Handbook for Sound Engineers

104 Chapter 5

Fig. 5-7 shows the effect of air space behind a 2.5 cm
(1 in) thick Owens Corning Linear Glass Board. As the
air space is increased in steps from 0 to 12.7 cm (0 to
5 in), the lower-frequency absorption increases progres-
sively.^15 It is sometimes cost-effective to use thinner
glass fiber and arrange for air space behind it; it is some-
times cost-effective to use glass fiber of greater thick-
ness. At other times, the need for low-frequency
absorption is so great that both thick material and air
space are required.

In acoustical applications, mineral wool (or rock

wool) is another popular variation of mineral fiber

board, Fig. 5-8. Figs. 5-9 and 5-10 provide an overview

of absorption coefficients for materials available from

Roxul.^17 The main difference between glass fiber and

mineral wool is that mineral wool is generally made

from basalt (glass fiber comes from silicates), which

leads to a higher heat tolerance.

Natural fiber materials used in acoustical applications

include wood fibers and cotton fibers. Tectum, Inc.

manufactures a variety of ceiling and wall panels from

aspen wood fibers, which produces a durable acoustical

treatment. Absorption coefficients for some Tectum, Inc.

materials are shown in Fig. 5-11.^18 There are also an

Figure 5-6. The effect of thickness on the absorption of
Owens Corning 703 glass fiber boards 48 kg/m^3 (3 lb/ft^3 ),
Type A mounting.^15

Figure 5-7. The effect of mounting over an air space on the
absorption of Owens Corning Linear Glass Cloth faced
board of 2.5 cm (1 in) thickness.^15

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

6 in

5 in

3 in

2 in

1 in

4 in

2.5 cm (1 in)

5.1 cm (2 in)

7.6 cm (3 in)

10.2 cm (4 in)

12.7 cm (5 in)

15.2 cm (9.6 in)

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 0 cm (0 in)

2.5 cm (1 in)

5.1 cm (2 in)

7.6 cm (3 in)

12.7 cm (5 in)

Air space depth

Figure 5-8. Mineral wool absorbers.

Figure 5-9. The effect of thickness on the absorption of

Roxul RockBoard 40 64 kg/m^3 (4 lb/ft^3 ) mineral wool

boards.^17

A. Raw material. B. Framed and fabric-

finished panel.

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)

7.6 cm (3 in)

10.2 cm (4 in)