Page 284 - The Master Handbook Of Acoustics

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REFRACTION OF SOUND

Denser medium B
Higher sound
speed

A C
D
Less dense medium
Lower sound speed

FIGURE 12-2
Rays of sound traveling from a denser medium having a certain sound speed into a less
dense medium having a lower sound speed. The wavefront AB is not parallel to wave-
front DC because the direction of the wave is changed due to refraction.

than that in the less dense one (Table 12-1). As one ray reaches the
boundary between the two media at A, the other still has some dis-
tance to go. In the time it takes one ray to travel from B to C, the other
ray has traveled a shorter distance from A to D in the new medium.
Wavefront A-B represents one instant of time as does wavefront D-C an
instant later. But these two wavefronts are no longer parallel. The rays
of sound have been refracted at the interface of the two media having
unlike sound speeds.
An analogy may assist memory and logic. Assume that the shaded
area is paved and that the lower-density 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, has been making good progress on the