Page 255 - The Master Handbook Of Acoustics

230 CHAPTER NINE

grill cloth covers mounted on the walls in regular patterns. All modules
can be made to appear identical, but the similarity is only skin deep.
There are commonly three, or perhaps four, different types of mod-
ules, each having its own distinctive contribution to make acoustically.
Figure 9-40 shows the radically different absorption characteristics
obtained by merely changing the covers of the standard module. This is
for a 2 3 ft module having a 7″ air space and a 1″ semirigid glass fiber
board of 3 lb/cu ft density inside. The wideband absorber has a highly
perforated cover (25% or more perforation percentage) or no cover at
all, yielding essentially complete absorption down to about 200 Hz.
Even better low-frequency absorption is possible by breaking up the air
space with egg-crate type dividers of corrugated paper to discourage
1
unwanted resonance modes. A cover 4″ thick with a 5% perforation
percentage peaks in the 300–400 Hz range. A true bass absorber is
obtained with a low-perforation cover (0.5% perforation). If essentially

1.0
A

0.8
B
Absorption coefficient 0.6 C

0.4

0.2 D

0
100 1,000 10,000
Frequency - Hz
FIGURE 9-40

Modular absorber having a 7″ air space and 1″ semirigid glass fiber board of 9 to 10
lb/cu ft density behind the perforated cover. (A) No perforated cover at all, or at least
more than 25% perforation. (B) 5% perforated cover. (C) 0.5% perforated cover. (D)
17
3 4″ plywood cover, essentially to neutralize the module. (Data from Brown. )