Page 246 - The Master Handbook Of Acoustics

221
ABSORPTION OF SOUND

In commonly available perforated materials, such as pegboard, the
holes are so numerous that resonances at only the higher frequencies
can be obtained with practical air spaces. To obtain much needed low-
frequency absorption, the holes can be drilled by hand. Drilling 32″
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holes 6″ on centers gives a perforation percentage of about 1.0%.
Figure 9-35 shows the effect of varying hole area from 0.18% to
8.7% in a structure of otherwise fixed dimensions. The plywood is
5 32″ thick perforated with 16″ holes, except for the 8.7% case in
3
3
which the hole diameter is about 4″. The perforated plywood sheet is
spaced 4″ from the wall and the cavity is half filled with glass fiber and
half is air space.

1.0
C

0.8
D
2" 4"
Absorption coefficient 0.6 A

0.4

0.2 B

0
100 1,000 4,000
Frequency - Hz
FIGURE 9-35
Actual absorption measurements of perforated panel absorbers of 4″ air space, half
5
filled with mineral wool and for panel thickness of 32″. (A) Perforation 0.18%. (B)
Perforation 0.79%. (C) Perforation 1.4%. (D) Perforation 8.7%. Note that the presence
of the mineral wool shifts the frequency of resonance considerably from the theoretical
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values of Eq. 9-4 and Fig. 9-34. (Data from Mankovsky. )