Page 32 - The Master Handbook Of Acoustics

7
FUNDAMENTALS OF SOUND

How a Sound Wave Is Propagated
How are air particles jiggling back and forth able to carry beautiful
music from the loudspeaker to our ears at the speed of a rifle bullet?
The little dots of Fig. 1-6 represent air molecules. There are more than
a million molecules in a cubic inch of air; hence this sketch is greatly
exaggerated. The molecules crowded together represent areas of com-
pression in which the air pressure is slightly greater than the prevail-
ing atmospheric pressure. The sparse areas represent rarefactions in
which the pressure is slightly less than atmospheric. The small arrows
indicate that, on the average, the molecules are moving to the right of
the compression crests and to the left in the rarefaction troughs
between the crests. Any given molecule will move a certain distance to
the right and then the same distance to the left of its undisplaced posi-
tion as the sound wave progresses uniformly to the right.

C R C R C R C

A

B

C Compression (region of high pressure)
R Rarefaction (region of low pressure)

Direction of sound wave
FIGURE 1-6
In (A) the wave causes the air particles to be pressed together in some regions and
spread out in others. An instant later (B) the wave has moved slightly to the right.