Page 342 - NS-2 Textbook

Sound and Sonar

In chapter 4 of this unit we described the different forms temperature increases, temperature of the medium also
that wave energy can take. Classified by type, these are affects sound transmission. Sound travels better within a
material and electromagnetic waves. Sotmd is a material given material if its temperatme is higher as opposed to
·wave. Like all -vvaves, it originates at a source of energy, when its temperature is lower. The table below gives the
which in the case of sound causes matter to vibrate. speed of sound in meters per second at sea level for dif-
These vibrations are passed along into the material sur- ferent materials at different temperatures:
rounding the source-the medium-in the form of a se-
ries of longitudinal (in the direction of travel) pressure
Material Speed of sOUlld (m/sec)
"\,\Taves. Each wave carries with it a certain amount of en-
ergy imparted to it by the source as it vibrates. Once Air at a degrees C 332
started, if the medium through which it travels is of uni- Air at 20 degrees C 344
form temperature and density, the individual waves Air at 100 degrees C 392
spread through the medium in the form of expanding Kerosene at 25 degrees C 1,324
three-dimensional spheres, much like ripples expanding Water at 25 degrees C 1,498
over a two-dimensional water surface from the point of Wood (oak) 3,850
impact of a stone. Steel 5,200
Because the available energy in the wave is spread
over an ever-increasing area as each sphere expands, Sound waves have the same general behavior as other
2
with the area of a sphere being 4m , the energy per unit
types of waves. They can be reflected by media having a
area falls off rapidly as the distance (the radius r) from
greater density than the medium they originate in, as for
the sound SOlU'ce increases. The amount of energy or
example when a sound wave traveling through air hits
power in a sound wave at any given location is called the the wall of a room. The reflected sOlmd is called an echo.
sound intensity. It is expressed in terms of watts per
They can be bent or refracted as they pass from one
square centimeter or per square meter. In order for a
medium to another, if the densities are not too dissimilar.
human to hear a sound, it must hit the eardrum with an Sound waves can also be diffracted, spreading after they
intensity of at least 10- 12 watts per square meter. Any-
pass through a narrow opening. They obey the formula
thing less will not deflect the eardrum sufficiently for the
sound to be heard. v = fA
A human's ability to hear a sound also depends on
the frequency of the sound, or the number of times per where v is the velocity of the wave, f its frequency, and A
is the wavelength. Thus, if we know the speed of sound
second that a sound wave passes by. As was stated in
chapter 4 of this unit, the audible frequency range for the for a given medium, and either the frequency or vvav€-
length, we can easily calculate the unknown quantity.
human ear is 20 to 20,000 Hz. Sounds in the extreme high
When a sound wave is reflected from an object creating
and low ends of this frequency range require more
an echo, one can easily compute the distance to the object
power per unit area to be heard than do sounds in the
if the speed of sound in the mediwn containing it is
mid-range.
known, using the simple formula
THE PHYSICS OF SOUND Distance = rate x time
Because sound is a material V\Tave, it stands to reason that For instance, if the speed of sOlmd in air were 344 m/sec,
the more material there is per unit volume in the and it took 4 seconds for an echo to rehlrn to a soW'ce,
medimn, the better sound will travel through it. Because then the one-way distance would be (4 sec + 2) x 344
of the increase in molecular motion within a material as m/sec = 688 meters. Besides specifying the intensity of a

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