Page 216 - The Master Handbook Of Acoustics

191
ABSORPTION OF SOUND

1.0

0.8

Absorption coefficient 0.6

0.4

0.2

0
125 250 500 1 kHz 2 kHz 4 kHz
Frequency - Hz
FIGURE 9-7
3
The average absorption characteristics of 8 acoustical tile brands of 4″ thickness. The
vertical lines show the spread of the data.

going from 1 to 2 inches than going from 2 to 3 inches or 3 to 4 inches.
A 4-inch thickness of glass fiber material of 3-lb/cu ft density has
essentially perfect absorption over the 125-Hz to 4-kHz region.

Effect of Airspace Behind Absorbent

Low-frequency absorption can also be improved by spacing the
absorbent out from the wall. This is an inexpensive way to get
improved performance—within limits. Figure 9-9 shows the effect on
the absorption coefficient of furring 1-inch glass-fiber wallboard out
from a solid wall. Spacing 1-inch material out 3 inches makes its
absorption approach that of the 2-inch material of Fig. 9-8 mounted
directly on the wall.