Page 204 - The Master Handbook Of Acoustics

CHAPTER
9

Absorption of Sound

he law of the conservation of energy states that energy can neither
Tbe created nor destroyed, but it can be changed from one form to
another. If we have some sound energy in a room to get rid of, how
can it be done? Sound is the vibratory energy of air particles, and it
can be dissipated in the form of heat. If it takes the sound energy of
a million people talking to brew a cup of tea, we must give up any
idea of heating our home with sound from the high-fidelity loud
speakers.

Dissipation of Sound Energy

When sound wave S hits a wall (such as in Fig. 9-1), what happens to
the energy it contains? If the sound wave is traveling in air and it
strikes a concrete block wall covered with an acoustical material, there
is first a reflected component A returned to the air from the surface of
the acoustical material. Of course, there is a certain heat loss E in the
air that is appreciable only at the higher audio frequencies.
Some of the sound penetrates the acoustical material represented
by the shaded layer in Fig. 9-1. The direction of travel of the sound is
refracted downward because the acoustical material is denser than
air. There is heat lost (F) by the frictional resistance the acoustical
material offers to the vibration of air particles. As the sound ray
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