Page 288 - The Master Handbook Of Acoustics

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REFRACTION OF SOUND

Wind speed near the surface of the earth is usually less than that at
greater heights. A wind gradient exists in such a case that has its effect
on propagation of sound. This is not a true refraction but the effect is
the same. Plane waves from a distant source traveling with the wind
would bend the sound down toward the earth. Plane waves traveling
against the wind will be bent upward.
It is possible, under unusual circumstances, that sound traveling
upwind may actually be favored. For instance, upwind sound is kept
above the surface of the ground, minimizing losses at the ground sur-
face. After all, does not the sportsman approach his prey upwind?
Doing so keeps footstep noises from being heard by the prey until the
sportsman is quite close.

Refraction of Sound in the Ocean

In 1960 some oceanographers devised an ambitious plan to see how far
underwater sound could be detected. 1,2 Charges of 600 lb were dis-
charged at various depths in the ocean off Perth, Australia. Sounds
from these discharges were detected near Bermuda. The great circle
path the sound presumably followed is shown in Fig. 12-6. Even
though sound in sea water travels 4.3 times faster than in air, it took
13,364 seconds (3.71 hr) for the sound to make the trip. This distance
is over 12,000 miles, close to half the circumference of the earth. Inter-
esting, but what has this to do with refraction? Everything!

Wind
direction

Sound
shadow
S
FIGURE 12-5

Wind gradients refract (not a true refraction) sound. A shadow sound is created upwind
and good listening conditions downwind.