Handbook for Sound Engineers

112 Chapter 5

To calculate the effective thickness for a perforated
absorber, a correction factor, G, is required. This factor is
often approximated to 0.85, but can be calculated for low
values of H (typically <0.16) using

(5-3)

Next, the effective panel thickness t' for a panel of
thickness t is calculated from Eq 5-3 using G

(5-4)

Finally, fR for a panel over an air space of depth D is
found with

(5-5)

Care should be taken to be consistent with units. For
example, if inches are used to calculate H, etc., c (the
speed of sound) should be in inches per second.
The fR of perforated absorbers is generally adjusted
by changing H. Increasing H (larger holes, smaller
spacing, or both), decreasing D, or using thinner panels
will all increase the fR. The fR can be lowered by
decreasing H, by increasing D, or by using thicker panels.
The fR from Eq. 5-5 is not exact, but is close enough for
use in the design stage. The air space is often partially or
completely filled with porous absorption. The only
drawback to this is that absorptive material in contact
with the perforated panel can reduce the absorber’s
effectiveness.
One of the more obvious perforated membranes that
can be used is common pegboard. Standard pegboard
tends to create an absorber with an fR in the 250–500 Hz
range, as shown in Fig. 5-25.^18 Since perforated
absorbers are often considered for low frequency
control, it is not uncommon to fabricate customized
perforated boards. For example, a hardboard membrane
with d= 6.4 mm (¼ in), S= 102 mm (4 in), t=3.2mm
( in), and D= 51 mm (2 in), a perforated absorber
tuned to roughly 125 to 150 Hz can be created. The
absorption coefficients of such an absorber with
96 kg/m³ (6.0 lb/ft³) of glass fiber filling the air space are
shown in Fig. 5-26.
Microperforated materials are one of the most recent
developments in the area of acoustical treatments.
Extremely thin materials with tiny perforations
(<<1 mm) are stretched over an air space and absorption
occurs by means of boundary layer effects.^2 Because
they are so thin, microperforated absorbers can be fabri-
cated from visually transparent material. RPG offers the

ClearSorber, which can be installed by stretching it over

glass without significantly altering the light throughput.

The absorption coefficients, dependent on the depth of

air space between the microperforated foil and the glass,

are shown in Fig. 5-27.^28

5.2.4.4 Slat Absorbers

Helmholtz resonators can also be constructed by using

spaced slats over an air space (with or without absorptive

G= 0.8 1 1.4 – H

tc t+= Gd

fR c

2 S

------ H

tcD

--------=

(^1) » 8
Figure 5-25. The effect of pegboard facing on the absorp-
tion of different thicknesses of Owens Corning 703 glass
fiber boards, Type A mounting.^18
Figure 5-26. The absorption of a perforated membrane
absorber “tuned” to 125–150 Hz, Type A mounting, cavity
filled with 96 kg/m^3 (6 lb/ft^3 ) glass fiber.
2.5 cm (1 in)
5.1 cm (2 in)
7.6 cm (3 in)
10.2 cm (4 in)
12.7 cm (5 in)
15.2 cm (6 in)
5 in
6 in
4 in
3 in
2 in
1 in
125 250 500 1k 2k 4k
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Octave band center frequency—Hz
ASAB
d = 6.4 mm (0.25 in)
S = 102 mm (4 in)
t = 3.2 mm (0.125 in)
D = 51 mm (2 in)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
ASAB
(^10012516020025031540050063080010001250160020002500315040005000)
(^1) / 3 Octave band center frequency—Hz