Page 33 - The Master Handbook Of Acoustics

8 CHAPTER ONE

Why does the sound wave move to the right? The answer is
revealed by a closer look at the arrows of Fig. 1-6. 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. When the
arrows point away from each other the density of molecules will
decrease. Thus, the movement of the higher pressure crest and the
lower pressure trough accounts for the small progression of the
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. 1-7. These fluctuations of pressure are very small indeed. The
faintest sound the ear can hear (20 Pascal) is some 5,000 million
times smaller than atmospheric pressure. Normal speech and music
signals are represented by correspondingly small ripples superim-
posed on the atmospheric pressure.

A

Compression

Pressure Atmospheric Rarefaction B
pressure

Time
FIGURE 1-7
(A) An instantaneous view of the compressed and rarefied regions of a sound wave in
air. (B) The compressed regions are very slightly above and the rarefied regions very
slightly below atmospheric pressure. Pressure variations of sound waves are thus super-
imposed on prevailing barometric pressure.