1-23
Table 1-1.—Comparison of Velocity of Sound in Various Mediums
MEDIUM
TEMPERATURE
VELOCITY
ºF
ºC
(FT/SEC)
AIR
32
0
1,087
AIR
68
20
1,127
ALUMINUM
68
20
16,700
CARBON DIOXIDE
32
0
856
FRESH WATER
32
0
4,629
FRESH WATER
68
20
4,805
HYDROGEN
32
0
4,219
LEAD
32
20
4,030
SALT WATER
32
0
4,800
SALT WATER
68
20
4,953
STEEL
32
0
16,410
STEEL
68
20
16,850
Q25. How does density and temperature affect the velocity of sound?
ACOUSTICS
The science of sound is called ACOUSTICS. This subject could fill volumes of technical books, but
we will only scratch the surface in this chapter. We will present important points that you will need for a
better understanding of sound waves.
Acoustics, like sound, relates to the sense of hearing. It also deals with the production, control,
transmission, reception, and the effects of sound. For the present, we are concerned only with the last
relationship—the effects of sound. These same effects will be used throughout your study of wave
propagation.
Echo
An ECHO is the reflection of the original sound wave as it bounces off a distant surface. Just as a
rubber ball bounces back when it is thrown against a hard surface, sound waves also bounce off most
surfaces. As you have learned from the study of the law of conservation of energy, a rubber ball never
bounces back with as much energy as the initial bounce. Similarly, a reflected sound wave is not as loud
as the original sound wave. In both cases, some of the energy is absorbed by the reflecting surface. Only a
portion of the original sound is reflected, and only a portion of the reflected sound returns to the listener.
For this reason, an echo is never as loud as the original sound.
Sound reflections (echoes) have many applications in the Navy. The most important of these
applications can be found in the use of depth finding equipment (the fathometer) and sonar. The
fathometer sends sound-wave pulses from the bottom of the ship and receives echoes from the ocean floor
to indicate the depth of the ocean beneath the ship. The sonar transmits a pulse of sound energy and
receives the echo to indicate range and bearing of objects or targets in the ocean depths.
Refraction
When sound waves traveling at different velocities pass obliquely (at an angle) from one medium
into another, the waves are refracted; that is, their line of travel is bent. Refraction occurs gradually when
one part of a sound wave is traveling faster than the other parts. For example, the wind a few feet above
