Page 210 - The Master Handbook Of Acoustics

185
ABSORPTION OF SOUND

The advantages of the impedance tube method are offset by the disad-
vantage that the absorption coefficient so determined is truly only for nor-
mal (perpendicular) incidence. In a room, sound impinges on the surface
of a material at all angles. Figure 9-4 is a graph for obtaining the random-
incidence coefficient from the absorption coefficient for normal inci-
dence as measured by the standing-wave method. The random-incidence
coefficients are always higher than the coefficients for normal incidence.

Tone-Burst Method

The utilization of short pulses of sound has made it possible to perform
anechoic acoustical measurements in ordinary rooms. It takes time for

1.0

0.8
Absorption coefficient - random incidence 0.6

0.4

0.2

0
0 0.2 0.4 0.6 0.8 1.0
Absorption coefficient - normal incidence
FIGURE 9-4
An approximate relationship between the absorption coefficients at normal incidence
and those with random incidence.