Handbook for Sound Engineers

110 Chapter 5

results in a flexing of the fibers, and a certain amount of
frictional loss results in absorption of some of the sound
energy. The mass of the panel and the springiness of the
air constitute a resonant system. In resonant systems,
peak absorption occurs at the resonance frequency ( fR).
The approximate fR for a membrane absorber is given by
Eq. 7-65 in Section 7.3.4.4.2. It should be emphasized
that this equation yields an approximate result. Errors in
calculated versus measured fR as high as 10% have been
measured.^2 Nonetheless, membrane absorbers have been
successfully used to control specific resonant modes in
small rooms. To control room modes, they must be
placed on the appropriate surfaces at points of maximum
modal pressure. (For a detailed discussion of room
modes see Chapter 6.2.) Adding porous absorption, such
as a mineral fiber panel (typically glass fiber or mineral
wool), to the cavity dampens the resonance and effec-
tively broadens the bandwidth or Q factor of the
absorber. If the Q factor is broadened, the absorber will
be somewhat effective, even if the desired frequency is
not precisely attained.
Additionally, care should be taken during design and
construction of membrane absorbers. Changes as small
as 1 to 2 mm to, for example, the cavity depth can alter
the performance significantly. Fig. 5-21 shows how the
calculated resonant frequency varies with air space for
various membranes. Other design tips can be found in
Section 7.3.4.4.2.
Since membrane absorbers require a high level of
precision to perform at the desired frequency, they are
often customized for a specific application. Mass

production is often uneconomical, although some

companies offer membrane absorbers, one of which is

the Modex Corner Bass Trap from RPG, Inc., with

absorption coefficients as shown in Figure 5-22.^23

Since there have not been many mass-produced

membrane absorbers, there is far less empirical test data

available on membrane absorbers relative to porous

absorbers. Nonetheless, some formal testing of commer-

cially available membrane traps has been undertaken, for

example, by Noy et al.^24 Results were mixed; some

membrane absorbers performed as designed, others

performed well (if not exactly how the designer

intended), and some did not work at all.

Putting theory into practice, Fig. 5-23 shows a pair of

small room response measurements before and after the

addition of a membrane absorber. Frequency is plotted

linearly on the x axis (horizontal) with the resonance

showing up at about 140 Hz. The y axis, going into the

page, is the time axis showing the decay of the room

coming towards the viewer. The time span on the y axis

was about 400 ms. A pair of membrane absorbers was

built with fR= 140 Hz. One was placed on the ceiling

and one on a side wall.

Membrane absorbers are often inadvertently built

into the structure of a room. Wall paneling, ceiling tiles,

windows, coverings for orchestra pits, and even

elements of furniture and millwork can all be membrane

absorbers; the only question is at what frequency they

resonate. Remember that everything in a room, including

the room itself, has some impact on the acoustics of the

room. One of the most common inadvertent membrane

Figure 5-21. Variation of fR versus depth of air space for
membrane absorbers consisting of common building
materials.

0 1 2 3 4 5

0 25 50 75 100 125 150

dL—mm

dL–in

250

200

150

100

50

0

fR

—Hz

12.7 mm (^1 / 2 in) plywood

m’ = 7.13 kg/m^2 (1.46 lb/ft^2 )

12.7 mm (^1 / 2 in) plywood

m’ = 8.74 kg/m^2 (1.79 lb/ft^2 )

15.9 mm (^5 / 8 in) plywood

m’ = 10.9 kg/m^2 (2.24 lb/ft^2 )

12.7 mm (^3 / 4 in) plywood

m’ = 10.7 kg/m^2 (2.19 lb/ft^2 )

Figure 5-22. The absorption of a commercial membrane

absorber, the Modex Corner Bass Trap from RPG, Inc.^23

Absorption coefficients of

Modex Corner Bass Trap

(^80100125160200250315400500630800)
1000125016002000250031504000
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
Octave band center frequency—Hz
ASAB