Page 218 - The Master Handbook Of Acoustics

193
ABSORPTION OF SOUND

The sound absorption coefficients of
Sonex for thicknesses of 2″, 3″, and 4″ are 1.0
shown in Fig. 9-12 for Mounting A. The 2″
glass fiber of Fig. 9-8 is considerably supe- 0.8
rior acoustically to the 2″ Sonex but a few
things should be considered in this com- 0.6 Density 1.6 lb/cu ft
parison. These are: Absorption coefficient 0.5 3.0 lb/cu ft
6.0 lb/cu ft
The Type 703 has a density of 3
lb/cu ft while Sonex is 2 lb/cu ft. 0.2
The 2″ Sonex is the wedge height
0
and the average thickness is far less, 125 250 500 1 kHz 2 kHz 4 kHz
while the 703 thickness prevails Frequency - Hz
throughout. FIGURE 9-10
Comparing the two products is, in a The density of glass-fiber absorbing material has rel-
sense, specious because the much atively little effect on absorption in the range of 1.6
higher cost of Sonex is justified in to 6 lb/cu ft. The material is mounted directly on
solid wall.
the minds of many by appearance
and ease of mounting rather than
straight acoustical considerations.

Drapes as Sound Absorbers

Drapes are a porous type of sound absorber because air can flow
through the fabric under pressure. Variables affecting absorbency
include weight of material, degree of drape, and distance from the
wall. Data are scarce, but Fig. 9-13 compares the absorption of 10, 14,
and 18 oz/sq yd velour hung straight and presumably at some dis-
tance from the wall. One intuitively expects greater absorption with
heavier material. However, the greater absorption in going from 14
to 18 oz/sq yd than in going from 10 to 14 oz/sq yd is difficult
to explain. The effect, whatever it is, is concentrated in the 500- to
1-kHz region.
The amount of fullness of the drape has a great effect as shown in
7
Fig. 9-14. The “draped to 8 area” means that the entire 8/8 area is
1
drawn in only slightly ( 8) from the flat condition. The deeper the
drape fold, the greater the absorption.