Handbook for Sound Engineers

Acoustical Treatment for Indoor Areas 103

5.2.2.1 Mineral and Natural Fibers

Of the varieties of mineral fiber, one of the most popular
is the glass fiber panel or board, Fig. 5-3. The absorption
of sound for various densities of Owens Corning 700
Series boards is shown in Figs. 5-4 and 5-5.^15 Fig. 5-4
shows the absorption for 2.5 cm (1 in) thick boards.
None of the three densities absorbs well at frequencies
below 500 Hz. At the higher audio frequencies, the
boards of 48 kg/m³ and 96 kg/m³ (3.0 lb/ft³ and 6.0 lb/ft³,
respectively) densities are slightly better than the lower
density 24 kg/m³ (1.5 lb/ft³) material. Fig. 5-5 shows a
comparison between different densities of the 10.2 cm
(4.0 in) thick fiberglass boards. In this case, there is little
difference in absorption between the three densities.^15

Boards of medium density have a mechanical advan-
tage in that they can be cut with a knife and press-fitted
into place. This is more difficult with materials that have
a 24 kg/m³ (1.5 lb/ft³) density and lower, such as
building insulation. The denser the board, the greater the
cost. Most acoustical purposes are well served by glass
fiber of 48 kg/m³ (3.0 lb/ft³) density, although some
consultants specify a 96 kg/m³ (6.0 lb/ft³) material. A

number of consultants regularly specify absorbers that

are composed of multiple densities, for example, a

combination of Owens Corning 701, 703, and 705. In

theory, a multidensity absorber (assuming the least dense

material is exposed to the sound source with gradually

increasing densities toward the wall) will be as good as

or better than a single-density absorber of the same

thickness.^16 In practice, this tends to hold true.

Fig. 5-6 explores the effect of thickness of 703 Fiber-

glas on absorption. The absorption of low-frequency

sound energy is much greater with the thicker maerials.^15

Figure 5-3. Glass fiber absorbers.

A. Raw material.

B. Fabric finished panels.

Figure 5-4. The effect of density on the absorption of

Owens Corning 700 series glass fiber boards of 2.5 cm

(1 in) thickness, Type A mounting.^15

Figure 5-5. The effect of density on the absorption of

Owens Corning 700 series glass fiber boards of 10.2 cm

(4 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 701 - 24 kg/m^3

(1.5 lb/ft^3 )

703 - 48 kg/m^3

(3.0 lb/ft^3 )

705 - 96 kg/m^3

(6.0 lb/ft^3 )

701 - 24 kg/m^3

( 1. 5 lb/ft^3 )

703 - 48 kg/m^3

( 3. 0 lb/ft^3 )

705 - 96 kg/m^3

( 6. 0 lb/ft^3 )

125 250 500 1 k 2 k 4 k

1. 40

1. 20

1. 00

0. 80

0. 60

0. 40

0. 20

0. 00

Octave band center frequency—Hz

A

SA

B