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Edwards Signaling Catalog

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Science of Sound<br />

Physical Principles of Sound<br />

The sources of sound are many and nearly infinitely varied.<br />

Sound can be described in terms of pitch– from the low rumble<br />

of distant thunder to the high-pitched buzzing of a mosquito–<br />

and loudness. However, it is important to understand that these<br />

are subjective qualities that depend in part on the hearer's<br />

sense of hearing. Objective, measurable qualities of sound<br />

include frequency and intensity, which are related to pitch and<br />

loudness. These terms, as well as others used in discussing<br />

sound, are best understood through an examination of the<br />

physical principles of sound.<br />

To understand the basics of sound, it is important to first<br />

understand some basic related scientific principles.<br />

At its most fundamental level, sound is the mechanical<br />

disturbance of a medium, either gas, liquid or solid. All sounds<br />

are created by causing a medium to vibrate, be it a bell, horn,<br />

siren, whistle or the wings of a cicada. Sound is propagated<br />

through mediums by causing adjacent particles to vibrate in<br />

a similar fashion– a bell vibrates at a given frequency and<br />

thereby displaces adjacent air molecules. This process continues,<br />

and eventually air particles in our ears bump into tiny hairs<br />

in our inner ear; these hairs send electrical impulses to our<br />

brain, which tells us that we are hearing a particular tone.<br />

Put more simply, sound can be described as the transmission<br />

of pressure, from an initial source to a listener through the air<br />

(or other medium).<br />

The most popular analogy to sound wave propagation is the<br />

example of a pebble that is dropped into a pond. The pebble,<br />

on its initial collision with the water’s surface, produces ripples<br />

originating from the “point source” of the rock’s entry, spreading<br />

in all directions. Due to the mass of the water molecules, energy<br />

is expended in making the water ripple. So as the ripples<br />

travel further and further away from the rock's point of entry,<br />

the ripples lose intensity. Sound behaves in the same manner.<br />

As sound travels, it loses energy, sounding “softer”. A law<br />

known as the inverse-square law dictates the amount of energy<br />

lost per unit distance— in a free-field, doubling the distance<br />

quarters the sound energy, given a point-source.<br />

Speed of sound in various mediums<br />

Medium<br />

Speed<br />

in feet<br />

per second<br />

Speed<br />

in meters<br />

per second<br />

Air at 59°F (15°C) 1,116 340<br />

Aluminum 16,000 5,000<br />

Brick 11,980 3,650<br />

Distilled water at 77°F (25°C) 4,908 1,496<br />

Glass 14,900 4,540<br />

Seawater at 77°F (25°C) 5,023 1,531<br />

Steel 17,100 5,200<br />

Wood (maple) 13,480 4,110<br />

The rate at which variations in air pressure occur is referred<br />

to as frequency and is expressed in cycles per second (cps)<br />

or Hertz (Hz). The human ear is typically capable of hearing<br />

sounds produced in the 20 to 20,000 Hertz range.<br />

The range of sound pressure to which the ear will respond<br />

is extremely wide (on the order of several million to one).<br />

Because of this, a linear scale to compare different sound<br />

pressures becomes as impractical as the use of a yardstick to<br />

measure miles. A logarithmic scale, is a far more suitable scale<br />

for this type of measurement. Therefore, the decibel, which is<br />

a logarithmic unit, is used to measure sound pressure levels.<br />

A very important concept to note when using decibels, is that<br />

for each additional 3 dBs, the sound pressure doubles! (e.g. a<br />

signaling device rated 83 dB at 10 ft. is TWICE as powerful as<br />

one rated at 80 dB at 10 ft.).<br />

Loudness is determined by the magnitude of these variations.<br />

Greater variations in pressure produce louder sounds. The<br />

louder the sound, the more our ear drum moves. The volume<br />

(or magnitude) of any particular sound is referred to as its<br />

“sound pressure.” Under normal atmospheric conditions, the<br />

maximum sound pressure level attainable is 194 dB. Sound<br />

pressure levels in excess of 120 dB may be painful. Above 150<br />

dB they can result in ear damage. A distance must generally<br />

be specified along with the dB rating to fully describe a sound.<br />

The sound pressure level changes 6 dB for each halving or<br />

doubling of distance. For a change in distance of ten times, the<br />

sound pressure level changes 20 dB.<br />

Here are some common sound levels to give you a framework<br />

for understanding the different sound levels:<br />

Source in dB Activity<br />

170-180 dB stun grenades (hearing tissue death occurs)<br />

140-150 dB firearms, jet engine, rock concert<br />

120-130 dB<br />

jackhammers, heavy construction tools,<br />

loud car stereo<br />

100-110 dB motorcycles, chainsaws, loud nightclub<br />

90 dB a hair dryer<br />

85 dB city traffic<br />

80 dB alarm clock<br />

60 dB normal conversation<br />

50 dB<br />

30-40 dB<br />

moderate rainfall, the average ambient sound level in<br />

one’s home.<br />

a quiet room or library, a whisper, the bedroom while<br />

sleeping.<br />

TM<br />

www.edwardssignaling.com 27

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