Page 166 - Master Handbook of Acoustics

FIGURE 8-3 Refraction of sound paths resulting from temperature gradients in the atmosphere. (A)

Air temperature constant with height. (B) Cool air near the surface of the earth and warmer air
above. (C) Warm air near the earth and cooler air above.

In Fig. 8-3B, a thermal gradient exists between the cool air near the surface of the earth and the
warmer air above. This affects the wavefronts of sound. Sound travels faster in warm air than in cool
air causing the tops of the wavefronts to go faster than the lower parts. The tilting of the wavefronts
directs the sound rays downward. Under such conditions, sound from source S is continually bent
down toward the surface of the earth and may follow the earth’s curvature and be heard at relatively
great distances.

In Fig. 8-3C, the thermal gradient is reversed as the air near the surface of the earth is warmer than
the air higher up. In this case the lower parts of the wavefronts travel faster than the tops, resulting in
an upward refraction of the sound rays. The same sound energy from source S would now be
dissipated in the upper reaches of the atmosphere, reducing the chances of it being heard at any great

distance at the surface of the earth.
Figure 8-4A presents a distant view of the downward refraction scenario of Fig. 8-3B. Sound
traveling directly upward from the source S penetrates the temperature gradient at right angles and