Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 105

increasing number of suppliers of natural cotton absorp-

tion panels. The absorption of natural cotton panels—so

far as they have been developed—appears to be compa-

rable to mineral fiber panels of similar density.

Most fibrous absorbers will be covered with some

sort of acoustically transparent fabric finish that is both

decorative and practical. The fabric finish is decorative

because the natural yellow or green of the glass fiber and

mineral wool panels tends to be less than aesthetically

pleasing; the finish is practical because airborne fibers

from mineral fiber materials can be breathing irritants.

Perforated metal (with or without powder-coated finish)

and plastic coverings with a high percent of open area

(much higher than resonant perforated absorbers

discussed below) can also be used with fibrous

absorbers. Perforated coverings are typically employed

for decorative purposes, maintenance purposes, or to

protect the panels from high impacts, such as might

occur in a gymnasium. Foil and paper finishes are also

sometimes available as low-cost means of containing

fibers for glass fiber or mineral wool panels. Because of

reflections from the foil or paper, the high-frequency

absorption of the faced side of the absorber is signifi-

cantly lower than that of the unfaced side. (The thin foil

or paper used is sometimes referred to as a membrane.

This has led to confusion with resonant membrane, or

diaphragmatic absorbers. For clarity, foil or paper

facings as they are applied to fibrous absorber panels are

not resonant membranes in the strict sense, but do

provide some nominal increases in low-frequency

absorption when the foil or paper is exposed to the inci-

dent sound.)

To provide some impact resistance, as well as to

provide a surface conducive for some office applications

(such as for office partitions), a thin (usually 3 mm)

glass fiber board of high density (usually 160 to

290 kg/m³ [10 to 18 lb/ft³]) can be applied over the face

of a fibrous absorber before the fabric finish is applied.

This is often referred to as a tackable surface finish since

it can readily accept push pins and thumbtacks.

In terms of installation ease, natural fibers hold some

promise since they will offer relief from the itch associ-

ated with the handling of mineral fiber boards. Natural

fiber products can also be installed without covering,

and Tectum, Inc. states that their wood fiber panels can

be repainted several times without significant degrada-

tion of acoustical performance.

5.2.2.2 Acoustical Foams

There are various types of reticulated open cell foams for

acoustical applications, Fig. 5-12. Closed cell foams also

find applications in acoustics, but largely as substrates

from which acoustical diffusers can be formed. The most

common foams used as open cell acoustical absorbers in

architectural applications are polyurethane (esters and

ethers) and melamine foams. Unlike fibrous boards,

foam panels are easy to cut and can be sculpted into

shapes and patterns. Besides the ubiquitous wedges and

pyramids, acoustical foams have been created with

various square, saw tooth, and even curved patterns

sculpted into the faces. While removing material gener-

ally serves to decrease absorption, creating more exposed

Figure 5-10. The effect of density on the absorption of
Roxul RockBoard mineral wool boards of 5.1 cm (2 in)
thickness.^17

Figure 5-11. The absorption of different thicknesses of

Tectum Wall Panels, Type A mounting.^18

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

RockBoard 35

56 kg/m^3 (3.5 lb/ft^3 )

RockBoard 40

64 kg/m^3 (4 lb/ft^3 )

RockBoard 60

96 kg/m^3 (6 lb/ft^3 )

RockBoard 80

128 kg/m^3 (8 lb/ft^3 )

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

2.5 cm (1 in)

3.8 cm (1.5 in)

5.1 cm (2 in)