Page 26 - Master Handbook of Acoustics

FIGURE 1-4 Particles involved in the propagation of sound waves can move with (A) longitudinal

motion in air, (B) transverse motion on a string, or (C) circular motion on the water surface.

Propagation of Sound

How are air particles, moving slightly back and forth, able to carry music from a loudspeaker to our
ears? The dots of Fig. 1-5 represent air molecules with different density variations. In reality, there
are more than a million molecules in a cubic inch of air. The molecules crowded together represent
areas of compression (crests in the wave shape) in which the air pressure is slightly greater than the

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prevailing atmospheric pressure (typically about 14.7 lb/in at sea level). The sparse areas represent
rarefactions (troughs in the wave shape) in which the pressure is slightly less than atmospheric
pressure. The arrows (see Fig. 1-5) indicate that, on 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, because of elasticity, after an initial displacement, will return toward its original position.
It will move a certain distance to the right and then the same distance to the left of its undisplaced

position as the sound wave progresses uniformly to the right. Sound exists because of the transfer of
momentum from one particle to another.