Refraction of Sound

the bending of sound rays in a nonuniform medium, such as the atmosphere or the ocean, in which the speed of sound depends on the coordinates. Sound rays always turn toward the layer where the speed of sound is lower. The more rapid the change in the speed of sound, the more pronounced the refraction.

Figure 1. (a) Paths of sound rays when temperature decreases with altitude, (b) paths of sound rays when temperature increases with altitude

The refraction of sound in the atmosphere is due to spatial variations in the air temperature, wind speed, and wind direction. The temperature usually decreases with altitude up to an altitude of 15–20 km. Since the speed of sound also decreases, rays from a sound source located near the earth’s surface are bent upward; beyond a certain distance, the sound can no longer be heard (Figure 1, a). If, however, the air temperature increases with altitude (a temperature inversion, which often occurs at night), then the rays are bent downward and the sound propagates over greater distances (Figure 1, b). When sound propagates against the wind, the rays are bent upward. When sound propagates with the wind, the rays are bent toward the earth, with the result that the audibility of the sound is substantially improved (Figure 2). The refraction of sound in the upper layers of the atmosphere can lead to the formation of zones of silence and zones of abnormal audibility.

Figure 2. Influence of wind on paths of sound rays

The refraction of sound in the ocean is due to spatial variations in the temperature, salinity, and hydrostatic pressure. Refraction is responsible for a number of characteristics of the propagation of sound in the ocean, such as propagation over long distances, the formation of shadow zones, and the focusing of sound (seeHYDROACOUSTICS).