into the second gate (Fig. 60c). Thus, the test was a fail-safe test 

that signaled "no test" (no signal in the first gate), "go" (signals in the first gate 

only), and "no go" (signals in both gates).

Fig. 60 Method of determining correct oil level in on automobile differential 

housing by use of an ultrasonic pulse-echo system. See text.

In another position measurement system, a set of two contact-type 4-MHz 

search units was utilized in a through transmission pitch-catch arrangement to 

determine the movement of a piston in a hydraulic oil accumulator as both 

precharge nitrogen-gas pressure and standby oil pressure varied (Fig. 61). 

The two search units were placed 180° apart on the outside surface of the 

accumulator wall at a position on the oil side of the piston, as shown in Fig. 

61. 

When a high energy pulse was sent from the transmitting unit, the beam was 

able to travel straight through the oil, and a strong signal was picked up by 

the receiving unit. However, as the search units were moved toward the 

piston (see locations drawn in phantom in Fig. 61), the sloping sides of the 

recess in the piston bottom deflected the beam so that very little signal was 

detected by the receiving unit.

Fig. 61 Setup for determining the position of a piston in a hydraulic oil 

accumulator by use of two contact search units utilizing a through 

transmission arrangement

Q144. A thin sheet may be inspected with the ultrasonic wavw direction 

normal to the surface by observing: 

A. The amplitude of the front surface reflection 

B. The multiple reflection pattern 

C. All front surface reflection 


6.10: In-Service Inspection

In-Service Inspection 

The methods described above are applied in the course of and immediately 

after the production process and are therefore called production tests. To 

survey highly stressed parts, especially in power plants, repeated tests or inservice 

inspections are becoming more and more important. In these 

inspections any defects identified earlier but not being a cause for rejection 

can be observed for any changes caused by the service conditions. In 

addition service-produced defects must be detected, these being mainly 

cracks caused by thermal shock, fatigue or creep, or by corrosion attack.

In-Service Inspection- Testing for fatigue cracks on crankshafts and 

crankpins. a Without bore; b with bore

In-Service Inspection- Oblique or skewed fatigue cracks on crankpins

In-Service Inspection- (a) Crack test on press columns, pump rods, etc. 

(b) Crack test on thread in the shadow of a sound beam; schematic screen 

picture above

In-Service Inspection- (a) Probe for detecting fatigue cracks in turbine discs 

(design Krautkriimer-Branson) (b) Detection of cracks in riveted turbine 

blades

In-Service Inspection- (a) Testing methods for conical defects in a bolt 

(b) Testing for fatigue cracks in bolts

In-Service Inspection- (a) Cross-section through a leaf spring for railway 

cars with quenching crack showing testing with small angle probe or normal 

probe. The use of surface waves is unfavorable due to roughness 

(b)Testing a helical spring for quenching cracks, using surface waves

6.11: Casting

Casting 

In castings flaw detection is almost exclusively concerned with manufacturing 

defects and only rarely as in-service inspection. Suitable testing techniques 

and the subsequent evaluation of indications in castings is very different from 

the testing of forged and worked material so that the differences must not be 

forgotten or difficulties can occur. In-service inspection, as in the case of 

forgings, depends on the local stresses and the piece geometry so it is not 

necessary to treat it specially in this section.

Casting- Typical casting defects and their detection methods

Casting

Casting- Detection of shrinkage cavities with normal and angle probes

6.12: Bonded Joint

Inspection of Bonded Joints 

If the shape of a joint is favorable, ultrasonic inspection can be used to 

determine the soundness of joints bonded either adhesively or by any of the 

various metallurgical methods, including brazing and soldering. Both pulseecho 

and resonance techniques have been used to evaluate bond quality in 

brazed joints. 

A babbitted sleeve bearing is a typical part having a metallurgical bond that is 

ultrasonically inspected for flaws. The bond between babbitt and backing 

shell is inspected with a straight-beam pulse-echo technique, using a contacttype 

search unit applied to the outside of the steel shell. A small-diameter 

search unit is used to ensure adequate contact with the shell through the 

couplant. Before inspection, the outside of the steel shell and the inside of the 

cast babbitt liner are machined to a maximum surface roughness of 3.20 μm 

(125 μ in.) (but the liner is not machined to final thickness).

During inspection, the oscilloscope screen normally shows three indications: 

the initial pulse, a small echo from the bond line (due to differences in 

acoustical impedance of steel and babbitt), and the back reflection from the 

inside surface of the liner. Regions where the bond line indication is minimum 

are assumed to have an acceptable bond. Where the bond line signal 

increases, the bond is questionable. Where there is no back reflection at all 

from the inside surface of the liner (babbitt), there is no bond. 

Inspection of other types of bonded joints is often done in a manner similar to 

that described above for babbitted bearings. An extensive discussion of the 

ultrasonic inspection of various types of adhesive-bonded joints (including 

two-component lap joints, three component sandwich structures, and 

multiple-component laminated structures) is available in the article "Adhesive- 

Bonded Joints" in this Volume.

6.13: Corrosion Monitoring

Corrosion Monitoring 

Ultrasonic inspection can be used for the in situ monitoring of corrosion by 

measuring the thickness of vessel walls with ultrasonic thickness gages. The 

advantage of this method is that internal corrosion of a vessel can be 

monitored without penetration. 

There are, however, some disadvantages. Serious problems may exist in 

equipment that has a metallurgically bonded internal lining, because it is not 

obvious from which surface the returning signal will originate. A poor surface 

finish, paint, or a vessel at high or low temperature may also complicate the 

use of contact piezoelectric transducers (although this difficulty might be 

addressed by noncontact in situ inspection with an EMA transducer).

Despite these drawbacks, ultrasonic thickness measurements are widely 

used to determine corrosion rates. To obtain a corrosion rate, a series of 

thickness measurements is made over an interval of time, and the metal loss 

per unit time is determined from the measurement samples. Hand-held 

ultrasonic thickness gages are suitable for these measurements and are 

relatively easy to use. 

However, depending on the type of transducer used, the ultrasonic thickness 

method can overestimate metal thicknesses when the remaining thickness is 

under approximately 1.3 mm (0.05 in.). Another corrosion inspection method 

consists of monitoring back-surface roughness with ultrasonic techniques. 

The following example describes an application of this method in the 

monitoring of nuclear waste containers.

6.14: Crack Monitoring

Crack Monitoring 

Laboratory and in-service monitoring of the initiation and propagation of 

cracks that are relatively slow growing (such as fatigue cracks, stress-rupture 

cracks, and stress-corrosion cracks) has been accomplished with ultrasonic 

techniques. An example of the ultrasonic detection of stress-rupture cracks 

resulting from creep in reformer-furnace headers is given in the article 

"Boilers and Pressure Vessels" in this Volume. A relatively new and improved 

approach for monitoring the growth of cracks is done with ultrasonic imaging 

techniques.

Monitoring of fatigue cracks in parts during laboratory tests and while in 

service in the field has been extensively done using ultrasonic techniques. 

Reference 13 describes the use of surface waves to detect the initiation of 

cracks in cylindrical compression-fatigue test pieces having a circumferential 

notch. The surface waves, which were produced by four angle-beam search 

units on the circumference of each test piece, were able to follow the contour 

of the notch and detect the cracks at the notch root. 

Monitoring the crack-growth rate was accomplished by periodically removing 

the cracked test piece from the stressing rig and measuring the crack size by 

straight-beam, pulse-echo immersion inspection. It was found necessary to 

break open some of the cracked test pieces (using impact at low temperature) 

and visually measure the crack to establish an accurate calibration curve of 

indication height versus crack size.

The use of pulse-echo techniques for monitoring fatigue cracks in pressure 

vessels in laboratory tests is described in Ref 14. These techniques use 

several overlapping angle-beam (shear wave) search units, which are glued 

in place to ensure reproducible results as fatigue testing proceeded. The inservice 

monitoring of fatigue cracking of machine components is often 

accomplished without removing the component from its assembly.

For example, 150 mm (6 in.) diam, 8100 mm (320 in.) long shafts used in 

pressure rolls in papermaking machinery developed fatigue cracks in their 

500 mm (20 in.) long threaded end sections after long and severe service. 

These cracks were detected and measured at 3-month intervals, using a 

contact-type straight-beam search unit placed on the end of each shaft, 

without removing the shaft from the machine. 

When the cracks were found to cover over 25% of the cross section of a shaft, 

the shaft was removed and replaced. In another case, fatigue cracking in a 

weld joining components of the shell of a ball mill 4.3 m (14 ft) in diameter by 

9.1 m (30 ft) long was monitored using contact type angle-beam search units. 

The testing was done at 3-month intervals until a crack was detected; then it 

was monitored more frequently. When a crack reached a length of 150 mm (6 

in.), milling was halted and the crack repaired.

6.15: Stress Measurements

Stress Measurements 

With ultrasonic techniques, the velocity of ultrasonic waves in materials can 

be measured and related to stress (Ref 16). These techniques rely on the 

small velocity changes caused by the presence of stress, which is known as 

an acousto-elastic effect. The technique is difficult to apply because of the 

very small changes in velocity with changes in stress and because of the 

difficulty in distinguishing stress effects from material variations (such as 

texture; see Ref 17). However, with the increased ability to time the arrival of 

ultrasonic pulses accurately (±1 ns), the technique has become feasible for a 

few practical applications, such as the measurement of axial loads in steel 

bolts and the measurement of residual stress (Ref 5). 

.

The real limitation of this technique is that in many materials the ultrasonic 

pulse becomes distorted, which can reduce the accuracy of the measurement. 

One way to avoid this problem is to measure the phase difference between 

two-tone bursts by changing the frequency to keep the phase difference 

constant (Ref 5). Small specimens are used in a water bath, and the pulses 

received from the front and back surfaces overlap. The presence of stress 

also rotates the plane of polarization of polarized shear waves, and there is 

some correlation between the angle of rotation and the magnitude of the 

stress. Measurement of this rotation can be used to measure the internal 

stress averaged over the volume of material traversed by the ultrasonic beam.

6.16: Bond Testing

The real limitation of






6.App-1: TOFD Introduction 

NOTE: Not in the exam syllabus or BOK

6.App-1.1 TOFD Basic Theory 

TOFD is usually performed using longitudinal waves as the primary detection 

method. Ultrasonic sensors are placed on each side of the weld. One sensor 

sends the ultrasonic beam into the material and the other sensor receives 

reflected and diffracted ultrasound from anomalies and geometric reflectors.

TOFD provides a wide area of coverage with a single beam by exploiting 

ultrasonic beam spread theory inside the wedge and the inspected material. 

When the beam comes in contact with the tip of a flaw, or crack, diffracted 

energy is cast in all directions. Measuring the time of flight of the diffracted 

beams enables accurate and reliable flaw detection and sizing, even if the 

crack is off-oriented to the initial beam direction. 

During typical TOFD inspections, A-scans are collected and used to create B- 

scan (side view) images of the weld. Analysis is done on the acquisition unit 

or in post-analysis software, positioning cursors to measure the length and 

through-wall height of flaws. 

Keywords: 

■ 

■ 

■ 

■ 

■ 

Tip Diffraction 

Off-oriented to the initial beam direction 

Time of Flight 

A-scan / B-scan 

Post analysis software

6.App-1.2 

Main Benefits of TOFD for Weld Inspection 

• Based on diffraction, so relatively indifferent to weld bevel angles and flaw 

orientation 

• Uses time of arrival of signals received from crack tips for accurate defect 

positioning and sizing 

• Precise sizing capability makes it an ideal flaw monitoring method 

• Quick to set up and perform an inspection, as a single beam offers a large 

area of coverage 

• Rapid scanning with imaging and full data recording 

• Can also be used for corrosion inspections 

• Required equipment is more economical than phased array, due to 

conventional nature (single pulser and receiver) and use of conventional 

probes 

• Highly sensitive to all weld flaw types

TOFD offers rapid weld inspection with excellent flaw detection and sizing 

capacities. The diffraction technique provides critical sizing capability with 

relative indifference to bevel angle or flaw orientation. TOFD can be utilized 

on its own or in conjunction with other NDT techniques.

6.App-1.3 

6.App-1.3.1 The Theory 

More Reading on Time of Flight Diffraction (TOFD) 

Time of flight diffraction (TOFD) detects flaws using the signals diffracted from 

the flaw’s extremities. Two angled compression wave probes are used in 

transmit-receive mode, one each side of the weld. The beam divergence is 

such that the majority of the thickness is inspected, although, for thicker 

components, more than one probe separation may be required. When the 

sound strikes the tip of a crack, this acts as a secondary emitter which 

scatters sound out in all directions, some in the direction of the receiving 

probe. A ‘lateral wave’ travelling at the same velocity as the compression 

waves, travels directly from the transmitter to the receiver. The time difference 

between the lateral wave and the diffracted signal from the flaw 

provides a measure of its distance from the scanned surface. 

If the flaw is large enough in the through wall dimension, it may 

be possible to resolve the tip diffracted signals from its top and 

bottom, thereby allowing the through wall height of the flaw to be 

measured. 

http://www.iteglobal.com/services/advanced-ndt/time-of-flight-diffraction-tofd/

Due to the low amplitude of the diffracted signals, TOFD is usually carried out 

using a preamplifier and hardware designed to improve signal-to-noise 

performance. As the probes are scanned along the weld, the RF A-Scan 

signals are digitised and displayed in the form of a grey-scale image showing 

flaws as alternating white and black fringes. 

Depending on which direction the probes are moved over the component 

surface, it is possible to construct ‘end-view’; (B-scan TOFD) or ‘side-view’ 

(D-scan TOFD) cross-sectional slices. TOFD can also utilise Synthetic 

Aperture focusing or beam modelling software to minimise the effects of 

beam divergence, thereby providing more accurate location and sizing 

information.

TOFD is generally recognised as the most accurate ultrasonic technique for 

measuring the through-wall height of planar flaws that lie perpendicular to the 

surface and as a method for detecting and quantifying crevice corrosion at the 

weld root. At present, national standards for the application of TOFD exist, 

however, no acceptance criteria have been agreed upon. 

The TOFD technique is suited for the detection and sizing of all types of 

embedded flaws, especially those planar in nature. However, the detection of 

small near the scan surface flaws can be more difficult due to the presence of 

the lateral wave response which often occupies several millimeters of the 

depth axis on images.

Tips Diffractions

TOFD 

Transmitter 

Receiver 

Crack 

Back-wall echo 

Diffracted wave from upper end of crack 

Diffracted wave from lower end of crack 

Crack height can be calculate by measuring propagation 

delayed time of diffraction wave 

Diffracted 

wave from 

upper end of 

crack 

Lateral wave 

Diffracted wave from lower end of crack

TOFD

6.App-1.2 Application Examples 

■ 

TOFD for Weld Root Corrosion and Erosion 

For piping and other flow systems, certain conditions exist that lead to 

corrosion and erosion in the weld root and the heat-affected zone (HAZ) of 

the weld. The contributing factors are often metallurgical, chemical, or flow 

related, and the resulting metal loss can lead to failure of the weld/base metal. 

The shape of the corroded or eroded weld or base metal can make ultrasonic 

inspection extremely difficult to apply, thus impeding accurate detection and 

measurement of anomalies. 

The time-of-flight diffraction (TOFD) technique proves to be a valid option for 

evaluating weld root corrosion and erosion, as well as similar conditions such 

as FAC (flow-accelerated corrosion). The goal of any of these inspections is 

to accurately measure the wall thickness, the weld, and the HAZ. The 

unpredictable shape of the remaining material often makes pulse-echo 

ultrasonic inspection ineffective. 

http://www.olympus-ims.com/en/applications/tofd-for-weld-root-corrosion-and-erosion/

TOFD has been used for some time for general weld inspections. It has 

proven to be a rapid and easily deployable method with an excellent capacity 

for sizing. One of the inherent strengths of TOFD for detection and sizing 

purposes is its relative indifference to the orientation of defects because of its 

primary use of diffracted versus reflected energy. 

The TOFD technique utilizes two transducers: a transmitter transducer floods 

the inspected region with sound in the forward direction; on the opposite side 

of the weld, a receiver transducer is positioned to receive diffracted and 

reflected energy from the back wall or from anomalies present in the region. 

Common pulse-echo techniques can be misdirected by the shape of the 

region, resulting in imprecise measurement and assessment.

Figure 5-3 – Preferential weld corrosion in lean amine (Reference 5)

Figure 5-2 – Hot Lean Amine Corrosion of Carbon Steel:

Weld Root Corrosion and Erosion 

Pulse-echo shear wave beam being reflected at an off angle. 

Illustration of diffracted energy reflecting off weld root/HAZ in all directions.

For these types of weld inspections, TOFD is typically performed from three 

positions for each weld: (1) centered on the weld, (2) offset to the left, and (3) 

offset to the right. 

Scanning from these particular positions helps to achieve the best results. 

This method ensures detection of the highest point of material loss, 

determines from which side of the weld the erosion/corrosion indications are 

originating, and eliminates any masking caused by the back wall signal. 

Depending on the instrument, these scans can be run concurrently or in 

separate acquisitions.

TOFD is deployed by scanning the weld with a semiautomatic or fully 

automatic scanner. Scan settings are set to determine scan resolution. The 

resulting data file can be saved indefinitely for review and comparison to 

future scans. After data is acquired, it is analyzed to identify any areas of 

concern, either directly on the instrument or in post-analysis software. Shifts 

in data (time/depth) are measured in order to assess the severity of metal 

loss. The cursors can then be positioned to define areas for depth or 

thickness measurement readings. Weld defects such as porosity, lack of 

fusion, and cracking can also be detected when scanning for corrosion and 

erosion.

Scan of weld with cursor positioned on an uncorroded area; A-scan shows 

good lateral wave and back wall signal with no indications in between.

Scan of weld with cursor positioned on a corroded area; A-scan shows shift in 

time of back wall signal from material loss.

Measurement of good area shows thickness as 7.39 mm; TOFD (m-r) reading 

shows the distance between the positioned cursors.

Measurement of corroded area shows thickness as 5.28 mm; cursors are 

positioned at top of plate (0) and highest point of material loss. In this 

example, there is 2.11 mm of material loss due to corrosion.

6.11.3.3TOFD for Corrosion Measurement Equipment (Typical) 

• OmniScan SX or MX2 (PA or UT models, depending on the number of 

channels desired and if phased array capability is needed). 

• TOFD circumferential scanner (HST-Lite or similar, depending on the 

desired number of probe holders and other application specifics; for 

example, pipe versus plate). 

• TOFD probe and wedges (various frequencies, angles, and materials). 

• Couplant delivery system, WTR-SPRAYER-8L or similar. 

• TomoView Analysis or OmniPC post-analysis software (optional).

6.App-1.3.4 

TOFD Benefits for Corrosion/Erosion Measurement 

• Rapid scanning. 

• Cost effective. 

• Auditable and retrievable permanent data sets. 

• Accurate sizing capability. 

• Excellent detection, even on irregularly shaped areas of metal loss. 

• Fast post-acquisition analysis results. 

• Portable and user-friendly TOFD scanning packages.

TOFD for Weld- TOFD Parallel Scanning

6.App-1.3.5 

Overview on Scanning Direction 

Most typical TOFD inspections are performed with the send and receive 

transducers on opposite sides of the weld and scanning movement parallel to 

the weld axis. The main purpose of this “perpendicular” (defined by beam to 

weld relationship) scanning is to quickly perform weld inspection with the weld 

cap or re-enforcement in place. This technique can give location in the scan 

axis, the indication length, height of indication and flaw characterization 

information. One of the weaknesses of this technique is the lack of index 

positioning (or where between the probes) the indication is located. This 

information is usually obtained with complimentary pulse echo ultrasonics 

when the weld is left in place.

■ 

Perpendicular Scanning 

Scanning direction “parallel” to the weld axis. Beam direction “perpendicular” 

to the weld axis. 

? Carriage movement 

direction 

One of the weaknesses of this technique is the lack of index positioning (or 

where between the probes) the indication is located.

■ 

Parallel TOFD scanning: 

Where the scan direction and beam direction are the same is less used, for 

obvious reasons of not being able to cover the entire length of weld rapidly, 

more complex movement pattern required of scanner mechanisms, and 

complexity of the data output of an entire weld inspected. This technique does 

have advantages when it is possible to be performed.

Typical “Perpendicular” Weld Scanning Setup and Data Collected. Data is 

side view of weld from scan start to scan finish down the weld. Position of 

encoder and scanning direction are highlighted.

Typical “Parallel” Weld Scanning Setup and Data Collected. Data is side view 

of weld from scan start to scan finish across the weld. Position of encoder and 

scanning direction are highlighted.

■ 

Benefit of TOFD Parallel Scanning 

Although perpendicular TOFD scanning down the weld can give highly 

accurate depth measurement, generally speaking a parallel scan will give 

more accurate depth information as well as flaw information, and location in 

the index position in the weld. With perpendicular scanning, no index position 

is possible without multiple offset scans being performed or complimentary 

NDT techniques to position the flaw. In parallel scanning Index position is 

ascertained by locating the minimum time peak, which corresponds to when 

the indication is centered between the two probes. For these reasons this 

technique is often used in critical crack sizing inspections, as well as change 

monitoring, in other words, monitoring a crack or other defect for growth until 

it reaches a critical level at which time it is repaired or replaced. For these 

reasons the technique is often performed on critical components that are 

costly to shut down for repair, often in the Power Generation industry. More 

information is often gathered from the flaw as diffraction occurs across the 

flaw instead of just down the flaw.

6.App-1.3.6 Further Reading- Introduction to Phased Array 

• http://www.olympus-ims.com/en/ndt-tutorials/intro/ut/

The Experts at work.

Break Time 

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/39440/v0001/reflector:20587?BBC- 

UID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c540f1856834&SSO2-UID

Break Time 



Break Time 



Break Time

Sail Off

Section 7: 

Reference Materials

Content: Section 7: Reference Material 



7.3: Video Time


Acoustic Properties - Piezoelectric Materials 

Acoustic Properties - Transducers 

Acoustic Properties - Metals 

Acoustic Properties - Powdered Metals 

Acoustic Properties - Liquid Metals 

Acoustic Properties - Plastics, Resins 

Acoustic Properties - Rubber 

Acoustic Properties - Ceramics 

Acoustic Properties - Wood 

Acoustic Properties - Liquids 

Acoustic Properties - Liquid Gases 

Acoustic Properties - Gases 

Acoustic Properties - Vapors 

Acoustic Properties - Body Tissue 

https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Reference%20Information/matproperties.htm


Auld, B.A., Acoustic Fields and Waves in Solids, Vol I & II, 2nd edition Krieger 

Publishing Company, February 1990; ISBN: 089874783X 

Cartz, Louis, Nondestructive Testing : Radiography, Ultrasonics, Liquid 

Penetrant, Magnetic Particle, Eddy Current, ASM Intl; ISBN: 0871705176 

Krautkramer, Josef and Krautkramer, Herbert, Ultrasonic Testing of Materials, 

4th/revised edition, Springer Verlag, November 1990, ISBN: 0387512314 

Diederichs, Rolf and Ginzel, Ed, Nondestructive Testing Encyclopedia, UT 

Formulae, NDT net 

http://www.ndt.net/ndtaz/ndtaz.php 

Ultrasonic Characterization of Materials, NIST, Materials Reliability Division

7.3: Video Time

Calibrating 70° Probe with IIW Block (50%FSH on 1.5mm SDH) to AWS D1.1 

(Repeat-Code1) 

www.youtube.com/embed/Qr0dGNuq9yY

Section 8: Ultrasonic Inspection Quizzes

Content: Section 8: Ultrasonic Inspection Quizzes 




http://www-pub.iaea.org/MTCD/publications/PDF/TCS-45_web.pdf

Ultrasonic Inspection Quizzes

Ultrasonic Inspection Quizzes 

http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/mineralsmetals/files/pdf/ndt-end/rad-rad-eng.pdf


https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Quiz/UT%20Quizzes.htm

http://www.ndtcalc.com/index.php?page=quiz&method=ut&qs=10

http://www.studyblue.com/notes/note/n/ut-asnt-level-ii/deck/6278710

Addendum-01a 

Equipment Calibrations 

My ASNT Level III UT Study Notes. 

2014-June

Normal Beams Calibration Techniques

Attenuation due to Beam spread for: 

• Large Reflector 

• Small Reflector

Attenuation Due to Beam Spread: Large Reflector

Attenuation Due to Beam Spread: Small Reflector 

D1 

SH1 

D2 

SH2

Material Attenuation Determination:

Material Attenuation Determination: Actual BWE display

IF zero Material attenuation: The second BWE at twice the distance will be 

exactly 6dB less (50% less), half the 1st BWE height ( ½ FSH). However this 

is never the case!

Δ dB = total Material attenuation at twice distance travel. 

Material attenuation = 

ΔdB 

D1 

D2

Material Attenuation in 100mm = YdB-XdB-6dB 

Material attenuation in dB/mm = (YdB-XdB-6dB)/100 

YdB-XdB-6dB 

X dB 

Y dB

Construction of beam edges plot- Normal Transducer

Construction of Beam Edges:

20dB drop to find edges of beam

The other edge:

Construction of beam spread at 13mm:



Angle Beams Calibration Techniques

Perspex as Matching Layer/Wedge 

Tunsten impregnated 

epoxy resin 

θs 1 

2730m/s 

θs 2 

3250m/s

Perspex as Matching Layer/Wedge 

1. The Shear wave velocity of Perspex is 2730m/s, the shear wave velocity 

od steel is 3250m/s. The refracted angle of Perspex ϴ S1 is always smaller 

than ϴ S2 

2. Pespex is very absortive and attenuated efficiently, thus reflected 

compressional wavw will be dampen.

First/ Second Critical Angles 

V L1 = 2730m/s, 

V S2 = 3250m/s, V L2 = 5900m/s 

1 st / 2 nd critical angle 

27.56° 

57.14° 

° 

I st Critical angle= 27.56° 

2 nd Critical angle= 57.14° 

B 

33.42°

First/ Second Critical Angles 

27.56° 

57.14° 

° 

33.42°

Finding the probe index

Finding the probe index

Checking the probe Angle:

Calibration for range:

Calibration for range:

Angle Beam- Beam edges Proving (Vertical Axis) using IOW Block 

Stand Off Measurement Techniques. 

Stand-off 2 

Stand-off 1 

Stand off 2


Botoom edge.


Bottom Edge.

The IOW Block: The Institute of Welding Block

The Proofing: 

Plot out the Stand-Off1 & 2 readings on a transparent slide, superimposed the 

ploted transparent slide on IOW Block

Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block


Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block 

Scanned at 

½, 1, 1 ½Skips


Angle Beam- Beam edges Proving (Horizontal Axis) using IIW Block 

½Skip 

1 Skip 

1½ Skip

The DAC

The DAC

DAC Curve

DAC Curve

DAC Curve Plot 

1. Obtained the signal from the refernce reflector and mark on the 

graticule/traspatrent sheet with gain setting at 80% FSH. 

2. Set the gain control -6bB and marks the 50% mark. 

3. Set the gain contril to the 

4. Obtained the signal at the gain setting in item 1 and repeat the process at 

different sound paths. 

5. Plot the curves at the gain setting and -6dB. 

6. Determined the transfer correction. 

7. Scanned the work pieces at the “Gain Setting + Transfer Correction”

FLAT Bottom Holes FBH

FLAT Bottom Holes FBH

Reading on: FLAT Bottom Holes FBH 

https://www.cnde.iastate.edu/ultrasonics/grain-noise

FLAT Bottom Holes FBH 

A type of reflector commonly used in reference standards. The end (bottom) 

surface of the hole is the reflector.E 

quivalent:, the size of a flat-bottom hole which at the same range, gives an 

ultrasonic indication equal to the one from the discontinuity. This reflector is 

used in DGS curves, or many calibration blocks, or standards such as the GE 

specification.

Transfer Corection

Transfer Correction: Reference surface are smooth and scale free unlike 

the actual work pieces. These call for transfer correction to account for 

transfer loss resulting from actual scanning.







Transfer Correction:

Transfer Correction: Comparison of BWE for Compression Probe 

Test Material curve 

Reference Block curve 

Gain Setting 

Transfer correction 

at thickness 

Measured point 

Beam path

Transfer Correction: Compression Probe Method, Plot a curve of gain 

setting for FSH at different south paths for actual and reference block, the 

different in gain control at thickness is the transfer correction.

Transfer Correction: Angle Probes Methos, used 2 eaqual angle probes, 

pitch and catch in the test material ans using the reference block. The 

differences in gain setting is the transfer correction,

DGS- Distance Gain Size 

http://www.sonostarndt.com/EnProductShow.asp?ID=198

FLAT Bottom Holes FBH 

■ 

DGS/AVG 

DGS is a sizing technique that relates the amplitude of the echo from a 

reflector to that of a flat bottom hold at the same depth or distance. This is 

known as Equivalent Reflector Size or ERS. DGS is an acronym for 

Distance/Gain/Size and is also known as AVG from its German name, 

Abstand Verstarkung Grosse. Traditionally this technique involved manually 

comparing echo amplitudes with printed curves, however contemporary 

digital flaw detectors can draw the curves following a calibration routine and 

automatically calculate the ERS of a gated peak. The generated curves are 

derived from the calculated beam spreading pattern of a given transducer, 

based on its frequency and element diameter using a single calibration point. 

Material attenuation and coupling variation in the calibration block and test 

specimen can be accounted for. 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/dgs-avg/

DGS is a primarily mathematical technique originally based on the ratio of a 

circular probe’s calculated beam profile and measurable material properties 

to circular disk reflectors. The technique has since been further applied to 

square element and even dual element probes, although for the latter, curve 

sets are empirically derived. It is always up to the user to determine how the 

resultant DGS calculations relate to actual flaws in real test pieces. 

An example of a typical DGS curve set is seen below. The uppermost curve 

(Curve #1) represents the relative amplitude of the echo from a flat plate 

reflector in decibels, plotted at various distances from the transducer, and the 

curves below (Curve #2) represent the relative amplitude of echoes from 

progressively smaller disk reflectors over the same distance scale.

(Curve #1) represents the relative amplitude of the echo from a flat plate 

reflector in decibels, plotted at various distances from the transducer

(Curve #2) represent the relative amplitude of echoes from progressively 

smaller disk reflectors over the same distance scale.

As implemented in contemporary digital flaw detectors, DGS curves are 

typically plotted based on a reference calibration off a known target such as a 

backwall reflector or a flat bottom hole at a given depth. From that one 

calibration point, an entire curve set can be drawn based on probe and 

material characteristics. Rather than plotting the entire curve set, instruments 

will typically display one curve based on a selected reflector size (registration 

level) that can be adjusted by the user. In the example below, the upper curve 

represents the DGS plot for a 2 mm disk reflector at depths from 10 mm to 50 

mm. The lower curve is a reference that has been plotted 6 dB lower. 

In the screen at left, the red gate marks the reflection from a 2 mm diameter 

flat bottom hole at approximately 20 mm depth. Since this reflector equals the 

selected registration level, the peak matches the curve at that depth. In the 

screen at right, a different reflector at a depth of approximately 26 mm has 

been gated. Based on its height and depth in relation to the curve the 

instrument calculated an ERS of 1.5 mm.

(Curve #2) represent the relative amplitude of echoes from progressively 

smaller disk reflectors over the same distance scale.

More reading on DGS

DGS- Different sizes of FBH at different distance

DGS 

# of near field

What is DGS 

TCG is a time-corrected DAC so that equal dimension reflectors give equal amplitude 

responses for all sound path distances. Used for PAUT Sectorial scans where it would 

be otherwise impossible to set every angle and sound path to the same sensitivity 

level using DAC's. 

ASTM E-1316: DGS (distance gain size-German AVG) distance amplitude curves 

permitting prediction of reflector size compared to the response from a back surface 

reflection. 

The probe manufacturer supplies data sheet diagams for each probe which shows the 

amplitude response curves from the backwall and a range of diameters of flat-bottom 

holes along the length of the soundfield. 

Have a look at EN 583-2:2001 Sensitivity and range setting for excellent authoritative 

descriptions of DAC/TCG and DGS. You'll have to look at AWS D1.1. for instance 

for knowledge of their sensitivity setting requirements. 

Knowledge of these techniques is desirable but will such knowledge really improve 

your inspection method? You use DAC because the Codes and standards you work to 

require you to assess indications to those DAC's. A report that a reflector was 3,5 mm 

equivalent FBH size to DGS would most probably be rejected.

DGS-If you have a signal feom a flaw at a certain depth, you can compare the 

signal of BWE from the FBH at that depth. The defect then could be sized as 

equivalent of the size of the FBH. 

Size 0.24 

Size 0.24 

2.4depth 

http://www.ndt.net/article/berke/berke_e.htm

Locating & Sizing Flaws

Locating reflectors with an angle-beam probe 

Fig. 53 Scanning a reflector using an angle beam probe 

The echo of a discontinuity on the instrument display does not now give us 

any direct information about its position in the material. The only available 

information for determination of the reflector position is the scale position and 

therefore the sound path s, this means the distance of the discontinuity from 

the index point (sound exit point) of the probe, Fig. 53. 

The mathematics of the right-angled triangle helps us to evaluate the Surface 

Distance and the Depth of a reflector which are both important for the 

ultrasonic test, Fig. 54a. We therefore now have the possibility to instantly 

mark a detected flaw's position on the surface of the test object by 

measurement of the surface distance from the sound exit point and to give 

the depth. For practical reasons, the reduced surface distance is used 

because this is measured from the front edge of the probe. The difference 

between the surface distance and the reduced surface distance corresponds 

to the x-value of the probe, this is the distance of the sound exit point to the 

front edge of the probe, Fig. 54b.

With ultrasonic instruments having digital echo evaluation these calculations 

are naturally carried out by an integrated microprocessor and immediately 

displayed so that the operator does not need to make any more timeconsuming 

calculations, Fig. 55. This is of great help with weld testing 

because with the calculation of the flaw depth an additional factor must be 

taken into account, namely: whether the sound pulses were reflected from the 

opposing wall. If this is the case then an apparent depth of the reflector is 

produced by using the depth formula which is greater than the thickness T of 

the test object. The ultrasonic operator must acertain whether a reflection 

comes from the opposite wall and then proceed with calculating the reflector 

depth, Fig. 56b.

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns

Scanning Patterns 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/common-test-practices/


81. The 100 mm radius in an IIW block is used to: 

(a) Calibrate sensitivity level 



(d) Check beam angle 

80. The 50 mm diameter hole in an IIW block is used to: 

(a) Determine the beam index point 





35. The 2 mm wide notch in the IIW block is used to: 

(a) Determine beam index point 




Addendum-01b 

Equipment Calibration 

My ASNT Level III UT Study Notes 

2014-June.

Pulse-Echo Instrumentation

The Circuitry: 

• Voltage activation of the PE crystal 

• Ultrasound formation 

• Propagation 

• Reflection 

• Charge formation of crystal 

• Processing 

• Display

Pulse-Echo Instrumentation 

Transmitter 

TRX 

Receiver 

Amplifier 

Detector 

Scan 

Converter 

Display 

TGC 

TGC – Time Gain Compensation Circuit


Pulser Components 

1. HV pulse generator 

2. The clock generator 

3. The transducer


Applied Voltage 

Generated Wave 

V 

+ + 

P 

TIME 

- 

TIME 

-


The Pulser rate is known as the pulse repetition frequency 

(PRF). 

Typical PRF 3,000 – 5,000. 

PRF automatically adjusted as a function of imaging depth.


Switch that controls the output power of the HV generator is 

the attenuator.


PULSER 

TRX 

ATTENUATOR


CLOCK GENERATOR 

Controls the actual number of pulses which activate the crystal. 

Responsible for sending timing signal to the 

1. Pulse generator 

2. TGC circuitry 

3. Memory


CLOCK 

GENERATOR 

TGC UNIT 

HV 

GENERATOR 

MEMORY 

TRS 

TRX 

CRT 

DISPLAY


Sensitivity refers to the weakest echo signal that the 

instrument is 

capable of detecting and displaying. 

Factors that determine sensitivity are 

1. Transducer frequency 

2. Overall and TGC receiver gain 

3. Reject control 

4. Variable focal zone on array real-time instruments.


Increasing the voltage causes 

1. Greater amplitude – greater penetration 

2. Longer pulses – degrades axial resolution 

3. Increase exposure


Transducer has dual roles; transmitting and receiving signals. 

The transducer is capable of handling a wide range of 

voltage amplitude. 

The Receiver is capable of handling only smaller signals 

Therefore it is desirable to isolate the pulser circuit from the 

receiver circuit.


The Transmit Receive Switch 

TRS – positioned at the input of the receiver and is designed to 

pass only voltages signals originating at the transducer by the 

returning echoes.


The Receiver Unit consist of 

1. Radiofrequency Amplifier 

2. Time gain compensation TGC unit 

3. Demodulation Circuit 

4. Detector Circuit 

5. Video Amplifier

PULSER 

TGC UNIT 

MEMORY 

TRX 

TRS 

RF 

RECEIVER 

CRT 

DISPLAY 

DEMODULATOR 

DETECTOR 

VIDEO 

AMPLIFIER


Radio-Frequency Amplifier 

• Amplify weak voltage signals. 

• This is called GAIN


Electric signals generated by the transducer are weak and 

needs amplification. 

The gain is the ratio of the output to input Voltage or Power. 

Gain = Voltage Out 

Voltage In


The Imaging effect of adjusting gain are: 

1. Increasing the gain - increased sensitivity, better 

penetration 

2. Decreasing the gain – decreased sensitivity, less 

penetration 

3. Too high a gain – overloads the display, loss or spatial 

resolution


Amplitude 

Saturation Level 

Normal Gain 

Distance


Excess Gain 


Amplitude 

Distance


Primary objective of grayscale pulse-echo imaging is to make 

all like reflectors appear the same in the Image regardless 

where they are located in the sound beam.


Time Gain Compensation TGC 

TGC - electronic process of adjusting the overall system 

gain as a function of the transmit time.


TGC Controls 

• Near Gain 

• Slope Delay 

• Slope 

• Knee 

• Far Gain 

• Body Wall


KNEE 

MAX GAIN 

Gain 

dB 

NEAR GAIN 

SLOPE 

DELAY 

Depth cm


KNEE 

MAX GAIN 

Gain 

dB 

NEAR GAIN 

SLOPE 

Body wall 

Depth cm


KNEE 

Gain 

dB 

SLOPE 

CUT-OFF 

DELAY 

Depth cm


The slide potentiometer allows adjustment of receiver gain for 

small discrete depth increments.


Slide Potentiometer 

Gain 

dB 

Depth (Time)


Frequency Tuning of the Receiver 

The frequency band width of the receiver refers to the range 

of ultrasound signal frequencies that the receiver can amplify 

with a maximum gain.


Types of Amplifiers 

• Wide-Band 

• Narrow-Band


Wide-band amplifier 

Narrow-band amplifier 

Gain 

Gain 

Frequency MHz 

Frequency MHz


Receiver Unit 

Receiver A 

TRX 

Receiver B 

Receiver C 

Output 

To 

System 

Frequency 

Selector 

Switch 

Receiver D


DYNAMIC RANGE 

The dynamic range is a measure of the range of echo signal 

amplitudes. 

The dynamic range can be measured at any point. 

The dynamic range decreases from transducer, to receiver to 

scan converter and finally to display.


Large range in signal amplitudes is due to: 

1. Normal variation in the reflection amplitude. 

2. Frequency dependent tissue attenuation.


RF amplifier can handle a wide range of signal amplitude at its 

input – but cannot accommodate the corresponding output using 

linear amplification.


Linear amplification - all voltages amplitudes, regardless of 

size at the point of input are amplified with the same gain 

factor.


LOGARITHMIC AMPLIFICATION 

In Logarithmic amplification weak echoes amplitudes are 

amplified more than strong echoes. 

This can reduced the dynamic range by as much as 50%. 

The process of reducing the signal DR by electronic means is 

called COMPRESSION


Gain 

A 

Linear Amplification 

B 

Logarithmic Amplification 

Input signal


R-F amplifier can also set the electronic level in the machine. 

S-N level – compares real echo signals the system can handle 

versus the non-echo signals presents (Noise). 

The Higher the SN ratio – better the operation of the system.


Pre-amplification is a technique to reduce system noise. 

Positioning of part of the amplifier circuitry in the transducer 

housing reduces system noise.


REJECTION 

Rejection is the receiver function that enables the operator to 

systematically increase or decrease the minimum echo signal 

amplitude which can be displayed. 

Alternate names = Threshold, Suppression.



Rejection Level 

Dynamic 

Range 

Zero Signal Level 

Noise 

Level


SIGNAL PROCESSING 

RF waveform – oscillating type of voltage signal (AC) 

First Step in processing the signal is Demodulation. 

Demodulation is the process of converting the electric 

signal from one form to another.


DEMODULATION 

• Rectification 

• Detection


RECTIFICATION 

• Rectification results in the elimination of the negative 

portion of the RF signals 

• Half Wave Rectification 

• Full wave Rectification


Half-Wave 

Rectification


Full-Wave 

Rectification


DETECTION 

The main effect of detecting the rectified RF signal is to 

round out or smooth the signal as to have a single broad 

peak. 

The rectified RF signal following detection is referred to as a 

Video Signal.


Smoothing


The video signal is then further amplified by the 

VIDEO AMPLIFIER. 

The output from the video amplifier is forwarded to 

1. CRT or 

2. Scan converter


DIGITAL SCAN CONVERTER 

The device that stores the echo signal is called a Scan 

converter.


All Scan Converters are designed to 

1. Store echoes in appropriate location 

2. Encode echoes in shade of gray 

3. Read out echoes in a horizontal raster format


4. Digital Memory is divided into small squares = Pixel. 

5. The Pixels form the Image Matrix 

6. Total # of storage location = rows x columns 

7. x and y location = ADDRESS

Matrix 

Rows x, coordinates

Matrix 

Columns, y coordinates

Matrix 

Pixel

10x 

10y 

X, Y ADDRESS 

8x 

7y 

5x 

5y 

3x 

3y 

1x 

1y


In the Scan converter the echoes are processed on a firstcome 

first-in basis.

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X 

X

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50

Raster 

Process 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50 

50


DIGITAL SCAN CONVERTER 

• Convert echo voltage signal into a numerical value. 

• Each numerical value corresponds to a shade of gray.


The number of shades of gray is determined by the BIT 

CAPACITY. 

# of shades of gray = 2


Echoes 

dB


Bit 

1 

2 

3 

4 

5 

6 

7 

8 

Shades of Gray 

2 

4 

8 

16 

32 

64 

128 

256


Gray Scale Resolution = dynamic range (dB) 

# of gray shades


Operator can select different A/D conversion scheme 

(Preprocessing). 

Each preprocessing curve is called an algorithm and assigns a 

specific percentage amount of shades of gray to regions of 

the echo amplitude.


% Available 

Shade of gray 

100% 

1 

2 

50% 

3 

4 

0% 

Echo Strength


POST PROCESSING 

Assignment of specific display brightness 

to numerical echo amplitudes read out of 

the digital memory.


9 

7 

8 

8 

8 

8 

8 

8 

8 

9 

8 

7 

8 

8 

8 

8 

7 

8 

8 

9 

8 

8 

8 

8 

SMOOTHING


The DSC is not necessary for image display, but is needed for 

the following post-processing functions. 

• Video Invert 

• Display Invert 

• Display Subdivision 

• Zoom Magnification


Zoom Magnification 

• Read Zoom 

• Write Zoom


Resolution at the DSC 

1. Find Matrix size 

2. Determine FOV ( width/length) 

3. Calculate pixels/cm 

4. Find linear distance/pixel = resolution


Data 

Pre- 

Processing 

RAM 

Data 

Post- 

Processing 

ADC 

Data 

Collection 

& 

Formatting 

Data 

Reformatting 

Echo 

Signal 

Positional 

Data 

Display


1. ROM 

2. PROM 

3. RAM

65. In Figure 3, transducer A is being used to establish: 

A. Verification of wedge angle 

B. Sensitivity calibration 

C. Resolution 

D. An index point

66. In Figure 3, transducer C is being used to check: 

A. Distance calibration 

B. Resolution 

C. Sensitivity calibration 

D. Verification of wedge angle 

67. In Figure 3, transducer D is being used to check: 

A. Sensitivity calibration 

B. Distance calibration 

C. Resolution 

D. Verification of wedge angle

68. When the incident angle is chosen to be between the first and second 

critical angles, the ultrasonic wave generated within the part will be: 


B. Shear 

C. Surface 

D. Lamb

69. In Figure 4, transducer B is being used to check: 

A. The verification of wedge angle 

B. Resolution 


D. Distance calibration

Q: In a UT test system where signal amplitudes are displayed on a CRT, an 

advantage of a frequency-independent attenuator over a continuously 

variable gain control is that: 

A. the pulse shape distortion is less 

B. the signal amplitude measured using the attenuator is independent 

of frequency 

C. the dynamic range of the system is decreased 

D. the effect of amplification threshold is avoided 

Q: An amplifier in which received echo pulses must exceed a certain 

threshold voltage before they can be indicated might be used to: 

A. suppress amplifier noise, unimportant scatter echoes, or small flaw 

echoes which are of no consequence 

B. provide a screen display with nearly ideal vertical linearity characteristics 

C. compensate for the unavoidable effects of material attenuation loss 

D. provide distance amplitude correction automatically

Q: The output voltage from a saturated amplifier is: 

A) 180 degrees out of phase from the input voltage 

B) lower than the input voltage 

C) nonlinear with respect to the input voltage 

D) below saturation 

Q: The transmitted pulse at the output of the pulser usually has a voltage of 

100 to 1000V, whereas the voltages of the echo at the input of the amplifier 

are on the order of: 

A) 10 Volts 

B) 50 Volts 

C) .001 to 1 Volts 

D) 1 to 5 Volts

Q: The intended purpose of the adjustable calibrated attenuator of a UT 

instrument is to: 

A) control transducer dampening 

B) increase the dynamic range of the instrument 

C) broaden the frequency range 

D) attenuate the voltage applied to the transducer

Addendum-02 

Equations & Calculations 


2014-June.

Trigonometry 

http://www.mathwarehouse.com/trigonometry/sine-cosine-tangent.php

Contents: 

1. Material Acoustic Properties 

2. Ultrasonic Formula 

3. Properties of Acoustic Wave 

4. Speed of Sound 

5. Attenuation 

6. What id dB 

7. Acoustic Impedance 

8. Snell’s Law 

9. S/N Ratio 

10. Near / Far Field 

11. Focusing & Focal Length 

12. Offsetting for Circular Specimen 

13. Quality “Q” Factors 

14. Inverse Law & Inverse Square Law 


1.0 Material Acoustic Properties 

Material 

Logitudinal wave 

Shear wave 

Z Acoustic 

mm/μs 

mm/μs 

Impedence 

Acrylic resin 

2.74 

1.44 

3.23 

(Perspex) 

Steel - SS 300 

5.613 

3.048 

44.6 

Series 

Steel - SS 400 

5.385 

2.997 

41.3 

Series 

Steel 1020 

5.893 

3.251 

45.4 

Steel 4340 

5.842 

3.251 

45.6 

http://www.ndtcalc.com/utvelocity.html

2.0 Ultrasonic Formula 


Ultrasonic Formula

Ultrasonic Formula 

α = Transducer radius

3.0 Properties of Acoustic Plane Wave 

Wavelength, Frequency and Velocity 

Among the properties of waves propagating in isotropic solid materials are 

wavelength, frequency, and velocity. The wavelength is directly proportional 

to the velocity of the wave and inversely proportional to the frequency of the 

wave. This relationship is shown by the following equation.

4.0 The Speed of Sound 

Hooke's Law, when used along with Newton's Second Law, can explain a few 

things about the speed of sound. The speed of sound within a material is a 

function of the properties of the material and is independent of the amplitude 

of the sound wave. Newton's Second Law says that the force applied to a 

particle will be balanced by the particle's mass and the acceleration of the the 

particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's 

Law then says that this force will be balanced by a force in the opposite 

direction that is dependent on the amount of displacement and the spring 

constant (F = -kx). Therefore, since the applied force and the restoring force 

are equal, ma = -kx can be written. The negative sign indicates that the force 

is in the opposite direction. 

F= ma = -kx

What properties of material affect its speed of sound? 

Of course, sound does travel at different speeds in different materials. This is 

because the (1) mass of the atomic particles and the (2) spring constants are 

different for different materials. The mass of the particles is related to the 

density of the material, and the spring constant is related to the elastic 

constants of a material. The general relationship between the speed of sound 

in a solid and its density and elastic constants is given by the following 

equation:

V is the speed of sound 

Eleatic constant 

→ spring constants 

Density 

→ mass of the atomic particles

Where V is the speed of sound, C is the elastic constant, and p is the material 

density. This equation may take a number of different forms depending on the 

type of wave (longitudinal or shear) and which of the elastic constants that are 

used. The typical elastic constants of a materials include: 

• Young's Modulus, E: a proportionality constant between uniaxial stress 

and strain. 

• Poisson's Ratio, n: the ratio of radial strain to axial strain 

• Bulk modulus, K: a measure of the incompressibility of a body subjected to 

hydrostatic pressure. 

• Shear Modulus, G: also called rigidity, a measure of a substance's 

resistance to shear. 

• Lame's Constants, l and m: material constants that are derived from 

Young's Modulus and Poisson's Ratio.

E/N/G

5.0 Attenuation 

The amplitude change of a decaying plane wave can be expressed as: 

In this expression A o is the unattenuated amplitude of the propagating wave 

at some location. The amplitude A is the reduced amplitude after the wave 

has traveled a distance z from that initial location. The quantity α is the 

attenuation coefficient of the wave traveling in the z-direction. The α 

dimensions of are nepers/length, where a neper is a dimensionless 

quantity. The term e is the exponential (or Napier's constant) which is equal 

to approximately 2.71828. 

http://www.ndt.net/article/v04n06/gin_ut2/gin_ut2.htm

Spreading/ Scattering/ adsorption (reflection is a form of scaterring) 

Adsoprtion 

Scaterring 

Spreading 

Scaterrring

Attenuation can be determined by evaluating the multiple backwall reflections 

seen in a typical A-scan display like the one shown in the image at the bottom. 

The number of decibels between two adjacent signals is measured and this 

value is divided by the time interval between them. This calculation produces 

a attenuation coefficient in decibels per unit time Ut. This value can be 

converted to nepers/length by the following equation. 



Amplitude at distance Z 

Where v is the velocity of sound in meters per second and Ut is in decibels 

per second (attenuation coefficient). 

α is the attenuation coefficient of the wave traveling in the z-direction. The 

α dimensions of are nepers/length (nepers constant).

Attenuation is generally proportional to the square of sound frequency. 

Quoted values of attenuation are often given for a single frequency, or an 

attenuation value averaged over many frequencies may be given. Also, the 

actual value of the attenuation coefficient for a given material is highly 

dependent on the way in which the material was manufactured. Thus, quoted 

values of attenuation only give a rough indication of the attenuation and 

should not be automatically trusted. Generally, a reliable value of attenuation 

can only be obtained by determining the attenuation experimentally for the 

particular material being used. 

Attenuation ∝ Frequency 2 (f ) 2

Which U t ? 

U 0 t , A 0 o 

U 1 t , A 1 o , α 1 

1 1

7.0 Acoustic Impedance 

Sound travels through materials under the influence of sound pressure. 

Because molecules or atoms of a solid are bound elastically to one 

another, the excess pressure results in a wave propagating through the 

solid. 

The acoustic impedance (Z) of a material is defined as the product of its 

density (p) and acoustic velocity (V). 

Z = pV 

Acoustic impedance is important in: 

1. the determination of acoustic transmission and reflection at the boundary 

of two materials having different acoustic impedances. 

2. the design of ultrasonic transducers. 

3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any 

material, so long as its density (p) and acoustic velocity (V) are known. The 

applet also shows how a change in the impedance affects the amount of 

acoustic energy that is reflected and transmitted. The values of the reflected 

and transmitted energy are the fractional amounts of the total energy incident 

on the interface. Note that the fractional amount of transmitted sound energy 

plus the fractional amount of reflected sound energy equals one. The 

calculation used to arrive at these values will be discussed on the next page. 



Reflection and Transmission Coefficients (Pressure) 

• This difference in Z is commonly referred to as the impedance 

mismatch. 

• The value produced is known as the reflection coefficient. Multiplying 

the reflection coefficient by 100 yields the amount of energy reflected as a 

percentage of the original energy. 

• the transmission coefficient is calculated by simply subtracting the 

reflection coefficient from one. 

Ipedence 

mismatch 

Reflection coefficient

Using the above applet, note that the energy reflected at a water-stainless 

steel interface is 0.88 or 88%. The amount of energy transmitted into the 

second material is 0.12 or 12%. The amount of reflection and transmission 

energy in dB terms are -1.1 dB and -18.2 dB respectively. The negative sign 

indicates that individually, the amount of reflected and transmitted energy is 

smaller than the incident energy.

If reflection and transmission at interfaces is 

followed through the component, only a small 

percentage of the original energy makes it back 

to the transducer, even when loss by attenuation 

is ignored. For example, consider an immersion 

inspection of a steel block. The sound energy 

leaves the transducer, travels through the water, 

encounters the front surface of the steel, 

encounters the back surface of the steel and 

reflects back through the front surface on its way 

back to the transducer. At the water steel 

interface (front surface), 12% of the energy is 

transmitted. At the back surface, 88% of the 

12% that made it through the front surface is 

reflected. This is 10.6% of the intensity of the 

initial incident wave. As the wave exits the part 

back through the front surface, only 12% of 10.6 

or 1.3% of the original energy is transmitted back 

to the transducer.


Following are the data:

Q1: What is the percentage of initial incident sound wave that will reflected 

from the water/Aluminum interface when the sound first enter Aluminum? 

R= (Z 1 -Z 2 ) 2 / (Z 1 +Z 2 ) 2 = (0.149-1.72) 2 /(0.149+1.72) 2 

R= 0.707, Answer= 70.7%

Q2: What is the percentage of sound energy that will finally reenter the water 

after reflected from the backwall of Aluminum? (Do not consider material 

attenuation and other factors) 

Answer: 6% 

0.706 – initial Back wall 

0.2934 

0.207x 0.2934=0.0609 

Second Backwall echo 

0.2934x 0.706 = 

0.207

8.0 Snell’s Law 

Snell's Law holds true for shear waves as well as longitudinal waves and can 

be written as follows 

= 

Where: 



VS1 is the shear wave velocity in material 1. 

VS2 is the shear wave velocity in material 2.

Snell’s Law 

http://education-portal.com/academy/lesson/refraction-dispersion-definition-snells-law-index-of-refraction.html#lesson


5. For an ultrasonic beam with normal incidence, the reflection coefficient is 

given by: 

(a) [(Z 1 +Z 2 ) 2 ]/[(Z 1 -Z 2 ) 2 ] 

(b) (Z 1 +Z 2 )/(Z 1 -Z 2 ) 

(c) [(4) (Z 1 )(Z 2 )]/[(Z 1 +Z 2 ) 2 ] 

(d) [(Z 1 -Z 2 ) 2 ]/[Z 1 +Z 2 ) 2 ] 

6. For an ultrasonic beam with normal incidence the transmission coefficient 

is given by: 

(a) [(Z 1 +Z 2 ) 2 ]/[(Z 1 -Z 2 ) 2 ] 

(b) (Z 1 +Z 2 )/(Z 1 -Z 2 ) 

(c) [(4) (Z 1 )(Z 2 )]/[(Z 1 +Z 2 ) 2 ] 

(d) [(Z 1 -Z 2 ) 2 ]/[Z 1 +Z 2 ) 2 ]

Practice Made Perfect 

7. Snell's law is given by which of the following: 

(a) (Sin A)/(Sin B) = VB/VA 

(b) (Sin A)/(Sin B) = VA/VB 

(c) (Sin A)/ VB = V(Sin B)/VA 

(d) (Sin A)[VA] = (Sin B)[ VB] 

8. Snell's law is used to calculate: 

(a) Angle of beam divergence 

(b) Angle of diffraction 

(c) Angle of refraction 

(d) None of the above


9. Calculate the refracted shear wave angle in steel [VS = 0.323cm/microsec] 

for an incident longitudinal wave of 37.9 degrees in Plexiglas [VL = 0.267cm/ 

microsec] 

(a) 26 degrees 

(b) 45 degrees 

(c) 48 degrees 

(d) 64 degrees 

10. Calculate the refracted shear wave angle in steel [VS = 0.323cm/microsec] 

for an incident longitudinal wave of 45.7 degrees in Plexiglas [VL = 0.267cm/ 

microsec] 


(b) 45.7 degrees 


(d) 70 degrees


11. Calculate the refracted shear wave angle in aluminium [VS = 0.31cm/ 

microsec] for an incident longitudinal wave of 43.5 degrees in Plexiglas [VL = 

0.267cm/microsec] 




(d) 68 degrees 

12. Calculate the refracted shear wave angle in aluminium [VS = 

0.31cm/microsec] for an incident longitudinal wave of 53 degrees in Plexiglas 

[VL = 0.267cm/microsec] 




(d) 68 degrees

9.0 S/N Ratio 


ratio (S/N) of a defect: 

FOM: Factor of merits at center frequency


ratio (S/N) of a defect:

Sound Volume: Area x pulse length Δt 

Material properties 

Flaw geometry at center frequency: 

Figure of merit FOM and 

amplitudes responds

10. Near/ Far Fields 

http://miac.unibas.ch/PMI/05-UltrasoundImaging.html

where α is the radius of the transducer and λ the wavelength. 

For beam edges at null condition K=1.22

Modified Near Zone 

T Perspex 

Modified Z f

Example: Calculate the modified Near Zone for; 

• 5 MHz shear wave transducer 

• 10mm crystal 

• 10 mm perspex wedge 

Perspex L-wave: 2730 m/s 

Steel S-wave: 3250 m/s 

Steel L-wave: 5900 m/s 

Modified NZ= (0.01 2 x f) / (4v) – 0.01(2730/3250) 

=0.0300m 

= 30mm

Apparent Near Zone distance

11.0 Focusing & Focal Length 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/beam-characteristics/

The focal length F is determined by following equation; 

Where: 

F = Focal Length in water 

R = Curvature of the focusing len 

n = Ration of L-velocity of epoxy to L-velocity of water

12.0 Offset of Normal probe above circular object 

θ 1 

V 1 

θ 1R 

θ 2 V 2

Calculate the offset for following conditions: 

Aluminum rod being examined is 6" diameter, what is the off set needed for (a) 

45 refracted shear wave (b) Longitudinal wave to be generated? 

(L-wave velocity for AL=6.3x10 5 cm/s, T-wave velocity for AL=3.1x10 5 cm/s, 

Wave velocity in water=1.5X10 5 cm/s) 

Question (a) 

Question (b)

13.0 “Q” Factor 

3dB down

14.0 Inverse Law and Inverse Square Law 

For a small reflector where the size of reflector is smaller than the beam width, 

the echoes intensity from the same reflector varies inversely to the square of 

the distance. 

5cm 

7.5cm 

75% FSH 33% FSH

Inverse Square Law 

http://www.cyberphysics.co.uk/general_pages/inverse_square/inverse_square.htm

Inverse Law: 

For large reflector, reflector greater than the beam width, e.g. backwall 

echoes from the same reflector at different depth; the reflected signal 

amplitude varies inversely with the distance. 

10cm 

7.5cm

DGS Distance Gain Sizing 

Y-axis shows the 

Gain 

size of reflector is given as 

a ratio between the size of 

the disc and the size of 

the crystal. 

X-axis shows the Distance from the probe in # of Near Field

DGS Distance Gain Sizing 

Y-axis shows the 

Gain 

size of reflector is given as 

a ratio between the size of 

the disc and the size of 

the crystal. 

X-axis shows the Distance from the probe in # of Near Field

– Distance Gain Size is a method of setting sensitivity or assessing the signal 

from an unknown reflector based on the theoretical response of a flatbottomed 

hole reflector perpendicular to the beam axis. (DGS does not size 

the flaw, but relate it with a equivalent reflector) The DGS system was 

introduced by Krautkramer in 1958 and is referred to in German as AVG. A 

schematic of a general DGS diagram is shown in the Figure. The Y-axis 

shows the Gain and X-axis shows the Distance from the probe. In a general 

DGS diagram the distance is shown in units of Near Field and the scale is 

logarithmic to cover a wide range.

The blue curves plotted show how the amplitudes obtained from different 

sizes of disc shaped reflector (equivalent to a FBH) decrease as the distance 

between the probe and the reflector increases.

In the general diagram the size of reflector is given as a ratio between the 

size of the disc and the size of the crystal. The red curve shows the response 

of a backwall reflection. The ratio of the backwall to the crystal is infinity (∞). 

Specific DGS curves for individual probes can be produced and so both the 

distance axis and the reflector sizes can be in mm. 

If the sensitivity for an inspection is specified to be a disc reflector of a given 

size, the sensitivity can be set by putting the reflection from the backwall of a 

calibration block or component to the stated %FSH. The gain to be added can 

be then obtained by the difference on the Y-axis between the backwall curve 

at the backwall range and the curve of the disc reflector of the given size at 

the test range. If the ranges of the backwall and the disc reflector are different, 

then attenuation shall be accounted for separately. Alternatively, the curves 

can be used to find the size of the disc shaped reflector which would give the 

same size echo as a response seen in the flaw detector screen.

20-4dB=16dB (deduced) 

Δ Flaw =30-16=14dB 

20dB 

(measured) 

Data: 

Probe frequency: 5MHz 

Diameter: 10mm compression probe 

Plate thickness: 100mm steel 

Defect depth: 60mm deep 

Gain for flaw to FSH: 30dB 

BWE at 100mm: 20dB

Example: If you has a signal at a certain depth, you can compare the signal of 

the flaw to what the back wall echo (BWE) from the same depth and estimate 

the FBH that would give such a signal at the same depth. The defect can then 

be size according to a FBH equivalent. 

Data: 

Probe frequency: 5MHz 

Diameter: 10mm compression probe 

Plate thickness: 100mm steel 

Defect depth: 60mm deep 

Gain for flaw to FSH: 30dB 

BWE at 100mm: 20dB 

------------------------------------------------------------------------- 

Near field: 21mm, flaw location= 3xNear Field 

From the chart BWE at 60mm will be 20-4dB=16dB 

Flaw signal Gain is 30dB-16dB= 14dB 

Used the flaw signal Gain and locate the equivalent reflector size is between 

0.4 to 0.48 of the probe diameter, say 0.44 x10mm = 4.4mm equivalent 

reflector size.

http://www.olympus-ims.com/en/atlas/dgs/

More on DGS/AVG by Olympus 

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/dgs-avg/ 

DGS is a sizing technique that relates the amplitude of the echo from a 

reflector to that of a flat bottom hole at the same depth or distance. This is 

known as Equivalent Reflector Size or ERS. DGS is an acronym for Distance- 

Gain-Size and is also known as AVG from its German name, Abstand 

Verstarkung Grosse. Traditionally this technique involved manually 

comparing echo amplitudes with printed curves, however contemporary 

digital flaw detectors can draw the curves following a calibration routine and 

automatically calculate the ERS of a gated peak. The generated curves are 

derived from the calculated beam spreading pattern of a given transducer, 

based on its frequency and element diameter using a single calibration point. 

Material attenuation and coupling variation in the calibration block and test 

specimen can be accounted for.

DGS is a primarily mathematical technique originally based on the ratio of a 

circular probe’s calculated beam profile and measurable material properties 

to circular disk reflectors. The technique has since been further applied to 

square element and even dual element probes, although for the latter, curve 

sets are empirically derived. It is always up to the user to determine how the 

resultant DGS calculations relate to actual flaws in real test pieces. 

An example of a typical DGS curve set is seen below. The uppermost curve 

represents the relative amplitude of the echo from a flat plate reflector in 

decibels, plotted at various distances from the transducer, and the curves 

below represent the relative amplitude of echoes from progressively smaller 

disk reflectors over the same distance scale.

As implemented in contemporary digital flaw detectors, DGS curves are 

typically plotted based on a reference calibration off a known target such as a 

backwall reflector or a flat bottom hole at a given depth. From that one 

calibration point, an entire curve set can be drawn based on probe and 

material characteristics. Rather than plotting the entire curve set, instruments 

will typically display one curve based on a selected reflector size (registration 

level) that can be adjusted by the user. 

In the example below, the upper curve represents the DGS plot for a 2 mm 

disk reflector at depths from 10 mm to 50 mm. The lower curve is a reference 

that has been plotted 6 dB lower. In the screen at left (figure 1), the red gate 

marks the reflection from a 2 mm diameter flat bottom hole at approximately 

20 mm depth. Since this reflector equals the selected registration level, the 

peak matches the curve at that depth. In the screen at right (Figure 2), a 

different reflector at a depth of approximately 26 mm has been gated. Based 

on its height and depth in relation to the curve the instrument calculated an 

ERS of 1.5 mm.

Figure1:

Figure2:

15.0 Pulse Repetitive Frequency/Rate and Maximum 

Testable Thickness 

Clock interval = 1/PRR 

Maximum testable length = ½ x Velocity x Clock interval 

Note: The Clock interval has neglected the time occupied by each pulse.

16.0 Immersion Testing of Circular Rod

Q4-12 

Answer: 

First calculate the principle offset d; ϴ = Sin-1(1483/3250 xSin45)=18.8 ° 

d=R.Sin18.8= 0.323 (Assume R=1). 

Wobbling ±10%; d’=0.355 ~ 0.290 

d’=0.355, ϴ = Sin-1(0.355)=20.8 ° 

giving inspection Φ = Sin-1(3250/1483xSin20.8)=51, 13.3% above 45 ° 

d’=0.290, ϴ = Sin-1(0.290)=16.9 ° 

giving inspection Φ = Sin-1(3250/1483xSin16.9)=39.6, 12% below 45 °

Maximum ϴ 

ϴ max = Sin -1 (ID/OD)

Addendum-03 

Questions & Answers I 

Collection of My Pitfalls

Uncertain Questions 

21. Which type of calibration block is used to determine the resolution of 

angle beam transducers per requirements of AWS and AASHTO 

a. An IIW block 

b. A DSC block 

c. A rompus block 

d. An RC block 

24. Resonance or standing waves are a result of: 

a. mode conversion 

b. interference from reflected waves 

c. beam divergence (spread) 

d. attenuation of the sound waves

Make mistakes now, 

not during exam!

RC- Resolution Calibration Block

30. On an A-scan display the dead zone refers to: 

a. the distance contained within the near field (incorrect) 

b. the area outside the beam spread 

c. the distance covered by the front surface pulse width and recovery 

time 

d. the area between the near field and the far field 

40. The second critical angle is the angle of the incident beam at which: 

a. the angle of the refracted compression wave is 900 

b. the angle of the reflected compression wave is 90° 

c. total reflection occurs 

d. surface waves are produced 

--------------------------------------------------------------------------------

17. Surface waves are used to detect discontinuities in the test materials: 

a. At half the depth. 

b. Above the lower surface. 

c. On the surface where the probe is in contact. 

d. None of the above. 

26. Which of the following probes is most commonly used for testing welded 

metals for laminations before angle beam inspection. 

a. Surface wave probe. 

b. Twin crystal 0° probe. 

c. Single crystal probe. 

d. An angle probe. 

29. Artificial flaws can be produced by using: 

Side drilled holes 

Flat bottom holes 

EDM notches (http://www.phtool.com/pages/edm.asp) 

All of the above

31. As the acoustic impedance ratio between two materials approaches 1 the 

amount of sound reflected at an interface: 

a. increases. 

b. decreases. 

c. is not affected. 

d. varies depending upon the velocity of the materials. 

34. Significant errors in ultrasonic thickness measurements can occur if; 

a. Test frequency is varying at a constant rate. 

b. The velocity of propagation deviates substantially from an assumed 

constant value for a given material. 

c. Water is employed as a couplant between the transducer and the part 

being measured. 

d. None of the above should cause errors.

45. When examining thin materials for planar discontinuities oriented parallel 

to the part surface, what testing method is most often used: 

a. Angle beam 

b. Through-transmission 

c. Straight beam - single crystal 

d. Straight beam - dual crystal 

7. The ultrasonic test method in which finger damping in most effective in 

locating a discontinuity is: 

a. shear wave 

b. longitudinal wave 

c. surface wave 

d. compressional wave

15. Which type of test block is used to check horizontal linearity and the dB 

accuracy per requirements of AWS and AASHTO? 

a. Distance/Sensitivity block 

b. A DSC block 

c. A rompus block 

d. A shear wave calibration block

Mistake Made -------------------------------------------------------------------------------- 

Question: Which probe will be used for critical examination in a forged 

component with a curved surface.: 

Your answer: 1 megahertz, 10mm dia. 

Correct answer: 10 megahertz, 25mm dia. 

Question: A general term applied to all cracks, inclusions, blow holes etc, 

which cause a reflection of sonic energy is: 

Your answer: a refractor 

Correct answer: a discontinuity 

Question: On an A-scan display the dead zone refers to: 

Your answer: the distance contained within the near field 

Correct answer: the distance covered by the front surface pulse width and 

recovery time

Mistake Made -------------------------------------------------------------------------------- 

Question: Dead zone size depends on: 

Your answer: construction of the probe. 

Correct answer: All of the above. 

Question: The second critical angle is the angle of the incident beam at which: 

Your answer: total reflection occurs 

Correct answer: surface waves are produced 

---------------------------------------------------------------------------------

Mistake Made -------------------------------------------------------------------------------- 

Question: When a longitudinal wave encounters an interface between two 

material with different accoustic impedances, what occurs when the Your 

answer: Reflection and refraction Correct answer: Reflection 

Question: In an ultrasonic instrument, the number of pulses produced by an 

instrument in a given period of time in known as the:Your answer: pulse 

length of the instrument Correct answer: pulse repetition rate 

Question: Which probe will be used for critical examination in a forged 

component with a curved surface.:Your answer: 10 megahertz, 10mm 

dia.Correct answer: 10 megahertz, 25mm dia.

Question: Which type of screen presentation displays a profile or crosssectional 

view of the test specimen? Your answer: A-scan Correct answer: 

B-scan 

Question: When a longitudinal wave encounters an interface between two 

material with different accoustic impedances, what occurs when the Your 

answer: Refraction Correct answer: Reflection

Questions & Answers

Table 1.2

Chapter 1: Physical Principles 

Q1-10 The acoustic energy reflected at a plexiglass-quartz interface is equal 

to? 

Answer: R= (Z 1 -Z 2 ) 2 / (Z 1 +Z 2 ) 2 = (3.2-15.2) 2 / (3.2+15.2) 2 = 42.53% 

Q1-11 The acoustic energy transmitted through a plexiglass-water interface is 

equal to? 

Answer: R= (Z 1 -Z 2 ) 2 / (Z 1 +Z 2 ) 2 = (3.2-1.5) 2 / (3.2+1.5) 2 = 13%, T= 1-R = 87% 

Q1-12 The first critical angle at a water-plexiglass interface will be? 

Answer: ϴ = Sin-1 (1483/2730) = 32.9°

Q1-13 The second critical angle at water-plexiglass interface will be? 

Answer: ϴ = Sin -1 (1483/1430) = Error! 

Q1-14 The incident angle need in immersion testing to develop a 70 shear 

wave in plexiglass is equal to? 

Answer: ϴ = Sin -1 (1483/1430 x sin70) = 77°

Q1-20 Two plate yield different back-wall reflections in pulse-echo testing 

(18dB) with their only apparent difference being in the second material void 

content. The plate are both 3” thick. What is the effective change in acoustic 

attenuation between the first and second plate? 

Answer: Sound path – 2 x thickness = 6” Attenuation = 18dB/6” = 3dB/in. 

Comment: 

The answer could be confused if the pulse-echo testing, 2-ways path length 

was not considered, arriving with the incorrect answer of 6dB/in

For evaluating material properties always remember to divide the result 

with the actual sweep distance if necessary! It was not a one-way–trip!

Q1-15 At a water-Aluminum interface, at an incident angle of 20°, the 

reflected and transmitted wave are? 

Answer: 60% transmitted and 40% reflected.

Q1-22 The beam spread half angle I the far field of a I” diameter transducer 

sending 5MHz longitudinal wave into Plexiglas block is? 

Answer: ϴ = Sin-1 (K λ/D) Assumed K=1.2 for null beam edge, 

ϴ = Sin -1 (K λ/D) =Sin-1(1.2V/DF)= Sin -1 [1.2x2730x10 3 / (25.4x5x10 6 )] 

=1.478° 

Q1-23 The near field of a round 1/2 “ diameter contact L-wave transducer 

being used on a steel test part operating at 3MHz is? 

Answer: Z= D2/4λ = 12.7 2 3x10 6 x / (4x5900x10 3 ) = 20.5mm

Chapter 2: Equipment 

Q2-5 A 5MHz 0.5” diameter flat search unit in water has a near field length of 

approximately? 

Answer: Z= D 2 /4λ = (12.7 2 x 5x10 6 ) / (4x 1480X10 3 ) = 136mm = 5.36” 

Q2-7 A 10MHz,0.5” diameter transducer placed on steel and acrylic in 

succession, the beam spread in these 2 material is? 

ϴ = sin-1(K λ/D). 

ϴ Fe = sin-1(1.2x5920x10 3 /10x10 6 x12.7) = 3.2°, 

ϴ Acrylic = sin -1 (1.2x2730x10 3 /10x10 6 x12.7) = 1.48°

Q2-12 An angle beam produce a 45° shear wave in steel, what is the incident 

angle? (V s for steel=0.125in/ms, V L for plastid=0.105in/ms) 

Answer: Snell’s Law; ϴ incident = Sin -1 [(0.105/0.125) xSin45] = 36.43° 

Q2-13 Aluminum rod 6” diameter being examined in immersion technique, 

what is the required offset to generate a 45° refracted shear wave? 

Answer: First find the incident angle using Snell’s Law 

ϴ incident = Sin -1 [(1.5/3.1) xSin45] = 20° 

Offset = rSin20 = 3Sin20 = 1.026”

Q2-14 What is the offset required, if 45 refracted longitudinal wave to be 

generated? 

Answer: First find the incident angle using Snell’s Law 

ϴ incident = Sin -1 [(1.5/6.3) xSin45] = 9.69° 

Offset = r.Sin9.69 ° = 3.Sin9.69 ° = 0.505” 

Q2-16 In a longitudinal wave immersion test of Titanium plate, an echoes 

pulse from an internal defect is observed 6.56μs following front echo. How 

deep is the defect below the front surface? 

Answer: Sound path travel= 6100000 x 6.56 x 10 -6 = 40mm 

The actual depth = sound path / 2 = 20mm

Q2-17 A change in echo amplitude from 20% of FSH to 40% of FSH is a 

change of how many dB? 

Answer: ΔdB= 20log(20/40) = 6dB drop or -6dB. 

Q2-20 What is the lens radius of curvature is needed in order to have a 20mm 

diameter 5MHz transducer focus in water at a distance of 40mm drom the 

lens face? 

Answer: 

R=F(n-1/n), n= V Lens /V water , n= 2.67/1.49= 1.792. 

R=40(0.792/1.792) = 17.7mm

Q2-18 In Fig.29 what is the rate of attenuation in dB/in of 5MHz transducer in 

Far Field, the horizontal scale is 0.5” per division and the vertical scale is 

linear. 

Answer: 

ΔI = 20log(1.25/2) D=

Q2-19 What is the rate of attenuation for 2.25MHz transducer? 

Answer: 

Δ I = 20log(0.9/2.2) D=2.5” , Attenuation = 3.11dB/in

Q2-21 Two signals were compared to each other. The second was found to 

be 14dB less than the first. This change could be represented by a change of? 

Answer: 

ΔI = 20Log(I/I o ), 

-14dB= 20Log(I/I o ), (I/I o )= 0.2 

2 answers could be confused: 

70% FSH to 14% FSH, a drop of 80% 

20% FSH to 100% FSH, an increase of 80%

Q2-11 A change in 16dB on the attenuator correspond to an amplitude ration 

of: 

Answer: 

ΔI = 20Log(I/Io), 

16dB= 20Log(I/Io), (I/Io)= 6.3

Charter 3: Common Practices 

Q3-6 In Fig. 3.7 the respond from 3.23mm FBH at a depth of 25mm is above 

that detected from 1mm FBH by? 

Answer: 

ΔdB= 20Log(2.1/0.6) = 10.88

Q3-7 The half angle beam spread of the reflected wave front from #8 FBH in 

an aluminum “A” block being immersion tested using 25MHz transducer is? 

Answer: 

Focal size = 8/64 x 25.4 =3.175mm diameter. 

The beam spread is in aluminum block, the wave velocity VL=6300 m/s 

The half angle beam spread ϴ= Sin -1 (Kλ/D) 

ϴ = Sin -1 [(1.2x6300x10 3 )/(3.175x 25 x 10 6 )] = 5.47° 

Comment: Be careful with the unit used, my mistake is: 

ϴ = Sin -1 [(1.2x6300x10 3 )/(3.175x 10 -3 x 25 x 106)]

Always Check the units correctly!!!! Only Donkey made such mistake!

Monkey made mistake too!

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you!

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not made mistake with UNIT 

USED too, so do you! 

http://www.tennenaturephotography.com/gallery/papua/Native_Dancer_Face?full=1

Q3-8 

Answer: The next SDH used will be 5/4T, 

first SDH after backwall echo. 

The node is 5/(4x2) = 5/8 node

Q3-11 When using a focued, straight beam search unit for lamination 

scanning in an immersion test of steel plate, a change in water path of 0.2” 

will result in the focal point moving in the steel a distance of? 

Answer: The change in water path=0.2” correspond to 0.2 x 1483/5900 = 0.05” 

Q3-12 A search unit with a foal length in water of 4” is used. A steel plate 8” 

thick velocity 0.230”/ms is place at a water depth of 2” from the search unit, At 

what depth is the focal point in steel? 

Answer: Focal depth in steel = 2 x V water / V steel = 2x1480/5900 = 0.5”

Q3-13 During examination, an indication of 25% FSH is detected and 

maximized. Foe better analysis the gain is increase by 12bB and the 

indication increase to 88% FSH. What value should be reached and what is 

the apparent problem? 

Answer: 

12dB= 20Log(I/25), I/25= 3.98, I=100%

Q3-23 A air filled #3 FBH 0.5” into the bottom of 4.5” aluminum block, will 

return to the 0.75” diameter sending immersion transducer ans echo signal 

equal to ? Of the initial pulse. Assume no attenuation to beam divergence or 

other causes. 

Answer: 

The size of reflector = 3/64” = 0.046875”. 

For a small reflector used inverse square law; 

Echo1/Echo2 = Area 2 / Area 1 

100/x= 0.046875 2 / 0.75 2 , x = 0.39%

Q3-15 In contact testing, the back surface signal from a 2” plate was set at full 

screen height. Passing over a coarse grained area, the back surface signal 

dropped to 10% FSH. What is the change in attenuation in this area? 

Answer: 

ΔI=20Log(10/100), the drop in dB= 20dB. 

The sweep distance = 4” 

The attenuation is 20/4 = 5dB/in. 

Comment: Remember that the attenuation is cause by the sound path 

traversing thru the sweep distance.

Q4-12 

Answer: 

First calculate the principle offset d; ϴ = Sin-1(1483/3250 xSin45)=18.8 ° 

d=R.Sin18.8= 0.323 (Assume R=1). 

Wobbling ±10%; d’=0.355 ~ 0.290 

d’=0.355, ϴ = Sin-1(0.355)=20.8 ° 

giving inspection Φ = Sin-1(3250/1483xSin20.8)=51, 13.3% above 45 ° 

d’=0.290, ϴ = Sin-1(0.290)=16.9 ° 

giving inspection Φ = Sin-1(3250/1483xSin16.9)=39.6, 12% below 45 °

Q4-13 

Answer: PRR = number of pulse per second N/s, 

Length generated by pulse per second = PRR x D 

For effective inspection Vp ≤ PRR x D 

Q4-14 

Answer: Effective inspection Length generated by the PRR x Width = 600in/s 

For a defect to be detected 3 time consecutively, the travel speed V p = 600/3 

= 200in/s

Q4-15 

Answer: Offset = T.tan70 x Number of ½ skip. 

Offset = 3x 1.5 tan70 

Comment: 1 skip= 2 legs 

Q4-16 

Answer: ?

Q4-16

Q4-17 

Answer: Total length of axial= 8x12x0.0254m 

L=2.438m, Sweep distance for a complete 

return loop =2 x L= 4.876m 

For PRR = 2000 

Distance travel by each pulse L p = 5920/2000 m 

L p =2.96m 

Since L p is less than the 4.876, the next pulse 

was found to be generated before the previous 

echo has returned to the receiver, thus reduce 

the PRR is required. 

Set PRR=1000, yield L p =5.92m > L=4.876m 

Will resolve the problem.

Q4-17 Illustrations 

Complete loop=4.876m 

Length of axial 8’ or 2.438m 

2 nd pulse 

generating 

Incoming & 

returning wave 

meet 

The previous pulse 

return position 

when 2 nd (next) 

pulse start to send 

0.958m 

0.958m 

0.522m 

₵

Q4-18 

Answer:

8. When testing a 30 mm diameter, 500 mm long shaft from the flat end of the 

shaft using longitudinal waves from a 20 mm diameter 2 MHz probe, 

numerous signals are seen on the screen after 500 mm. These are: 

a) ghost images 

b) side wall echoes 

c) internal thread indications 

d) none of the above

Break! 

mms://a588.l3944020587.c39440.g.lm.akamaistream. 

net/D/588/39440/v0001/reflector:20587?BBC- 

UID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5 

d6448f55674c540f1856834&SSO2-UID=

Q5-20 

Answer: None of above

Q5-22 

Answer: Class C

Q5-22 Table B-1

5. At a solid to free boundary, an obliquely incident longitudinal wave from the 

solid can result in, at most: 

a) a reflected longitudinal wave only 

b) a reflected longitudinal and reflected shear wave 

c) a refracted longitudinal long wave 

d) a reflected longitudinal and reflected shear and refracted longitudinal wave 

6. Geometric-optic treatment (?) of ultrasonic waves fails to account for: 

a) reflection 

b) refraction 

c) diffraction 

d) normal incidence 

34.The most useful range of incident longitudinal wave angles for ultrasonic 

testing is: 

(a) Normal incidence to the first critical angle 

(b) First critical angle to the second critical angle (?) 

(c) Second critical angle to the third critical angle 

(d) Above the third critical angle

38. The angle of a refracted shear wave generated as a sound wave passes 

at an angle through an acoustic interface is dependant on: 

a) The acoustic impedances of the materials of each side of the 

interface 

b) The frequency of the incident sound wave 

c) The wavelength of the incident sound wave 

d) The hardness of the materials on each side of the interface 

22. The three most common modes of sound vibration are: 

(a) Longitudinal, compressional, and transverse waves 

(b) Longitudinal, transverse and rayleigh waves 

(c) Transverse, longitudinal and shear waves 

(d) Transverse, shear waves and rayleigh waves

13. An oscilloscope display in which the screen base line is adjusted to 

represent the one way distance in a test piece is called a: 

(a) A scan display 

(b) B scan display 

(c) C scan display 

(d) D scan display 

12. Which of the following test frequencies would generally provide the best 

penetration in a 12 inch thick specimen of coarse-grained steel? 

(a) 1.0 MHz 

(b) 2.25 MHz 

(c) 5.0 MHz 

(d) 10 MHz (Incorrect – silly mistake)

48. A more highly damped transducer crystal results in: 

(a) Better resolution 

(b) Better sensitivity (mistake) 

(c) Lower sensitivity 

(d) Poorer resolution 

6. The portion of a test piece which is represented by the CRT screen area 

from zero to the rightmost edge of the initial pulse is called: 

(a) The dead zone (mistake) 

(b) The near field 

(c) The near zone 

(d) The far zone

17. Transducer focal lengths are normally specified as: 

(a) Distance in steel 

(b) Distance in aluminium 

(c) Distance in air 

(d) Distance in water (mistake) 

21. An advantage of using a ceramic transducer in search units is that: 



(c) It has a very low mechanical impedance 

(d) It can withstand temperatures as high as 700 o C

47. When a vertical indication has reached the maximum signal height which 

can be displayed or viewed on the CRT of an ultrasonic instrument, the 

indication is said to have reached its: 

(a) Distance-amplitude height (mistake) 

(b) Absorption level 

(c) Vertical level 

(d) Limit of resolution

53. An ultrasonic instrument control which is used to adjust the sharpness of 

the CRT screen display is called: 

(a) Astigmatism or focus 

(b) Pulse repetition rate 

(c) Pulse energy 

(d) Gain

63. The purpose of the couplant is to: 

(a) Match impedances between the transducer and test piece 

(b) Absorb stray reflectors 

(c) Clean the test piece so a more efficient test may be continued 

(d) Lock the ultrasonic scanner into place prior to testing 

Note: by exclude the air between the 2 interfaces.

72. When conducting an immersion test, the water path distance must be 

controlled so that: 

a) Spurious signals are not created by surface waves on the test piece 

b) The (water path distance)/(diameter) ratio does not result in asymmetric 

standing waves 

c) The test piece discontinuity indications appear between the first front 

and first back surface echoes 

d) The second front surface echo does not appear on the CRT screen 

between the first front and first back surface echoes (?)

Immersion Testing Method

Standards Answer: C

Standards Answer: B

Standards Answer: A

Standards Answer: A (or C?)

Standards Answer: A

Standards Answer: C

Standards Answer: B

Standards Answer: C

Standards Answer: C

Standards Answer: A?

Arrows shown standard correct answers: 

Level I Q&A


Level I Q&A

Study Blueeeeeeee… 

28 th July 2014 17:34

Arrows shown standard correct answers:

mms://a588.l3944020587.c39440.g.lm.akamaistre 

am.net/D/588/39440/v0001/reflector:20587?BBC- 

UID=e5203c9d59fef1a79c12d8c601e839f58db16f7 

d5d6448f55674c540f1856834&SSO2-UID=


Level II Q&A 

http://www.mtv123.com/mp3/45297/326534.shtml



R↑∝ F↑




3-Screen Height Linearity 

The ultrasonic testing instrument shall provide linear vertical presentation 

within ±5% (According to ASME Sec.V, Article 5 T-532) of the full screen 

height for 20% to 80% of the calibrated screen height. 

The procedure for evaluating screen height linearity is provided in appendix 1 

of article 5, ASME code Sec.V and shall be performed at the beginning of 

each period of extended use (or every 3 months, which ever is less). 

http://www.inspection-for-industry.com/ultrasonic-testing.html

Take a break 

mms://a588.l3944020587.c39440.g.lm.akamaistream.net/D/588/394 

40/v0001/reflector:20587?BBC- 

UID=e5203c9d59fef1a79c12d8c601e839f58db16f7d5d6448f55674c5 

40f1856834&SSO2-UID=

Calculation: Incident angle= 7° 

Refracted longitudinal wave = 29.11° 

Refracted shear wave = 15.49°



Q2. During ultrasonic inspection of a weld, having a thickness of 28 mm angle 

beam search units are to be used. The recommended angle of search unit 

Is: 

a. 70º 

b. 60º 

c. 45º 

d. any one

Practices Make Perfect


Click to Q&A 


Questions More Reading & on Answers Q&A 


Ultrasonic Formula

Inverse Law and Inverse Square Law 



the distance. 

5cm 

7.5cm 

75% FSH 33% FSH



Inverse Law: 




10cm 

7.5cm

Echo Amplitude- Reflector Size “D” & Depth “d” Relations: 

(small reflector- Inverse square law) 

Amplitude α D 2 

Amplitude α 1/d 2 

Amplitude = kD 2 /d 2 , k =constant 

Amplitude 1 / Amplitude 2 = D 12 d 22 / d 12 D 2 

2 

d 

Amplitude 

D


(large reflector- inverse law) 

Amplitude α 1/d 

Amplitude = k/d , k =constant, 

Amplitude 1 / Amplitude 2 = d 2 / d 1 

d 

Amplitude 

D


Scanner speed = (PRR / Number of hits) × Effective diameter of probe 

Speed of test part = (PRR / Number of hits) × Effective diameter of probe 

Where: 



PRR = Pulse Repetition Rate 

Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60) 

where r = radius of disc or pipe in mm 

RPM = Number of Rotation of pipe Per Minute = Revolution Per Minute

Addendum-04A 

Questions & Answers 

on Calculations 


2014-June.

Expert at Works

Content: 

Exercise 1 

Exercise 2 

Expert at Works too!

Content: 

Exercise 1 

Exercise 2 

Expert at Works – The real achiever!

Questions & Answers



Click to Q&A 


Questions More Reading & on Answers Q&A 




Ultrasonic Formula

Ultrasonic Formula

Inverse Law and Inverse Square Law 



the distance. 

5cm 

7.5cm 

75% FSH 33% FSH



Inverse Law: 




10cm 

7.5cm


(small reflector- Inverse square law) 

Amplitude α D 2 

Amplitude α 1/d 2 

Amplitude = kD 2 /d 2 , k =constant 

Amplitude 1 / Amplitude 2 = D 12 d 22 / d 12 D 2 

2 

d 

Amplitude 

D


(large reflector- inverse law) 

Amplitude α 1/d 

Amplitude = k/d , k =constant, 

Amplitude 1 / Amplitude 2 = d 2 / d 1 

d 

Amplitude 

D


Scanner speed = (PRR / Number of hits) × Effective diameter of probe 

Speed of test part = (PRR / Number of hits) × Effective diameter of probe 

Where: 



PRR = Pulse Repetition Rate 

Linear speed of disc or pipe in mm/ s = (2πr x RPM / 60) 

where r = radius of disc or pipe in mm 

RPM = Number of Rotation of pipe Per Minute = Revolution Per Minute

Addendum-04B 

Questions & Answers- I II III 


2014-June.

Offshore Lifts

Production Platform 


Top Scorer

Content: 

1. Exercise 01 - Studyblue 

2. Exercise 02 - Studyblue

Exercises 

Studyblue-01

1. The wave mode that has multiple or varying wave velocities is: 




D. Lamb waves 

2. Which of the following would be considered application(s) of ultrasonic 

techniques? 

A. Determination of a material’s elastic modulus 

B. Study of a material’s metallurgical structure 

C. Measurement of a material’s thickness 



3. The only significant sound wave mode that travels through a liquid is: 

A. Shear wave 

B. Longitudinal wave 

C. Surface wave 

D. Rayleigh wave 

4. The acoustic impedance of a material is used to determine the: 

A. Angle of refraction at an interface 

B. Attenuation within the material 

C. Relative amounts of sound energy coupled through and reflected 

at the interface 

D. Beam spread within the material

5. When angle beam contact testing a test piece, increasing the incident 

angle until the second critical angle is reached results in: 

A. Total reflection of a surface wave 

B. 45 degree refraction of the shear wave 

C. Production of a surface wave 


6. Acoustic energy propagates in different modes. Which of the following 

represents a mode? 

A. A longitudinal wave 

B. A shear wave 

C. A surface wave 

D. All of the above

7. The simple experiment where a stick in a glass of water appears disjointed 

at the water surface illustrates the phenomenon of: 

A. Reflection 

B. Magnification 

C. Refraction 

D. Diffraction 

8. The crystal thickness and transducer frequency are related. The thinner 

the crystal: 

A. The lower the frequency 

B. The higher the frequency 

C. There is no appreciable affect 


9. The random distribution of crystallographic direction in alloys with large 

crystalline structures is a factor in determining: 

A. Acoustic noise levels 

B. Selection of test frequency 

C. Scattering of sound 


10. The length of the zone adjacent to a transducer in which functions in 

sound pressure occur is mostly affected by: 

A. The frequency of the transducer 

B. The diameter of the transducer 

C. The length of transducer cable 

D. Both A and B

11. The differences in signals received from identical reflectors at different 

material distances from a transducer may be caused by: 

A. Material attenuation 

B. Beam divergence 

C. Near field effects 


12. It is possible for a discontinuity smaller than the transducer to produce 

indications of fluctuating amplitude as the transducer is moved laterally if 

testing is being performed in the: 

A. Fraunhofer zone 

B. Near field 

C. Snell field 

D. Shadow zone

13. In immersion testing, the near field effects of a transducer may be 

eliminated by: 

A. Increasing transducer frequency 

B. Using a larger diameter transducer 

C. Using an appropriate water path 

D. Using a focused transducer 

14. In the far field of a uniform ultrasonic beam, sound intensity is 

___________ the beam centerline. 

A. Minimum at 

B. Maximum at 

C. Maximum throughout twice the angle (sin Y=C/Df) Where is acoustic 

velocity, D is crystal diameter, and f is frequency at 

D. Not related to orientation of

15. Which of the following may result in a long narrow rod if the beam 

divergence results in a reflection from a side of the test piece before the 

sound wave reaches the back surface? 

A. Multiple indications before the first back reflection 

B. Indications from multiple surface reflections 

C. Conversion from the longitudinal mode to shear mode 

D. Loss of front surface indications 

16. Where does beam divergence occur? 

A. Near field 

B. Far field 

C. At the crystal 


17. As frequency increases in ultrasonic testing, the angle of beam 

divergence of a given diameter crystal: 

A. Decreases 

B. Remains unchanged 

C. Increases 

D. Varies uniformly though each wavelength 

18. As the radius of curvature of a curved lens is increased, the focal length of 

the lens: 

A. Increases 

B. Decreases 

C. Remains the same 

D. Cannot be determined unless the frequency is known

19. When examining materials for planar flaws oriented parallel to the part 

surface, what testing method is most often used? 

A. Angle beam 

B. Though-transmission 

C. Straight beam 

D. Dual crystal 

20. If a contact angle beam transducer produces a 45 degrees shear wave in 

steel, the angle produced by the same transducer in an aluminum 

specimen would be: 

A. Less than 45 degrees (correct answer) 

B. Greater then 45 degrees 

C. 45 degrees 

D. Unknown: more information is required 

Hint: V s Steel = 3250, V s Al=3130, however V L Steel = 5920, V L Al=6320 m/s

Popey 

http://192.81.248.91:8095/

21. Rayleigh waves are influenced most by defects located: 

A. Close to or on the surface 

B. 1 wavelength below the surface 

C. 3 wavelengths below the surface 

D. 6 wavelengths below the surface 

Hint: One wave length deep and not one wave length below! 

22. The ultrasonic testing technique in which finger damping is most effective 

in locating a discontinuity is the: 

A. Shear wave technique 

B. Longitudinal wave technique 

C. Surface wave technique 

D. Compressional wave technique

23. Lamb waves can be used to detect: 

A. Laminar-type defects near the surface of the thin material 

B. Lack of fusion in the center of a thick weldment 

C. Internal voids in diffusion bonds 

D. Thickness changes in heavy plate material 

24. The ratio of the velocity of sound in water compared to that of aluminum 

or steel is approximately: 

A. 1:8 

B. 1:4 

C. 1:3 

D. 1:2

25. Which of the following scanning methods could be classified as an 

immersion type test? 

A. Tank in which the transducer and test piece are immersed 

B. Squirter bubbler method in which the sound is transmitted in a column of 

flowing water 

C. Scanning with a wheel-type transducer with the transducer inside a liquid 

filled tire 


26. In an immersion test of a piece of steel or aluminum, the water distance 

appears on the display as a fairly wide space between the initial pulse and 

the front surface reflection because of: 

A. Reduced velocity of sound in water as compared to test specimen 

B. Increased velocity of sound in water as compared to test specimen 

C. Temperature of the water 

D. All of the above

27. Using the immersion method, a distance amplitude curve (DAC) for a 19 

mm (0.75 in) diameter, 5 MHz transducer shows the high point of the DAC at 

the B/51 mm (2 in) block. One day later, the high point of the DAC for the 

same transducer is at the J/102 mm (4 in) block. Assuming that calibration 

has not changed, this would indicate that the transducer: 

A. Is improving in resolution 

B. Is becoming defective 

C. Has the beam of a smaller transducer 

D. Both B and C (?) 

28. What law can be used to calculate the angle of refraction within a metal 

for both longitudinal and shear waves? 

A. Poisson’s ratio law 

B. Snell’s law 

C. Fresnel’s field law 

D. Charles’ law

29. At an interface between two different materials, an impedance difference 

results in: 

A. Reflection of the entire incident energy at the interface 

B. Absorption of sound 

C. Division of sound energy into transmitted and reflected modes 


30. When using focused transducers, non-symmetry in a propagated sound 

beam may be caused by: 

A. Backing material variations 

B. Lens centering or misalignment 

C. Porosity in lenses 

D. All of the above

31. Ultrasonic wheel units may be used for which of the following types of 

examination? 

A. Straight or longitudinal examination 

B. Angle beam or shear wave examination 

C. Surface wave or Rayleigh wave examination 


32. During straight beam testing, test specimens with non-parallel front and 

back surfaces can cause: 

A. Partial or total loss of back reflection 

B. No loss in back reflection 

C. A widened (broad) back reflection indication 

D. A focused (narrow) back reflection indication

33. In the immersion technique, the distance between the face of the 

transducer and the test surface (water path) is usually adjusted so that the 

time required to send the sound beam through the water: 

A. Is equal to the time required for the sound to travel through the test piece 

B. Is greater than the time required for the sound to travel through the 

test piece 

C. Is less than the time required for the sound to travel through the test piece 


34. In a B-scan display, the length of a screen indication from a discontinuity 

is related to: 

A. A discontinuity’s thickness as measured parallel to the ultrasonic beam 

B. The discontinuity’s length in the direction of the transducer level 



35. Which circuit triggers the pulser and sweep circuits in an A-scan display? 

A. Receiver-amplifier 

B. Power supply 

C. Clock 

D. Damping 

36. On an A-scan display, the “dead zone” refers to: 

A. The distance contained within the near field 

B. The area outside the beam spread 

C. The distance covered by the front surface pulse with and recovery 

time 

D. The area between the near field and the far field

37. On an A-scan display, what represents the intensity of a reflected beam? 

A. Echo pulse width 

B. Horizontal screen location 

C. Signal brightness 

D. Signal amplitude 

38. Of the following scan types, which one can be used to produce a 

recording of flaw areas superimposed over a plan view of the test piece? 

A. A-scan 

B. B-scan 

C. C-scan 

D. D-scan

39. In immersion testing in a small tank, a manually operated manipulator is 

use to: 

A. Set the proper water path 

B. Set the proper transducer angle 

C. Set the proper index function 

D. Complete both A and B 

40. In straight (normal) beam contact testing, a reduction in the back surface 

reflection amplitude could indicate: 

A. Inadequate coupling 

B. A flaw which is not normal to the beam 

C. A near surface defect that cannot be resolved from the main bang (initial 

pulse) 

D. All of the above

41. A 152 mm (6 in) diameter rod is being inspected for centerline cracks. The 

A-scan presentation for one complete path through the rod is as shown in 

Figure 2. The alarm gate should: 

A. Be sued between points A and E 

B. Be used at point D only 

C. Be used between points B and D 

D. Not be used for this application

42. In an automatic scanning immersion unit, the bridge or carriage serves to: 

A. Support the manipulator and scanner tube and to move it about 

transversely and longitudinally 

B. Control the angular and transverse positioning of the scanner tube 

C. Control the vertical and angular positioning of the scanner tube 

D. Raise and lower the transducer

Immersion Testing 

Bridge 

Manipulator 

Tube

43. When adjusting the flaw locating rule for a shear wave weld inspection, 

the zero point on the rule must coincide with the: 

A. Sound beam exit point of the wedge 

B. Point directly over the flaw 

C. Wheel transducer 

D. Circular scanner 

44. A special scanning device with the transducer mounted in a tire-like 

container filled with couplant is commonly called: 

A. A rotating scanner 

B. An axial scanner 

C. A wheel transducer 

D. A circular scanner

45. Which best describes a typical display of a crack whose major surface is 

perpendicular to the ultrasonic beam? 

A. A broad indication 

B. A sharp indication 

C. The indication will not show due to improper orientation 

D. A broad indication with high amplitude 

46. A primary purpose of a reference standard is: 

A. To provide a guide for adjusting instrument controls to reveal 

discontinuities that are considered harmful to the end use of the product. 

B. To give the technician a tool for determining exact discontinuity size 

C. To provide assurance that all discontinuities smaller than a certain 

specified reference reflector are capable of being directed by the test. 

D. To provide a standard reflector which exactly simulates natural 

discontinuities of a critical size.

47. Compensation for the variation in echo height related to variations in 

discontinuity depth in the material is known as: 

A. Transfer 

B. Attenuation 

C. Distance amplitude correction 

D. Interpretation 

48. Which of the following is a reference reflector that is not dependent on 

beam angle? 

A. A flat bottom hole 

B. A vee notch 

C. A side drilled hole which is parallel to the plate surface and 

perpendicular to the sound path 

D. A disc-shaped laminar reflector

49. During a straight beam ultrasonic test, a discontinuity indication is 

detected that is small in amplitude compared to the loss in amplitude of back 

reflection. The orientation of this discontinuity is probably: 

A. Parallel to the test surface 

B. Perpendicular to the sound beam 

C. Parallel to the sound beam 

D. At an angle to the test surface 

50. A discontinuity is located having an orientation such that its long axis is 

parallel to the sound beam. The indication from such a discontinuity will be: 

A. Large in proportion to the length of the discontinuity 

B. Small in proportion to the length of the discontinuity 

C. Representative of the length of the discontinuity 

D. Such that complete loss of back reflection will result

51. Gas discontinuities are reduced to flat discs or other shapes parallel to the 

surface by: 

A. Rolling 

B. Machining 

C. Casting 

D. Welding 

52. In which zone does the amplitude of an indication from a given 

discontinuity diminish exponentially as the distance increases? 

A. The far field zone 

B. The near field zone 

C. The dead zone 

D. The Fresnel zone

53. A smooth flat discontinuity whose major plane is not perpendicular to the 

direction of sound propagation may be indicated by: 





54. Using a pulse echo technique, if the major plane of a flat discontinuity is 

oriented at some angle other than perpendicular to the direction of sound 

propagation the result may be: 

A. Loss of signal linearity 

B. Loss or lack of a received discontinuity echo 

C. Focusing of the sound beam 

D. Loss of interference phenomena

55. As transducer diameter decreases, the beam spread: 

A. Decreases 

B. Remains the same 

C. Increases 

D. Becomes conical in shape 

56. A set of standard reference blocks with the same geometrical 

configuration and dimensions other than the size of the calibration reflectors, 

e.g., flat bottom holes, is called a set of: 

A. Distance amplitude standards 

B. Area amplitude standards 

C. Variable frequency blocks 

D. Beam spread measuring blocks

57. The angle at which 90 degrees refraction of a longitudinal sound wave is 

reached is called: 

A. The angle of incidence 

B. The first critical angle 

C. The angle of maximum reflection 

D. The second critical angle 

58. The control of voltage supplied to the vertical deflection plates of the 

instrument display in an A-scan UT setup is performed by the: 

A. Sweep generator 

B. Pulser 

C. Amplifier circuit 

D. Clock timer

59. Attenuation is a difficult quantity to measure accurately, particularly in 

solid materials, at the test frequencies normally used. The overall result 

usually observed includes other loss mechanisms which can include: 

A. Beam spread 

B. Couplant mismatch 

C. Test piece geometry 


60. The vertical linear range of a test instrument may be determined by 

obtaining ultrasonic responses from: 

A. A set of distance amplitude reference blocks 

B. Steel balls located at several different water path distances 

C. A set of area amplitude reference blocks 

D. All of the above

61. Large gains in a metallic test specimen usually result in: 

A. A decrease or loss of back surface reflection 

B. Large “hash” or noise indications 

C. A decrease in penetration 


62. The total energy losses occurring in all materials is called: 


B. Scatter 

C. Beam spread 

D. Interface

63. Delay-tip (stand-off) type contact transducer are primarily used for: 

A. Defect detection 

B. Sound wave characterization 

C. Thickness measurement or flaw detection in thin materials 

D. Attenuation measurements 

64. Acoustical lenses are commonly used for contour correction. When 

scanning the inside of a pipe section by the immersion method, use a: 

A. Focused cup lens 

B. Convex lens 

C. Concave lens 

D. Variable pitch lens

65. In Figure 3, transducer A is being used to establish: 

A. Verification of wedge angle 

B. Sensitivity calibration 

C. Resolution 

D. An index point

66. In Figure 3, transducer C is being used to check: 

A. Distance calibration 

B. Resolution 



67. In Figure 3, transducer D is being used to check: 

A. Sensitivity calibration 

B. Distance calibration 

C. Resolution 


68. When the incident angle is chosen to be between the first and second 

critical angles, the ultrasonic wave generated within the part will be: 


B. Shear 

C. Surface 

D. Lamb

69. In Figure 4, transducer B is being used to check: 

A. The verification of wedge angle 

B. Resolution 


D. Distance calibration

70. The angle at which 90 degrees refraction of the shear wave mode occurs 

is called the: 

A. First critical angle 

B. Second critical angle 

C. Third critical angle 

D. Angle of reflection 

71. In a water immersion test, ultrasonic energy is transmitted into steel at an 

incident angle of 14 degrees. What is the angle of the refracted shear wave 

within the material? 

A. 45 degrees 

B. 23 degrees 

C. 31 degrees 

D. 13 degrees

72. If you were requested to design a plastic shoe to generate a Rayleigh 

wave in aluminum, what would be the incident angle of the ultrasonic 

energy? 

A. 37 degrees 

B. 57 degrees 

C. 75 degrees 

D. 48 degrees 

73. Compute the wavelength of ultrasonic energy in lead at 1 MHz 

A. 0.21 cm 

B. 21 cm 

C. 0.48 cm 

D. 4.8x10-3 cm

74. For aluminum and steel, the longitudinal velocity is approximately 

_________ the shear velocity. 

A. Equal to 

B. Twice 

C. Half of 

D. Four times 

75. Water travel distance for immersion inspections should be: 

A. Such that the second front reflection does not appear between the 

first front and back reflections 

B. Exactly 76 mm (3 in) 

C. Less than 76 mm (3 in) 

D. Always equal to the thickness of the material being inspected

76. The electronic circuitry that allows selection and processing of only those 

signals relating to discontinuities that occur in specific zones of a part is 

called: 

A. An electronic gate 

B. An electronic attenuator 

C. A distance amplitude correction circuit 

D. A fixed marker 

77. When conducting a contact ultrasonic test, the “hash” or irregular signals 

that appear in the CRT display of the area being inspected could be caused 

by: 

A. Fine grains in the structure 

B. Dirt in the water couplant 

C. Coarse grains in the structure 

D. A thick but tapered back surface

78. In inspecting a 102 mm (4 in) diameter threaded steel cylinder for radial 

cracks extending from the root of the threads, it would be preferable to 

transmit: 

A. Shear waves at an angle to the threads 

B. Longitudinal waves from the end of the cylinder and perpendicular to 

the direction of the thread roots 

C. Surface waves perpendicular to the thread roots 

D. Shear waves around the circumference of the cylinder 

79. In an immersion inspection of raw material, the water travel distance 

should be: 

A. Exactly 76 mm (3 in) 

B. Equal to 76 mm (3 in) ± 13 mm (± 0.5 in) 

C. Equal to the water travel distance used in setting up on the reference 

standards 

D. Equal to the thickness of the material

80. The angle formed by an ultrasonic wave as it enters a medium of different 

velocity than the one from which it came and a line drawn perpendicular to 

the interface between the two media is called the angle of: 

A. Incidence 

B. Refraction 

C. Rarefaction 

D. Reflection

81. The process of adjusting an instrument or device to a reference standard 

is referred to as: 

A. Angulation 

B. Scanning 

C. Correcting for distance amplitude variation 

D. Calibration 

82. An electron tube in which a beam of electrons from the cathode is used to 

reproduce an image on a display at the end of the tube is referred to as: 

A. An amplifier tube 

B. A pulser tube 

C. A cathode ray tube 

D. A sweep tube

83. A grouping of a number of crystals in one transducer, with all contact 

surfaces in the same plane, and vibrating in phase with each other to act as a 

single transducer is called a: 

A. Focusing crystal 

B. Crystal mosaic 

C. Scrubber 

D. Single plane manipulator 

84. The angle of reflection is: 

A. Equal to the angle of incidence 

B. Dependent on the couplant used 

C. Dependent on the frequency used 

D. Equal to the angle of refraction

WPS/PQR

85. The angular position of the reflecting surface of a planar discontinuity with 

respect to the entry surface is referred to as: 

A. The angle of incidence 

B. The angle of refraction 

C. The orientation of the discontinuity 


86. A short burst of alternating electrical energy is called: 

A. A continuous wave 

B. A peaked DC voltage 

C. An ultrasonic wave 

D. A pulse

87. In ultrasonic testing, the time duration of the transmitted pulse is referred 

to as: 

A. The pulse length or pulse width 

B. The pulse amplitude 

C. The pulse shape 


88. The phenomenon by which a wave strikes a boundary and changes 

direction of its propagation within the same medium is referred to as: 

A. Divergence 

B. Impedance 

C. Angulation 

D. Reflection

89. The change in direction of an ultrasonic beam when it passes from one 

medium to another whose velocity differs from that of the first medium I called: 

A. Refraction 

B. Rarefaction 

C. Angulation 

D. Reflection 

90. The coated inside surface of the large end of a cathode ray tube which 

becomes luminous when struck by an electron beam is called: 

A. An electron gun 

B. An electron amplifier 

C. An ultrasonic instrument display 

D. An electron counter

91. Which of the following modes of vibration exhibits the shortest wavelength 

at a given frequency and in a given material? 

A. A longitudinal wave 

B. A compression wave 

C. A shear wave 

D. A surface wave 

92. In general, shear waves are more sensitive to small discontinuities than 

longitudinal wave for a given frequency and in a given material because: 

A. The wavelength of a shear wave is shorter than the wavelength of 

longitudinal waves 

B. Shear waves are not as easily dispersed in the material 

C. The direction of particle vibration for shear waves is more sensitive to 

discontinuities 

D. The wavelength of shear waves is longer than the wavelength of 

longitudinal waves

93. In general, shear waves are more sensitive to small discontinuities than 

longitudinal wave for a given frequency and in a given material because: 

A. The wavelength of a shear wave is shorter than the wavelength of 


B. Shear waves are not as easily dispersed in the material 

C. The direction of particle vibration for shear waves is more sensitive to 


D. The wavelength of shear waves is longer than the wavelength of 


94. In general, which of the following modes of vibration would have the 

greatest penetrating power in a coarse-grained material if the frequency of the 

waves is the same? 




D. All of the above modes would have the same penetrating power

95. A testing technique in which the crystal or transducer is parallel to the test 

surface and ultrasonic waves enter the material being testing in a direction 

perpendicular to the test surface is: 

A. Straight beam testing 

B. Angle beam testing 

C. Surface wave testing 


96. The distance from a given point on an ultrasonic wave to the next 

corresponding point is referred to as: 

A. Frequency 

B. Wavelength 

C. Velocity 

D. Pulse length

97. The speed with which ultrasonic waves travel through a material is known 

as its: 

A. Velocity 

B. Pulse repetition rate 

C. Pulse recovery rate 

D. Ultrasonic response 

98. The ultrasonic transducers most commonly used for discontinuity testing 

utilize: 

A. Magnetostriction principles 

B. Piezoelectric principles 

C. Mode conversion principles 


99. Mechanical and electrical stability, insolubility in liquids, and resistance to 

aging are three advantages of transducers made of: 

A. Lithium sulfate 

B. Barium titanate 

C. Quartz 

D. Rochelle salts 

100. The formula on below is referred to as: 

A. The acoustical impedance ratio formula 

B. The phase conversion formula 

C. The Fresnel zone formula 

D. Snell’s law

Barbecue Lamb

101. The formula on the right is used to determine: 

A. Angular relationships 

B. Phase velocities 

C. Amount of reflected sound energy 

D. Acoustic impedance 

102. The amount of energy reflected from a discontinuity is dependent on: 

A. The size of the discontinuity 

B. The orientation of the discontinuity 

C. The type of discontinuity 

D. All of the above

103. If ultrasonic wave is transmitted through an interface of two materials in 

which the first material has a higher acoustic impedance value but the same 

velocity value as the second material, the angle of refraction will be: 

A. Greater than the angle of incidence 

B. Less than the angle of incidence 

C. The same as the angle of incidence 

D. Beyond the critical angle 

104. Which of the following frequencies would probably result in the greatest 

ultrasonic attenuation losses? 

A. 1 MHz 

B. 20 MHz 

C. 10 MHz 

D. 25 MHz

105. The product of the sound velocity and the density of a material is known 

as the: 

A. Refraction value of the material 

B. Acoustic impedance of the material 

C. Elastic constant of the material 

D. Poisson’s ratio of the material 

106. The amplifier range over which the unsaturated signal response 

increases in amplitude in proportion to the discontinuity surface area is the: 

A. Sensitivity range 

B. Vertical linearity range 

C. Selectivity range 

D. Horizontal linearity range

107. When inspecting a rolled or forged surface with a thin scale that I 

generally tightly adhering to the part, before testing the part: 

A. Clean the surface of loose scale 

B. Have all scale removed 

C. Rough machine the surface 

D. Caustic etch the surface 

108. The angle of reflection of an ultrasonic beam at an aluminum-water 

interface is: 

A. 0.256 times the angle of incidence 

B. Approximately ½ the angle of incidence 

C. Equal to the angle of incidence 

D. Approximately 4 times the angle of incidence

109. What kind of waves travel at a velocity slightly less than shear waves 

and their mode of propagation is both longitudinal and transverse with respect 

to the surface? 

A. Rayleigh waves 

B. Transverse waves 

C. L-waves 

D. Longitudinal waves 

110. Which ultrasonic test frequency would probably provide the best 

penetration in a 30 cm (12 in) thick specimen of coarse-grained steel? 

A. 1 MHz 

B. 2.25 MHz 

C. 5 MHz 

D. 10 MHz

111. During immersion testing of an ASTM Ultrasonic Standard Reference 

Block, a B-scan presentation system will show a: 

A. “plan” view of the block, showing the area and position of the hole bottom 

as seen from the entry surface 

B. Basic test pattern showing the height of indication from the hole bottom 

and its location in depth from the entry surface 

C. Cross section of the reference block, showing the top and bottom 

surfaces of the block and the location of the hole bottom in the block 


112. Properties of shear or transverse waves used for ultrasonic testing 

include: 

A. Particle motion normal to propagation direction, and a propagation 

velocity that is about ½ the longitudinal wave velocity in the same 

material 

B. Exceptionally high sensitivity due to low attenuation resulting from longer 

wavelengths when propagating through water 

C. High Coupling efficiency because shear waves are less sensitive to 

surface variables when traveling from a coupling liquid to the part. 

D. None of the above statements apply to shear waves

115. One of the most common applications of ultrasonic tests employing 

shear waves is for the: 

A. Detection of discontinuities in welds, tube, and pipe 

B. Determination of elastic properties of metallic products 

C. Detection of laminar discontinuities in heavy plate 

D. Measurement of thickness of thin plate 

116. Significant errors in ultrasonic thickness measurement can occur if: 

A. The test frequency is varying at a constant rate 

B. The velocity of propagation deviates substantially from an assumed 

constant value for a given material 

C. Water is employed as a couplant between the transducer and the part 

being measured 

D. None of the above should cause errors

117. Generally, the best ultrasonic testing method for detecting discontinuities 

oriented along the fusion zone in a welded plate is: 

A. An angle beam contact method using surface waves 

B. A contact test using a straight longitudinal wave 

C. An immersion test using surface waves 

D. An angle beam method using shear waves 

118. An ultrasonic testing instrument that displays pulses representing the 

magnitude of reflected ultrasound as a function of time or depth of metal is 

said to contain: 

A. A continuous wave display 




119. At a water-steel interface the angle of incidence in water is 7 degrees. 

The principal mode of vibration that exists in the steel is: 


B. Shear 


D. Surface 

Hint: Keyword-Principle 

120. In a liquid medium, the only mode of vibration that can exist is: 


B. Shear 


D. Surface

121. In an ultrasonic instrument, the number of pulses produced by an 

instrument in a given period of time is known as the: 

A. Pulse length of the instrument 

B. Pulse recovery time 

C. Frequency 

D. Pulse repetition rate 


coordinates the action and timing of other components is called a: 

A. Display unit 

B. Receiver 

C. Marker circuit or range marker circuit 

D. Synchronizer, clock, or timer


produces the voltage that activates the transducer is called: 

A. An amplifier 

B. A receiver 

C. A pulser 

D. A synchronizer 

124. In basic pulse echo ultrasonic instrument, the component that produces 

the time base line is called a: 


B. Receiver 

C. Pulser 

D. Synchronizer


produces visible signals on the CRT which are used to measure distance is 

called a: 


B. Marker circuit 

C. Receiver circuit 

D. Synchronizer 

126. Most basic pulse echo ultrasonic instruments use: 





127. The instrument displays a plan view of the part outline and defects when 

using: 




D. A C-scan presentation 

128. The incident angles at which 90 degrees refraction of longitudinal and 

shear waves occurs are called: 

A. The normal angles of incidence 

B. The critical angles 

C. The angles of maximum reflection 


129. Compression waves whose particle displacement is parallel to the 

direction of propagation are called: 



C. Lamb waves 

D. Rayleigh waves 

130. The mode of vibration that is quickly damped out when testing by the 

immersion method is: 




D. Surface waves

131. The motion of particles in a shear wave is: 

A. Parallel to the direction of propagation of the ultrasonic beam 

B. Transverse to the direction of the beam propagation 

C. Limited to the material surface and elliptical in motion 

D. Polarized in a plane at 45 degrees to the direction of beam propagation 

132. An ultrasonic longitudinal wave travels in aluminum with a velocity of 

635,000 cm/s and has a frequency of 1 MHz. The wavelength of this 

ultrasonic wave is: 

A. 6..35 mm (0.25 in) 

B. 78 mm (3.1 in) 

C. 1.9 m (6.35 ft) 

D. 30,000 A

133. The refraction angle of longitudinal ultrasonic waves passing from water 

into a metallic material at angles other than normal to the interface is 

primarily a function of: 

A. The impedance ratio (r = Z w Z M ) of water to metal 

B. The relative velocities of sound in water and metal 

C. The frequency of the ultrasonic beam 

D. The density ratio of water to metal 

134. In contact testing, shear waves can be induced in the test material by: 

A. Placing an X-cut crystal directly on the surface of the materials, and 

coupling through a film of oil 

B. Using two transducers on opposite sides of the test specimen 

C. Placing a spherical acoustic lens on the face of the transducer 

D. Using a transducer mounted on a plastic wedge so that sound enters 

the part at an angle

135. As frequency increases in ultrasonic testing, the angle of beam 

divergence of a given diameter crystal: 

A. Decreases 

B. Remains unchanged 

C. Increases 

D. Varies uniformly through each wavelength 

136. Which of the following is not an advantage of contact ultrasonic 

transducers (probes) adapted with Lucite shoes? 

A. Most of the crystal wear is eliminated 

B. Adaption to curved surfaces is permitted 

C. Sensitivity is increased 

D. Ultrasound is allowed to enter a part’s surface at oblique angles

137. The velocity of sound is the lowest in: 

A. Air 

B. Water 

C. Aluminum 

D. Plastic 

138. A longitudinal ultrasonic wave is transmitted from water into steel at an 

angle of 5 degrees from the normal. In such a case, the refracted angle of the 

transverse wave is: 

A. Less than the refracted angle of the longitudinal wave 

B. Equal to the refracted angle of the longitudinal wave 

C. Greater than the refracted angle of the longitudinal wave 

D. Not present at all

139. The velocity of longitudinal waves is the highest in: 

A. Water 

B. Air 

C. Aluminum 

D. Plastic 

140. In steel, the velocity of sound is greatest in: 



C. Surface waves 

D. None of the above – sound velocity is identical in all modes, in a give 

material

141. The acoustic impedance is: 

A. Used to calculate the angle of reflection 

B. The product of the density of the material and the velocity of sound 

in the material 

C. Found by Snell’s law 

D. Used to determine resonance values 

142. Thin sheet may be inspected with the ultrasonic wave directed normal to 

the surface by observing: 

A. The amplitude of the front surface reflection 

B. The multiple reflection pattern 

C. All front surface reflections 


143. A diagram in which the entire circuit stage or sections are shown by 

geometric figures and the path of the signal or energy by lines and/or arrows 

is called a: 

A. Schematic diagram 

B. Blueprint 

C. Block diagram 


144. A hole produced during the solidification of metal due to escaping gases 

is called: 

A. A burst 

B. A cold shut 

C. Flaking 

D. A blow hole

145. A discontinuity that occurs during the casting of molten metal which may 

be caused by the splashing, surging, interrupted pouring, or the meeting of 

two streams of metal coming from different directions is called: 

A. A burst 

B. A cold shut 

C. Flaking 

D. A blow hole 

146. The ratio between the wave speed in one material and the wave speed 

in a second material is called: 

A. The acoustic impedance of the interface 

B. Young’s modulus 

C. Poisson’s ratio 

D. The index of refraction

147. The expansion and contraction of a magnetic material under the 

influence of a changing magnetic field is referred to as: 

A. Piezoelectricity 

B. Refraction 

C. Magnetostriction 

D. Rarefaction 

148. The ratio of stress to strain in a material with the elastic limit is called 

A. Young’s modulus 

B. The modulus of elasticity 


D. The index of refraction

149. A point, line, or surface of a vibrating body marked by absolute or 

relative freedom from vibratory motion is referred to as: 

A. A node 

B. An antinode 

C. Rarefaction 

D. Compression 

150. The factor that determines the amount of reflection at the interface of two 

dissimilar materials is: 

A. The index of rarefaction 

B. The frequency of the ultrasonic wave 

C. Young’s modulus 

D. The acoustic impedance

151. A quartz crystal cut so that its major faces are parallel to the Z and Y 

axes and perpendicular to the X axis is called: 

A. A Y-cut crystal 

B. An X-cut crystal (correct answer) 

C. A Z-cut crystal 

D. A ZY-cut crystal 

Keyword: Perpendicular to the axis 

Perpendicular to the X axis

152. The equation describing wavelength in terms of velocity and frequency is: 

A. Wavelength = velocity X frequency 

B. Wavelength = Z (frequency X velocity) 

C. Wavelength = velocity ÷ frequency 

D. Wavelength = frequency ÷ velocity

153. When an ultrasonic beam reaches the interface of two dissimilar 

materials it is: 

A. Reflected 

B. Refracted 

C. Mode converted 


154. When inspecting aluminum by the immersion method using water for a 

couplant, the following information is known: The angle of refraction for 

longitudinal wave is approximately 

• Velocity of sound in water = 1.49 X 10 5 cm/s 

• Velocity of longitudinal waves in aluminum = 6.32 X 10 5 cm/s, and angle of 

incidence = 5 degrees 

A. 22 degrees 

B. 18 degrees 

C. 26 degrees 

D. 16 degrees

155. Of the piezoelectric materials listed below, the most efficient sound 

transmitter is: 


B. Quartz 

C. Barium titanate 

D. Silver oxide 

156. Of the piezoelectric materials listed below, the most efficient sound 

receiver is: 


B. Quartz 

C. Barium titanate 

D. Silver oxide

157. The most common used method of producing shear waves in a test part 

when inspecting by the immersion method is: 

A. By transmitting longitudinal wave into a part in a direction perpendicular to 

its front surface 

B. By using two crystals vibrating at different frequencies 

C. By using a Y-cut quartz crystal 

D. By angulating the search tube to the proper angle 

158. Beam divergence is a function of the dimensions of the crystal and the 

wavelength of the beam transmitted through a medium, and it: 

A. Increases if the frequency or crystal diameter decreases 

B. Decreases if the frequency or crystal diameter decreases 

C. Increases if the frequency increases and crystal diameter decreases 

D. Decreases if the frequency is increases and crystal diameter decreases

159. The wavelength of an ultrasonic wave is: 

A. Directly proportional to velocity and frequency 

B. Directly proportional to velocity and inversely proportional to 

frequency 

C. Inversely proportional to velocity and directly proportional to frequency 

D. Equal to the product of velocity and frequency 

160. The fundamental frequency of a piezoelectric crystal is primarily a 

function of: 

A. The length of the applied voltage pulse 

B. The amplifying characteristics of the pulse amplifier in the instrument 

C. The thickness of the crystal 


161. Acoustic velocities of materials are primarily due to the material’s: 

A. Density 

B. Elasticity 


D. Acoustic impedance 

Inspection of castings is often impractical because of: 

A. Extremely small grain structure 

B. Coarse grain structure 

C. Uniform flow lines 

D. Uniform velocity of sound

163. Lamb waves may be used to inspect: 

A. Forgings 

B. Bar stock 

C. Ingots 

D. Thin sheet 

164. The formula used to determine the angle of beam divergence of a quartz 

crystal is: 

A. Sin ϴ = diameter ½ X wavelength 

B. Sin ϴ diameter = frequency X wavelength 

C. Sin ϴ = frequency X wavelength 

D. Sin ϴ /2 = 1.22 X wavelength/diameter

165. The resolving power of a transducer is directly proportional to its: 

A. Diameter 

B. Bandwidth (standard answer but how?) 

C. Pulse repetition 


166. Acoustic lens elements with which of the following permit focusing the 

sound energy to enter cylindrical surfaces normally or along a line focus? 

A. Cylindrical curvatures 

B. Spherical lens curvatures 

C. Convex shapes 

D. Concave shapes

167. The primary requirement of a paintbrush transducer is that: 

A. All crystals be mounted equidistant from each other 

B. The intensity of the beam pattern not vary greatly over the entire 

length of the transducer 

C. The fundamental frequency of the crystals not very more than 0.01% 

D. The overall length not exceed 76 mm (3 in) 

168. Heat conduction, viscous friction, elastic hysteresis, and scattering are 

four different mechanisms which lead to: 


B. Refraction 

C. Beam spreading 

D. Saturation

169. Because the velocity of sound in aluminum is approximately 245,000 in/s 

for sound to travel through 25 mm (1 in) of aluminum, it takes: 

A. 1/8 s 

B. 4 µs 

C. 4 ms 

D. ¼ X 10 4 s 

170. When testing a part with a rough surface, it is generally advisable to sue: 

A. A lower frequency transducer and more viscous couplant than is 

used on parts with a smooth surface 

B. A high frequency transducer and more viscous couplant than is used on 

parts with a smooth surface 

C. A high frequency transducer and a less viscous couplant than is sued on 

parts with a smooth surface 

D. A lower frequency transducer and a less viscous couplant than is used of 

parts with a smooth surface

171. Reflection indications from a weld area being inspected by the angle 

beam technique may represent: 

A. Porosity 

B. Cracks 

C. Weld bead 


172. During a test using A-scan equipment, strong indications that move at 

varying rates across the screen in the horizontal direction appear. It is 

impossible to repeat a particular screen pattern by scanning the same area. A 

possible cause of these indications is: 

A. Porosity in the test part 

B. An irregularly shaped crack 

C. A blow hole 

D. Electrical interference

173. In an A-scan presentation, the horizontal line formed by the uniform and 

repeated movement of an electron beam across the fluorescent screen of a 

cathode ray tube is called: 

A. A square wave pattern 

B. A sweep line 

C. A marker pattern 


174.The greatest amount of attenuation losses take place at: 

A. 1 MHz 

B. 2.25 MHz 

C. 5 MHz 

D. 10 MHz

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175. Waves that travel around gradual curves with little or no reflection from 

the curve are called: 

A. Transverse waves 

B. Surface waves 

C. Shear waves 

D. Longitudinal waves 

176. To evaluate and accurately locate discontinuities after scanning a part 

with a paintbrush transducer, it is generally necessary to use a: 


B. Scrubber 

C. Grid map 

D. Crystal collimator 

Hint: Paint brush is supposes to be a long elongated probe with a larger 

dimension, on detecting indication which requires further investigation, a 

smaller probe is a natural choice.

Choices

177. An ultrasonic instrument has been calibrated to obtain a 51 mm (2 in) 

indication from a 2 mm (0.08 in) diameter flat bottom hole located 76 mm (3 in) 

from the front surface of an aluminum reference block. When testing an 

aluminum forging, a 51 mm (2 in) indication is obtained from a discontinuity 

located 76 mm (3 in) from the entry surface. The cross sectional area of this 

discontinuity is probably: 

A. The same as the area of the 2mm flat bottom hole 

B. Greater than the area of the 2mm flat bottom hole 

C. Slightly less than the are of the 2mm flat bottom hole 

D. about 1/2 the area of the 2mm flat bottom hole 

178. As the impedance ratio of two dissimilar materials increases, the 

percentage of sound coupled through an interface of such materials: 

A. Decreases 

B. Increases 

C. Is not changed 

D. May increase or decrease

179. Low frequency sound waves are not generally used to test thin materials 

because of: 

A. The rapid attenuation of low frequency sound 

B. Incompatible wavelengths 

C. Poor near-surface resolution 

D. None of the above will actually limit such a test 

Hint: 

Zf = D 2 /4 λ = D 2 x f /4V , Lower the frequency shorter the Near Field. 

However the resolution is impaired with longer wavelength. 

During the above assessment, is the Near Field effect omitted due to probe 

set-up e.g. Delay Line etc.? Or the Near Field may not necessary 

undetectable except the “Dead Zone”.

Frequency = 2.25 MHZ, Wavelength λ = 2.6mm, Near Zone Z f = 16.25mm 

http://static1.olympus-ims.com/data/Flash/HTML5/beamSpread/BeamSpread.html?rev=6C43

Frequency = 5 MHZ, Wavelength λ = 0.585mm, Near Zone Z f = 36.11mm 

http://static1.olympus-ims.com/data/Flash/HTML5/beamSpread/BeamSpread.html?rev=6C43

180. When using tow separate transducers (one a transmitter, the other a 

receiver), the most efficient combinations would be a: 

A. Quartz transmitter and a barium titanate receiver 

B. Barium titanate transmitter and a lithium sulfate receiver 

C. Lithium sulfate transmitter and a barium titanate receiver 

D. Barium titanate transmitter and a quartz receiver

181. In immersion testing, the accessory equipment to which the search cable 

and the transducer are attached is called a: 

A. Crystal collimator 

B. Scrubber 

C. Jet-stream unit 

D. Search tube or scanning tube 

182. In general, discontinuities in wrought products tend to b oriented: 

A. Randomly 

B. In the direction of grain flow 

C. At right angles to the entry surface 

D. At right angles to the grain flow

Steel Making 

http://www.tatasteelindia.com//products-and-processes/processes/STEEL-MAKING-PROCESS.swf

183. In immersion testing, irrelevant or false indications caused by contoured 

surfaces are likely to result in a: 

A. Broad-based indication 

B. Peaked indication 

C. “hashy” signal 

D. Narrow-based indication 

184. In contact testing, defects near the entry surface cannot always be 

detected because of: 

A. The far-field effect 

B. Attenuation 

C. The dead zone 

D. Refraction

185. In cases where the diameter of tubing being inspected is smaller than 

the diameter of the transducer, what can be used to confine the sound beam 

to the proper range of angles? 

A. A scrubber 

B. A collimator 

C. An angle plane angulator 

D. A jet-stream unit 

186. The maximum scanning speed possible is primarily determined by: 

A. The frequency of the transducer 

B. Viscous drag problems 

C. The pulse repetition rate of the test instrument 

D. The persistency of the ultrasonic instrument display

187. The property of certain materials to transform electrical energy to 

mechanical energy and vice versa is called: 

A. Mode conversion 

B. Piezoelectric effect 

C. Refraction 

D. Impedance matching 

188. Surface waves are reduced to an energy level of approximately 1/25 of 

the original power at a depth of: 

A. 25 mm (1 in) 

B. 102 mm (4 in) 

C. 1 wavelength 

D. Impedance matching

189. To prevent the appearance of he second front surface indication before 

the first back reflection when inspecting aluminum by the immersion method 

(water is used as a couplant), it is necessary to have a minimum of at least 25 

mm (1 in) of water for every: 

A. 51 mm (2 in) of aluminum 

B. 102 mm (4 in) of aluminum 

C. 152 mm (6 in) of aluminum 

D. 203 mm (8 in) of aluminum 

190. Increasing the length of the pulse and used to activate the transducer will: 

A. Increase the strength of the ultrasound but decrease the resolving 

power of the instrument 

B. Increase the resolving power of the instrument 

C. Have no effect on the test 

D. Decrease the penetration of the sound wave

191. The lack of parallelism between the entry surface and the back surface: 

A. May result in a screen pattern that does not contain back reflection 

indications 

B. Makes it difficult to locate discontinuities that lie parallel to the entry 

surface 

C. Usually indicates that a porous condition exists in the metal 

D. Decreases the penetrating power of the test 

192. A discontinuity with a concave surface will: 

A. Diffuse the sound energy throughout the part 

B. Cause the reflected beam to focus at a point determined by the 

curvature of the discontinuity 

C. Cause mode reinforcement of the ultrasonic wave 

D. Cause none of the above

193. Rayleigh waves: 

A. Are generated at the first critical angle 

B. Are generated at the second critical angle 

C. Are generated at either critical angle 

D. Travel only in liquid 

E. Are another name for Lamb waves 

194. Angle beam testing of plate will often miss: 

A. Cracks that are perpendicular to the sound wave 

B. Inclusions that are randomly oriented 

C. Laminations that are parallel to the front surface 

D. A series of small discontinuities

195. Reducing the extent of the dead zone of a transducer by using a delay 

tip results in: 

A. Improved distance amplitude correction in the near field 

B. Reduced frequency of the primary ultrasonic beam 

C. Reduced ability to detect flaws in the near field 

D. Improved accuracy in thickness measurement of thin plate and sheet 

E. None of the above 

196. In plate, skip distance can be calculated from which of the following 

formulas where (t = plate thickness, Θ = angle of sound beam refraction, 

and V = sound velocity) 

A. S = (2 X t)/ tan Θ 

B. S = 2 X t X sin Θ 

C. S = 2 X t X tan Θ 

D. S = 2 X V X sin Θ 


197. The technique of examining an ultrasonic reflector from different 

directions might be used to enable the technician to: 

A. Distinguish between different types of flaws 

B. Predict the useful service life of the test specimen 

C. Distinguish between flaw indications and spurious or flat indications 


E. None of the above 

Discussion: Could “C” be a correct answer too? 

198. The principal application of ultrasonic techniques consists of: 

A. Flaw detection 

B. Thickness measurements 

C. Determination of elastic moduli 



199. Attenuation is the loss of the ultrasonic wave energy during the course of 

propagation in the material due to: 

A. Reflection and refraction 

B. Dispersion and diffraction 

C. Absorption and scattering 

D. Composition and shape 

E. All of the above 

Hint: The attenuation in the material. 

200. When setting up an ultrasonic inspection, the repetition frequency of the 

ultrasonic instrument should be set: 

A. So that its period is at least as long as the operating time 

B. The same as the transducer resonance frequency 

C. As low as possible to avoid over pulsing and distortion 

D. According to the instruction manual of the instrument 


201. In immersion shear wave testing, waves are normally generated by 

angulating the transducer beyond the first critical angle. What is the direction 

of the material’s particle motion? 

A. The same as the wave propagation 

B. Normal to the material surface 

C. Parallel to the direction of wave propagation 

D. Perpendicular to the direction f wave propagation 

E. Only surface waves existed beyond the first critical angle 

202. Which of the following modes of vibration are quickly dampened out 

when testing by the immersion method? 




D. Surface waves

203. The most commonly used method of producing shear waves in a test 

part when inspecting by the immersion method is: 

A. By transmitting longitudinal waves into a part in a direction perpendicular 

to its front surface 

B. By using two crystals vibrating at different frequencies 

C. By suing Y-cut quartz crystal 

D. By angulating the search tube or manipulator to the proper angle

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Exercises 

Studyblue-02 


1. The wave mode that has multiple or varying wave velocities is: 




D. Lamb waves 

2. Which of the following would be considered application (s) of ultrasonic 

techniques? 

A. Determination of a material’s elastic modulus 

B. Study of a material’s metallurgical structure 

C. Measurement of a material’s thickness 

D. All of the above

3. The only significant sound wave mode that travels through a liquid is: 

A. Shear wave 

B. Longitudinal wave 

C. Surface wave 

D. Rayleigh wave 

4. The acoustic impedance of a material is used to determine the: 

A. Angle of refraction at an interface 

B. Attenuation within the material 

C. Relative amounts of sound energy coupled through and reflected at 

the interface 

D. Beam spread within the material

Addendum-04C 

Questions & Answers- I II III 


2014-June.

Reading: 

http://www.freezingblue.com/iphone/flashcards/printPreview.cgi?cardsetID=1 

04091

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Some body make this up, He had never said that.

Assorted Exercises

Practice 1: 

Source: http://www.scribd.com/doc/9086290/Ultrasonic-Solution

Q23. Propagation of ultrasonic wave through the material medium can be 

treated as: 

a) Isothermal 

b) Adiabatic 

c) Both (i) & (ii) 

d) None of these 

Q26. In case of a wave propagating through an absorbing medium, the 

amplitude with distance; 

a) Increases linearly 

b) Decreases linearly 

c) Falls exponentially 

d) None of these

Q27. If E is the bulk modulus of a loss free gas and p is its density, the 

characteristic impedance offered by the gas to the sound wave traveling in it 

is given by; 

a) Z = p E 

b) Z = p 2 E 

c) Z = (p E) 0.5 

d) none of these 

Q29. The condition for which all the incident energy with the incident wave is 

transmitted with no reflection is that impedance of the coupling medium is 

a) Harmonic mean 

b) Arithmetic mean 

c) Product 

d) Sum of the two impedances to be matched

Q30. The sum of reflection & transmission coefficient at junction between two 

media is; 

a) Zero 

b) One 

c) Between zero and one 

d) None of these 

Q31. All the energy arriving at the boundary with the incident wave leaves the 

boundary with the 

a) Reflected wave 

b) Transmitted 

c) Both (i) & (ii) 

d) None of these

Q35. in case of ultrasonography, jelly used between probe and body surface 

for the purpose of 

a) pain relief 

b) removal of etching 

c) coupling 

d) None of these

Practice 2: 

Source: Lavender International: General Assessments: Module 5-2

Q1. Which of the following is a reference reflector that is not dependent on 

beam angle? 

a) A flat bottomed hole 

b) A vee notch 

c) A side drilled hole which is parallel to the plate surface and 

perpendicular to the sound path 

d) A disc shaped laminar reflector 

Q2. Where does beam divergence occur? 

a) Near field 

b) Far field 

c) At the crystal 

d) None of the above

Q3. On a scan display the dead zone refers to? 

a) The distance contained within the near field 

b) The area outside the beam spread 

c) The distance covered by the front surface pulse width and recovery 

time 

d) The area between the near field and far field 

Q4. Which of the following modes of vibration exhibits the shortest 

wavelength at a given frequency and 

a) in a given material? 

b) Longitudinal wave 

c) Compression wave 

d) Shear wave 

e) Surface wave

Q5. Look at diagram one at the foot of the page which illustrates four waves. 

Wave A strikes the surface of the specimen and produces waves B, C and 

D. The incident angle is? 

a) Angle A 

b) Angle B 

c) Angle C 

d) Angle D 

Q6. Diagram two at the foot of the page illustrates four waves. Wave A strikes 

the surface of the specimen and produces waves B, C and D. The 

refraction angle is? 

a) Angle A’ 

b) Angle B 

c) Angle C 

d) Angle D

Q7. In which zone does the amplitude of an indication from a given 

discontinuity diminsh exponentially as the distance increases? 

a) Far field zone 

b) Near field zone 

c) Dead zone 

d) Fresnel zone 

Q8. Rayleigh waves are influenced most by defects located? 

a) One wavelength below the surface 

b) Six wavelengths below the surface 

c) Close to or on the surface 

d) Three wavelengths below the surface

Q9. Of the following sound waves modes one has multiple or varying wave 

velocities? 

a) Longitudinal waves 

b) Shear waves 

c) Transverse waves 

d) Lamb waves 

Q10. Transducers used in ultrasonic testing exhibit which of the following 

effects? 

a) Ferromagnetic 

b) Piezoelectric 

c) Electromechanical 

d) Hyperacoustic

Q11. Of an A-scan display what represents the intensity of the refelected 

beam? 

a) Echo pulse width 

b) Horizontal screen location 

c) Signal brightness 

d) Signal amplitude 

Q12. A short burst of alternating energy is called? 

a) A continuous wave 

b) A peaked dc voltage 

c) An ultrasonic wave 

d) A pulse

Q13. Attenuation is a difficult quantity to measure accurately particularly in 

solid materials at the test frequencies normally used. The overall result 

observed includes other loss mechanisms which can include? 

a) Beam spread 

b) Couplant mismatch 

c) Test piece geometry 

d) All of the above 

Q14. The simple experiment where a stick in a glass of water appears 

disjointed at the water ? 

a) Reflection 

b) Magnification 

c) Refraction 

d) Diffraction

Q15. The ratio of the velocity of sound in water compared to that for 

aluminum or steel is approximately? 

a) 1:4 

b) 1:2 

c) 1:8 

d) 1:3 

Q16. Which of the following cannot be considered as a coupling agent? 

a) Grease 

b) Water 

c) Air 

d) Glycerin

Q17. The speed with which ultrasonic waves travel through a material is 

known as its? 

a) Velocity of sound energy 

b) Pulse repetition rate of sound energy 

c) Pulse recovery rate of sound energy 

d) Ultrasonic response of sound energy 

Q18. A testing technique in which the crystal or transducer is parallel to the 

surface and ultrasonic waves enter the material being tested in a direction 

perpendicular to the test surface is? 

a) Straight beam testing 

b) Angle beam testing 

c) Surface wave testing 

d) None of the above

19. The total energy losses occurring in all materials is called? 

a) Attenuation 

b) Scatter 

c) Surface wave testing 

d) None of the above 

20. Acoustic energy propagates in different modes. Which of the following 

represents a mode? 

a) Longitudinal mode 

b) Shear wave 

c) Surface wave 

d) All of the above

Here are answers: 

1. A side drilled hole which is 

parallel to the plate surface 

and perpendicular to the 

sound path 

2. Far field 

3. The distance covered by the 

front surface pulse width 

and recovery time 

4. Surface wave 

5. Angle D 

6. Angle A 

7. Far field zone 

8. Close to or on the surface 

9. Lamb waves 

10.Piezoelectric 

11. Signal amplitude 

12. A pulse 

13. All of the above 

14. Refraction 

15. 1:4 

16. Air 

17. Velocity of sound energy 

18. Straight beam testing 

19. Attenuation 

20. All of the above

Other Sources

Q35: Couplant used in contact testing is a good conductor for sound waves 

and acts as a: 

a) Noise suppressor 

b) Source to reduce surface irregularities on the test object 

c) Means to reduce signal strength 

d) Source to reduce reflection from edges on the test object. 

e) Source to reduce frictions and prevent excessive wearing to contact face.

Q47: A major (!) limitation of using a low test frequency is: 

a) The limit of depth penetration 

b) That a small search probe required 

c) That small discontinuities are hard to detect due to a larger angle of 

divergence. (standard answer) 

d) The low amplitude signals from disbonds and othe flat andd thin 


Question: The detectability drop with increase of wavelength. The smallest 

discontinuities could be detected is approximately ½ λ. Could the beam 

divergence contribution term as Major?

UT Testing-Self Study Notes- Rev1

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