Page 240 - Master Handbook of Acoustics

CHAPTER 12

Absorption

T he law of conservation of energy states that energy can neither be created nor destroyed.

However, energy can be changed from one form to another. If there is excessive sound energy

in a room, the energy itself cannot be eliminated, but it can be transformed into an innocuous form.
This is the function of sound-absorbing materials. Generally, sound absorbers can be considered as
belonging to one of these types: porous absorbers, panel absorbers, and volume or resonance
absorbers. Generally, porous absorbers are most effective at higher frequencies, whereas panel and
volume absorbers are most effective at lower frequencies.
All of these types of absorbers operate in fundamentally the same way. Sound exists as the

vibratory energy of air particles, and by using absorbers the vibratory energy can be dissipated in the
form of heat. Thus, sound energy is reduced. The amount of heat generated from sound absorption is
minuscule. It would take the sound energy of millions of people talking to brew a cup of tea, so we
must abandon any ecological hopes of warming our homes with sound—even the sound of heated
arguments.

Dissipation of Sound Energy

A sound wave S traveling in air strikes a concrete block wall covered with an acoustical material, as
shown in Fig. 12-1. What happens to the energy it contains? As a sound wave travels through air,
there is first a small heat loss E from air absorption that is appreciable only at higher audio

frequencies. When the sound wave hits a wall, there is a reflected component A returned to the air
from the surface of the acoustical material.