Page 287 - The Master Handbook Of Acoustics

262 CHAPTER TWELVE

Warm

Cool S

A

Cool

Warm

Sound Sound
shadow S shadow

B
FIGURE 12-4
Comprehensive illustration of refraction of sound from source S; (A) cool air near the
ground and warmer air above, (B) warm air near the ground and cooler air above. In
(B) note that sound shadow areas result from the upward refraction.

certain speed, it is to be expected that the speed of sound will
be affected. If sound travels 1,130 ft/sec and a 10 mi/hour (about
15 ft/sec) wind prevails, what will be the effect of the wind on the
sound? Upwind the sound speed with respect to the earth would be
increased about 1%, and downwind it would be decreased the same
amount. This seems like a very small change but it is enough to affect
refraction materially. Figure 12-5 illustrates the effect of wind on the
downward refraction case of Fig. 12-4A. A downwind shadow is cre-
ated and upwind sound is refracted downward.