Page 217 - The Master Handbook Of Acoustics
P. 217
192 CHAPTER NINE
Effect of Density of Absorbent
1.0
4" Glass fiber and other materials come in var-
3"
0.8 2" ious densities from the flimsy thermal insu-
Absorption coefficient 0.6 1" boards used widely in industry. All of these
lation batts to the semi-rigid and rigid
have their proper place in acoustical treat-
ment of spaces, but the question right now
0.4
is, “What effect does density—the packing
Thickness of
material
of the fibers—have on sound absorption
0.2
coefficient?” In other words, is the sound
0
125 250 500 1 kHz 2 kHz 4 kHz able to penetrate the interstices of the high-
density, harder surface material as well as
Frequency - Hz
one of the flimsy kind? The answer appears
FIGURE 9-8
in Fig. 9-10, which shows relatively little
The thickness of glass-fiber sound-absorbing mater- difference in absorption coefficient as the
ial determines the low-frequency absorption (den-
sity, 3 lb/cu ft). The material is mounted directly on density is varied over a range of almost 4 to
a hard surface. 1. For very low densities the fibers are so
widely spaced that absorption suffers. For
extremely dense boards, the surface reflec-
tion is high and sound penetration low.
1.0 Open-Cell Foams
0.8 3" 2" 1" Flexible polyurethane foams are widely
Absorption coefficient 0.6 0" Air space machinery, aircraft, and in various indus-
used in noise quieting of automobiles,
trial applications. After a slow start, foams
0.4
are finding some application as sound
0.2
including recording studios and home lis-
Thickness of absorbers in architectural applications,
material = 1" tening rooms. Figure 9-11 is a photograph
0 of one form of Sonex, a foam product con-
4
125 250 500 1 kHz 2 kHz 4 kHz
toured to simulate the wedges used in ane-
Frequency - Hz
choic rooms. These are shaped in male
FIGURE 9-9
and female molds and come in meshed
The low-frequency absorption of 1-inch glass fiber- pairs. This material can be cemented or
board is improved materially by spacing it out from
the solid wall. stapled to the surface to be treated.