Sound and its properties Human voice Biophysics of hearing Sound

Voice and SpeechHuman voice is specifically that part of human sound production in which the vocalfolds (vocal cords) are the primary sound source. Generally speaking, themechanism for generating the human voice can be subdivided into three parts; thelungs, the vocal folds within the larynx, and the articulators. The lung (the pump)must produce adequate airflow and air pressure to vibrate vocal foldsAdult men and women have different vocal folds sizes; reflecting the male-femaledifferences in larynx size. Adult male voices are usually lower-pitched and havelarger folds.http://www.youtube.com/watch?v=ajbcJiYhFKYVocal rangeThe voiced speech of a typical adult male has a fundamental frequency from about90 to 170 Hz, and that of a typical adult female from 170 to 260 Hz

Intensity of sound wavehas been defined as the energy which passes through a unit areaperpendicular to the direction of propagation per unit time.Its derived SI units are therefore joules per square meter per second (J m -2 s -1 ),or watts per square meter (W m -2 ).When we study the human ear - this is a rather cumbersome unit, because wecan hear sound intensities over an enormous range, from something of the orderof 1 x 10 -12 W m -2 to something in excess of 1 W m -2 . For convenience, soundintensity is often expressed in terms of a unit called the decibel (dB).Intensity level (Sound pressure level) in dB =10logII0• The decibel is actually a measure of the relative intensity of the soundcompared with a reference intensity.• The reference intensity is taken as I 0= 1 x 10 -12 W m -2 , which is the lowestintensity sound (of frequency 1 kHz) that the average human can hear.Loudness, a subjective measure, is often confused with objective measures ofsound strength such as sound pressure, sound pressure level (in decibels), soundintensity or sound power.Units used to measure loudness:Sone (loudness N)Phon (loudness level L) ((loudness level = intensity level i.e. 1dB = 1 Phon only for 1 kHz tone))The sensitivity of the human ear changes as a function of frequency.Each line on this graph shows the SPL required for frequencies to be perceived as equallyloud. It also shows that humans with good hearing are most sensitive to sounds around 2–4kHz, with sensitivity declining to either side of this region.

Acoustic impedanceAcoustic impedance Z of a medium, such as air, rock or water is a materialproperty. It determines the size of reflection of the acoustic energy on a boundarywith different acoustic impedance.Z = ρ c [kgs -1 m -2 ], [Nsm -3 ]I =p 22 Zρ 1c 1reflection coefficient:R =( Z ) 22 − Z1( Z + Z ) 221ρ 2c 2Relationship between intensity level, intensity and pressure of a sound waveAverage intensity level(dB)160140120100806040200Average intensity(Wm-2)1x10 41x10 21x10 01x10 -21x10 -41x10 -61x10 -81x10 -101x10 -12Average pressure(Pa)20002002020.20.020.0020.00020.00002Thus a sound at a level of 120 dB, which will cause physical discomfort, has amean pressure amplitude is about 20 Pa, or approximately 0.02% of 1 atm. Themean pressure amplitude generated by someone whispering in your ear (20 dB) isnot much more than one billionth of an atmosphere.

Anatomy and physiology of the earThe ear serves as a transducer, converting the mechanical energy of the incomingsound wave into electrical energy for onward transmission to the brain.Anatomy of the ear. This diagramis not to scale; the middle- andinner-ear structures have beenenlarged for clarity.The pressure fluctuations associated with the incoming sound waves are rathersmall, and that an amplification device is needed if sounds at our lowest thresholdof intensity are to be heard.Furthermore, the inner ear is filled with fluid, and the fluid has a rather differentimpedance to that of the air through which the incoming sound wave arrives. Thiswould mean that a large proportion of the energy of the incoming sound wave wouldsimply be reflected back from the interface.The middle ear serves both to amplify the incoming sound pressures and tomatch the impedance of the inner and outer ear structures.The outer ear is primarily a collecting and funnelling device, although itsphysical characteristics do give rise to a particularly efficient response to soundwaves with frequencies of around 3-4 kHz.

Outer earconsists of the pinna and the ear canal.The pinna (external auricle) of the human serves little useful purpose — apart frombeing somewhere to hang the spectacles and miscellaneous decorative devices.The ear canal (auditory canal, external canal) is roughly the shape of a tube,approximately 30 mm in length and 6 mm in diameter. The air in a tube willresonate at particular frequencies determined by the length of the tube and by theboundary conditions at the ends of the tube. The ear canal is closed at one end, bythe eardrum, and open at the other.Standing waves in a tube open at one end occur at frequencies given byf n= (2n-1)v4ln=1,2,3,…Assuming the velocity of sound in ambient air to be 340 m s-1, the fundamentalfrequency of the ear canal is approximately 2.8 kHz.The human ear is most sensitive to sounds at about this frequency.The eardrum (tympanic membrane, tympanum) forms the boundary between theouter ear and the middle ear. It vibrates in response to incoming sound waves, andtransmits the vibrations into the small bones in the middle ear. It is a thin membraneof tissue, approximately 0.1 mm thick, with a cross-sectional area of about 60 mm 2 .It can be damaged (ruptured or perforated) by sounds of high intensity, above about160dB, but in common with other tissues, it does have the capacity to heal over aperiod of time.Middle earThe middle ear is an air-filled cavity in the temporal bone of the skull. Its outerboundary is the eardrum and its inner boundaries are the oval window and the roundwindow at the entrance to the cochlea. The cavity of the middle ear has an air vent tothe pharynx through the Eustachian tube. The purpose of this venting is to equalizethe air pressures on either side of the eardrum.The primary mechanical components of the middle ear are the ossicles (smallbones). They are the hammer (malleus), the anvil (incus) and the stirrup (stapes).

The function of the middle ear is to amplify the pressures from the incoming soundwaves and to match the impedances of the air-filled ear canal and the fluid-filledcochlea.The lever system provided by the ossicles increases the force at the oval windowrelative to that at the eardrum.It works as an impedance-matching system, the purpose of which is to maximize the transferof power from the air in the ear canal, which is light and compliant, to the fluid in the inner ear,which is heavy and stiff.If it were not for the impedance-matching system, much of the energy of the sound wavewould simply be reflected back out of the ear.The middle ear is capable of producing a reasonable match of the impedances of the inner andouter ear over a range of frequencies from about 400 Hz to about 4000 Hz.The primary mechanism that the middle ear uses for impedance matching is amechanical pressure amplification system. The lever arm from the fulcrum to thecentre of the eardrum is greater than that to the oval window by about 30%, and sothe force is increased by 30%. Furthermore, the oval window has a cross-sectionalarea of only about 3 mm 2 , compared with the effective cross-sectional area of the ofeardrum of approximately 60 mm 2 . The anticipated pressure amplification thatmight be achieved by the is therefore1.3 x 60/3Typical pressure amplifications measured experimentally are around 17.

Inner earis a multi-chambered cavity in thetemporal bone.consists of several parts,including the cochlea, thesaccule, the utricle and the semicircularcanals.the function of the utricle andsemi-circular canals is associatedwith our senses of position andbalance.Only the cochlea has a role inthe hearing process. The role ofthe cochlea is to convert themechanical energy presentedas a mechanical vibration at theoval window into electricalenergy, transmitted onwardsthrough the auditory nerve.The cochlea is essentially a tapered tube approximately 30 mm in length. The tubeis not straight but is wound into a spiral shape, so that its external appearance ismuch like the shell of a snail.The diameter of the tube tapers downwards from the outside of the spiral, at theinterface to the middle ear, to the apex.The tube is divided along its length info three chambers. The two outerchambers, the scala vestibuli and the scala tympani, are connected at the apexof the cochlea through an opening called the helicotrema.The fluid in the scalae is called perilymph. The vibration of the oval window,induced by the stirrup, gives rise to a wave traveling inwards away from theoval window. The outer boundaries of the cochlea are rigid, and, since the fluid isbasically incompressible, some flexibility is required to allow the system to moveunder the applied loads. This flexibility is provided by the round window at the outerboundary of the scala tympani. The cochlear duct, sandwiched between the twoscalae, contains a fluid called endolymph.

The membrane separating the scala tympani from the cochlear duct is calledthe basilar membrane, and its motion plays a role in the generation of thenerve impulses. The membrane contains many fibres, each approximalely 1—2µm in diameter, the length of these fibers varies from approximately 75 µm near tothe oval window up to about 475 µm at the helicotrema. On the basilar membraneis the organ of Corti that accomplishes the transformation of mechanical toelectrical energy.The organ of Corti containshair cells, rooted on thebasilar membrane, which areconnected to nerve fibres.These cells have projectinghairs or fibers that bridgeacross to the tectorialmembrane. The hairs areapproximately 8-12 µm indiameter. As the basilarmembrane moves in responseto the pressure waves, it bendsor shears the hairs and thesestimulate the nerve cells toproduce electrical impulses.Vibrations of stapes and oval window displace perilymph fluid within scala vestibuli.Vibrations pass to the scala tympani. Movements of perilymph travel to the base of cochleawhere they displace the round windowperilymphscala vestibuliscala tympanyperilymphendolymphTheories of hearingthere is still no wholly satisfactory model to explore the transformer mechanism ofhearing to analyse intensity and frequency of the sound.Place theoriesA] Helmholtz’s resonator theorythe fibers in the basilar membrane vary in length, being shorter near the ovalwindow and longer near the apex. A comparison can be made between this arrayof fibers and the wires in a harp or piano.Assuming that the fibers are all in tension, then each fiber has a resonantfrequency that can be calculated from the laws of mechanics.Only certain fiber corresponding to certain input vibration frequency will resonate.However the theory does not explain the total frequency range of human hearing.

B] von Bakesy’s theoryMOTION OF THE BASILAR MEMBRANEthe cochlea is a criticallydamped mechanical system,and the motion of the basilarmembrane is in the form of awave moving away from theoval window.The amplitude of vibrationof the basilar membranevaries along its length, andthe position of maximumamplitude is a function offrequency. At low frequencies(25 Hz), the whole of themembrane is vibrating, andthe peak amplitude of vibrationis at the far end, near thehelicotrema. At highfrequencies (3 kHz), the peakamplitude of vibration occursnear to the oval window. B] von Bakesy’s theory

Organ of Corti• Stereocilia of the outer hair cellsare embedded in the tectorialmembrane.• When the cochlear duct isdisplaced, a shearing force iscreated, moving and bending thestereocilia.• Ion channels open, depolarizingthe hair cells, releasingglutamate that stimulates thesensory neuron.• Greater bending of stereocilia, theincreased frequency of APproduced.Auditory Hair Cells• Two types of hair cells are located within the human organ of Corti– Inner hair cells (approximately 3500) form a single line of cells along the basilarmembrane = primary sensors– Outer hair cells (approximately 12,000) are arranged in three rows along thebasilar membrane = amplifiersCilia project from the top of each haircellThe tectorial membrane isattached to the hair cell ciliaWhen sound waves move thebasilar and tectorialmembranes, the cilia bend inone direction or the otherShear of the cilia generates areceptor potential that releasesa neurotransmitter

Hearing deficiencynormal audiogramhearing impairment of the conductive typedB6040200-20Air conductionBone conduction60 250 1000 4000Hzhearing impairment of the sensorineural typeaffects the air and bone conduction by the same amplitudedB6040200-20Air conductionBone conduction60 250 1000 4000Hz6040dB200-20Air conductionBone conduction60 250 1000 4000HzQuantification of hearing lossMild:for adults: between 25 and 40 dBfor children: between 20 and 40 dBModerate: between 41 and 55 dBModerately severe: between 56 and 70 dBSevere: between 71 and 90 dBProfound: 90 dB or greaterHearing deficiencyPresbycusis is a progressive hearing impairment accompanying age, typicallyaffecting sensitivity to higher frequencies (above about 2 kHz).Tinnitus is the perception of sound within the human ear in the absence ofcorresponding external sound.Tinnitus is not a disease; but a symptom resulting from a range of underlyingcauses that can include: ear infections, foreign objects or wax in the ear, noseallergies that prevent (or induce) fluid drain and cause wax build-up. Tinnitus canalso be caused by natural hearing impairment (as in aging), as a side-effect of somemedications, and as a side-effect of genetic (congenital) hearing loss. However, themost common cause for tinnitus is noise-induced hearing loss.As tinnitus is usually a subjective phenomenon, it is difficult to measure usingobjective tests, about one in five people between 55 and 65 years old report tinnitussymptoms .

Correction of hearing lossesHearing implant is composed of a sensitive microphone, a miniature transistor amplifier, a powersource and a suitably adjusted loudspeaker system. This can have the shape of a small earphone with ahollow top which is put into the outer auditory duct. In most cases are only amplify those frequencyranges, which are heard worse.For non-hearing patients with preserved auditory nerve function there is hope for at least partialrenewal of hearing through electrode cochlear implant. A multielectrode system stimulates the auditorynerve at various places of the cochlea and partially replaces the function of Corti organ. The device iscomposed of an outer part and inner (implanted) part. The outer part is created by a special sensitivemicrophone with amplifier, speech processor which modulates the acoustic signal and the emitting coil.The implanted part is composed of a receiving coil, a demodulator and a multielectrode system. Thereceiving coil with the demodulator is implanted under the skin behind the auricle, the electrodesystem is introduced into middle part of the cochlea, the ductus cochlearisMain components of a cochlear implant (M microphone, SP speech processor, EC emitting coil,RC receiving coil, D demodulator, SE system of electrodes.http://www.youtube.com/watch?v=-WA7-k_UcWY