Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 109

absorption panels are often available as baffles as well.
Absorption is reported as the number of sabins per
baffle. Acoustical baffles are often hung vertically
(perpendicular to the floor), but they can also be hung
horizontal, or even at an angle. The pattern of hanging
can have an effect on the overall performance of the
treatments. For example, some applications will benefit
more from baffles hung in two or more directions,
versus simply hanging all the baffles parallel to each
other in one direction. Hanging is often accomplished
via factory- or user-installed grommets or hooks.
Acoustical banners are simply scaled-up versions of
acoustical baffles. The core absorptive material is some-
times of a slightly lower density to facilitate installing
the banners so that they can be allowed to droop from a
high ceiling. Sizes for banners tend to be large:
1.2m×15m (4.0ft×50ft) (larger sizes are not
uncommon).

5.2.3.3 Other Furnishings and Objects

Anyone who has moved into a new home has experi-
enced the absorptive power of furnishings. Rooms
simply do not sound the same when they aren’t filled
with chairs and bookshelves and end tables and
knick-knacks and so on. Even in the uncarpeted living
spaces in our homes, the addition of even a small number
of items can change the acoustical character of the room.
This concept was put to the test when a small room
with tile floor and gypsum wallboard walls and ceilings
was tested before and after the addition of two couches.
The couches in question were fabric—as opposed to
leather or leather substitute—and were placed roughly
where they eventually wound up staying even after
moving in the balance of the room’s furnishings. The
absorption—in sabins per couch—is shown in Fig. 5-20.
(Fig. 5-20 is for illustrative purposes only—i.e., the
absorption shown was not measured in a laboratory.)

5.2.4 Resonant Absorbers

In the most general sense, a resonant absorber employs
the resonant properties of a material or cavity to provide
absorption. Resonant absorbers are typically pressure
devices, contrasted with porous absorbers that are typi-
cally velocity devices. In other words, a porous absorber
placed at a point of maximum particle velocity of the
sound will provide maximum absorption. A resonant
absorber placed at a point of maximum particle pressure
will provide maximum absorption. This can become
important in applications where maximum
low-frequency performance is important. A broadband

porous absorber spaced away from a surface will be the

most effective method of maximizing low-frequency

absorption. In contrast, a resonant absorber placed on or

at the surface will provide maximum low-frequency

absorption.

Resonant absorbers are often described as having

been tuned to address a specific frequency range. The

meaning of this will become clear below from the equa-

tions involved in determining a resonant absorber’s

frequency of resonance. It should be noted that many

versions of the equations for resonant frequency exist in

the literature. Not all of these have been presented accu-

rately and, unfortunately, some equation errors have

been perpetuated. Unfortunately, calculating the resonant

frequency of a resonant absorber is not straightforward.

Careful research and review were undertaken for the

sections below. Unless otherwise noted, the Cox and

D’Antonio^2 method of utilizing the basic Helmholtz

equation as the starting point for resonance calculations

was implemented in the following sections.

5.2.4.1 Membrane Absorbers

Membrane absorbers—also called panel and diaphrag-

matic absorbers—utilize the resonant properties of a

membrane to absorb sound over a narrow frequency

range. Nonperforated, limp panels of wood, pressed

wood fibers, plastic, or other rigid or semi-rigid material

are typically employed when constructing a membrane

absorber. When mounted on a solid backing, but sepa-

rated from it by a constrained air space, the panel will

respond to incident sound waves by vibrating. This

Figure 5-20. Absorption spectrum of a fabric-covered

couch in a 5.6 m × 4.7 m × 2.3 m room.

63 125 250 500 1k 2k 4k

50

45

40

35

30

25

20

15

10

5

0

Octave band center frequency–Hz

Approximate couch

dimensions:

213 cm wide

104 cm deep

74 cm high

Absorption–Sab/couch