Page 285 - The Master Handbook Of Acoustics

260 CHAPTER TWELVE

pavement. As soldier A reaches the
Table 12-1. Speed of sound. ploughed ground he or she slows down
Speed of sound and begins plodding over the rough sur-
Medium Ft/sec Meters/sec face. Soldier A travels to D on the
ploughed surface in the same time that
Air 1,130 344
soldier B travels the distance BC on the
Sea water 4,900 1,500
pavement. This tilts the wavefront off in a
Wood, fir 12,500 3,800
new direction, which is the definition of
Steel bar 16,600 5,050
Gypsum board 22,300 6,800 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 of Sound in the Atmosphere

The 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. Horizontal changes
are taking place at the same time this vertical layering exists. All is a
wondrously intricate and dynamic system, challenging the meteorolo-
gists (as well as acousticians) to make sense of it.
In the absence of thermal gradients, a sound ray may be propagated
rectilinearly as shown in Fig. 12-3A. The sound ray concept is helpful
in considering direction of propagation. Rays of sound are always per-
pendicular to sound wavefronts.
In Fig. 12-3B a thermal gradient exists between the cool air near the
surface of the earth and the warmer air above. This affects the wave-
fronts of the 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 is such as to direct the sound rays downward.
Under such conditions, sound from the source is bent down toward the
surface of the earth and can be heard at relatively great distances.
The thermal gradient of Fig. 12-3C is reversed from that of Fig. 12-
3B as the air near the surface of the earth is warmer than the air higher
up. In this case the bottom parts of the wavefronts travel faster than
the tops, resulting in an upward refraction of the sound rays. The
same sound energy from the 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.