TC 9-64 _________________________________________________________________________
Echo
1-73. 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. 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 a 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
1-74. 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 the surface of the earth has a greater velocity than that near the
surface because friction retards the lower layers (see figure 1-16). The
velocity of the wind is added to the velocity of the sound through the air. The
result is that the upper portion of the sound wave moves faster than the
lower portion and causes a gradual change in the direction of travel of the
wave. Refraction causes sound to travel farther with the wind than against it.
Figure 1-16. Refraction of Sound
Reverberation
1-75. In empty rooms or other confined spaces, sound may be reflected
several times to cause what is known as reverberation. Reverberation is the
multiple reflections of sound waves. Reverberations seem to prolong the time
during which a sound is heard. Examples of this effect often occur in nature.
For instance, the discharge of lightning causes a sharp, quick sound. By the
time this sound has reached the ears of a distant observer, it is usually
drawn out into a prolonged roar by reverberations that we call thunder. A
similar case often arises with underwater sound equipment. Reverberations
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