Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 107

5.2.2.3 Acoustical Tiles

Acoustical tiles have the highest density of the porous

absorbers. They are widely used for suspended (lay-in)

ceiling treatments. Years ago, it was common to see

30 cm × 30 cm (12 in × 12 in) tile mounted directly to a

hard plaster (Type A mounting). This is not a very effi-

cient way to use this type of absorber and is no longer

popular.

The standard sizes for acoustical tiles are 61 cm

square (24 in × 24 in) or 61 cm × 122 cm (24 in × 48 in)

and the Sabine absorption coefficients are usually given

for Type E400 mounting, which mimics a lay-in ceiling

with a 400 mm (16 in) air space. Fig. 5-16 shows the

average absorption coefficients of a sampling of 39

different acoustical tiles. The vertical lines at each

frequency point indicate the spread of the coefficients

for each frequency. It is interesting to note the wide vari-

ance possible with different types of tiles.

5.2.2.4 Spray and Trowel Applied Treatments

Some acoustical treatments can be applied by spray

and/or trowel. Many are applied, finished, and detailed

much like standard plaster—and are even paintable.

Special bonding chemicals and processes give these

types of materials their absorptive qualities. Some have a

gypsum base, which can provide a look similar to normal

plaster or gypsum wallboard walls. Acoustical plasters

tend to provide high frequency absorption, with poor low

frequency performance, especially when applied thinly

(<2.5 cm thickness). Acoustical plasters can be an

economical option when considering spaces that require

large areas of absorption—e.g., a gymnasium ceiling.

Some spray applied treatments can provide fireproofing,

as well as thermal insulation. They are also popular in

historical preservation applications, where the aesthetic

appearance of a surface cannot be altered, but the acous-

tics must be improved to provide better communications

in the space.

5.2.2.5 Carpet and Draperies

Carpet is a visual and comfort asset, and it is a porous

absorber of sound, although principally at upper audio

frequencies. Carpet is what the electrical engineer might

call a low-pass filter. Because it is a high-frequency

absorber, carpet should be used cautiously as a room

treatment. Carpet can make a well-balanced room bass

heavy because of its excessive high frequency absorp-

tion. The various types of carpet have different sound

absorption characteristics. In general, sound absorption

increases with pile weight and height; cut pile has greater

absorption than loop pile. Pad material has a significant

effect on the absorption of a carpet. Generally, the

heavier the carpet pad, the more absorption. Imperme-

able backing should be used with care as it dramatically

reduces the effect of the carpet pad and thereby reduces

absorption. Due to the limited thickness of carpet, even

the deepest possible pile (with the thickest possible pad)

will not absorb much low-frequency sound. Fig. 5-17

shows the absorption coefficient for a typical

medium-pile carpet, with and without a carpet pad.^21

Figure 5-16. The average Sabine absorption coefficients
(DSAB) of 39 acoustical ceiling tiles of varying thickness,
Type E400 mounting.

125 250 500 1k 2k 4k NRC

1.40

1.20

1.00

0.80

0.60

0.40

0.20

0.00

Octave band center frequency—Hz

ASAB

Figure 5-17. The absorption of loop pile tufted carpet

(0.7 kg/m^2 ) with and without carpet pad (1.4 kg/m^2 ), Type

A mounting.^21

Carpet without pad

Carpet with pad

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