Page 165 - Master Handbook of Acoustics

TABLE 8-1 Examples of Speed of Sound in Different Materials

An analogy may be useful. Assume that the shaded area of Fig. 8-2 is paved and the unshaded area
is ploughed. Assume also that the wavefront A-B is a line of soldiers. The line of soldiers A-B,
marching in military order, makes good progress on the pavement. As soldier A reaches the ploughed
ground, he or she slows down and begins plodding over the rough surface. Soldier A travels to D on
the ploughed surface in the same time that soldier B travels the distance B to C on the pavement. This
tilts the column of soldiers off in a new direction, which is the definition of refraction. In any
homogeneous medium, sound travels rectilinearly (in the same direction). If a medium of another

density is encountered, the sound is refracted.

Refraction in the Atmosphere

The earth’s atmosphere is anything but a stable, uniform medium for the propagation of sound.
Sometimes the air near the earth is warmer than the air at greater heights; sometimes it is colder. At
the same time this vertical layering exists, other changes may be taking place horizontally. It is a
wondrously intricate and dynamic system, challenging acousticians (and meteorologists) to make
sense of it.
In the absence of thermal gradients, a sound ray may be propagated rectilinearly, as shown in Fig.

8-3A. As noted, rays of sound are perpendicular to sound wavefronts.