Page 163 - Master Handbook of Acoustics
P. 163
FIGURE 8-1 Partially immersing a stick in water illustrates refraction of light, created because the
speeds of propagation are different in air and water. Sound is another wave phenomenon that is also
refracted because of changes in media sound speed.
Refraction in Solids
The sound-ray concept is helpful in considering direction of propagation. Rays of sound are always
perpendicular to sound wavefronts. Figure 8-2 shows two sound rays passing from a dense medium to
a less dense medium. The sound speed in the denser medium is greater than that in the less dense one;
examples are listed in Table 8-1. As one ray reaches the boundary between the two media at A, the
other ray still has some distance 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 unequal sound speeds. In
particular, in this example, the speed of sound is faster in the denser medium and slower in the less
dense medium.
