Page 206 - The Master Handbook Of Acoustics

181
ABSORPTION OF SOUND

The sound ray S of Fig. 9-1 experiences many rather complex
events during its odyssey through this barrier, and every reflection and
passage through air or acoustical material dissipates some of its origi-
nal energy. The refractions bend the ray but do not necessarily dissi-
pate heat. Fortunately, all this minutia is not involved in practical
absorption problems.

Evaluation of Sound Absorption

The absorption coefficient is a measure of the efficiency of a surface
or material in absorbing sound. If 55 percent of the incident sound
energy is absorbed, the absorption coefficient is said to be 0.55. One
square foot of this material gives 0.55 absorption units (sabins). An
open window is considered a perfect absorber because sound passing
through it never returns to the room. It would have an absorption
coefficient of 1.0. Ten square feet of open window would give 10
sabins of absorbance.
The absorption coefficient of a material varies with frequency and
with the angle at which the sound wave or ray impinges upon the
material. In an established sound field in a room, sound is traveling in
every imaginable direction. What we need in our calculations are
sound absorption coefficients averaged over all possible angles of
incidence.
Young has pointed out a long-standing and widespread confusion
in the field of acoustics concerning the sound absorption coeffi-
1
cient. There really are two kinds, one based on the arithmetic mean
reflection coefficient of the several sound absorbing surfaces, a, and
the other the geometric mean reflection coefficient,
, which are
related by:
a log (1
) (9-1)
e
in which
a Sabine absorption coefficient

energy absorption coefficient

We can skirt this problem by concentrating our attention on the
Sabine coefficient, a, which is actually what is measured and pub-
lished in various tables.