Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 111

absorbers encountered in modern architecture is the
gypsum wallboard (GWB) (drywall or sheetrock) cavity.
Fortunately, the absorption of the GWB cavity can be
calculated, and the calculated results have been shown to
be in good agreement with laboratory measurements.^25
Section 9.2.3.1 provides discussion and calculation
methods for GWB cavity absorption.

5.2.4.2 Helmholtz Resonators

The ubiquitous cola bottle may be the acoustician’s most
cherished conversation piece. Bottles and jugs are prob-
ably the most common everyday examples of what are
referred to in acoustics as Helmholtz resonators. As part
of his exhaustive and painstakingly detailed work in
hearing, sound, and acoustics, Hermann von Helmholtz
determined and documented the acoustical properties of
an enclosed volume with a relatively small aperture.^26
Helmholtz resonators, as we now know them, have
specialized absorptive properties for acoustical applica-
tions. At the frequency of resonance, absorption is very

high. The frequency range of this absorption is very

narrow—only a few Hz wide, typically. When absorptive

material, such as loose mineral fiber, is used to partially

fill a Helmholtz resonator, the effective frequency range

is widened.

Eq. 7-69 in Section 7.3.4.4.3 can be used to calculate

fR for a Helmholtz resonator. Commercially, one of the

most common products utilizing Helmholtz resonator

theory is sound absorbing concrete masonry units

(CMU). For example, Fig. 5-24 provides the sound

absorption data for SoundBlox products available from

Proudfoot.^27

5.2.4.3 Perforated Membrane Absorbers

Membrane absorbers and Helmholtz resonators are

dependent on the size of the air space or cavity they

contain. Turning the former into the latter can be accom-

plished by cutting or drilling openings in the face of the

membrane. The tuned cavity of a membrane absorber

then becomes the cavity of a Helmholtz resonator. When

round holes are used for the openings in the face, a perfo-

rated absorber is created. To calculate the fR for a perfo-

rated membrane absorber, first the effective thickness

must be calculated. For perforated panels of having holes

of diameter d and regular hole spacing S (center-to-center

distance between holes), Eq. 5-2 yields the fraction of

open area, H

. (5-2)

Figure 5-23. The effect of a membrane absorber in a small
room.

A. No membrane absorber.

B. Addition of a membrane absorber.

Figure 5-24. The absorption of absorbent concrete blocks

from Proudfoot, Inc. Type RSC SoundBlox, utilizing Helm-

holtz resonance effects.^27

(^12516020025031540050063080010001250160020002500315040005000)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
(^1) / 3 Octave band center frequency–Hz
ASAB
102 mm (4 in) block
152 mm (6 in) block
203 mm (8 in) block
H
S
4
--- d
S

©¹
= §·^2