Page 27 - Master Handbook of Acoustics

FIGURE 1-5 Sound waves traveling through a medium change the localized air particle density. (A)
A sound wave causes the air particles to be pressed together (compression) in some regions and

spread out (rarefaction) in others. (B) An instant later the sound wave has moved slightly to the right.

In this example, why does the sound wave move to the right? The answer is revealed by a closer
look at the arrows (see Fig. 1-5). The molecules tend to bunch up where two arrows are pointing
toward each other, and this occurs a bit to the right of each compression region. When the arrows
point away from each other, the density of molecules decreases. Thus, the movement of the higher
pressure crest and the lower pressure trough accounts for the progression of the sound wave to the

right.
As mentioned previously, the pressure at the crests is higher than the prevailing atmospheric
barometric pressure and the troughs lower than the atmospheric pressure, as shown in the sine wave
of Fig. 1-6. These fluctuations of pressure are very small indeed. The faintest sound the ear can hear
(20 μPa) exists at a pressure some 5,000 million times smaller than atmospheric pressure. Normal
speech and music signals are represented by correspondingly small ripples superimposed on the

atmospheric pressure.