7.1 SURFACE ABSORPTION COEFFICIENTS

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7.1.1 Values for Surface Absorption Coe⁄cients

Sound-absorbing materials are used to reduce the sound levels in a room or

to reduce the reverberation, if either of these quantities are excessive. As will

be discussed in Sec. 7.3, surface absorption materials do not change the

sound coming directly from the source to the receiver. Instead, the surface

Copyright © 2003 Marcel Dekker, Inc.

treatment affects sound that has been reflected at least one time from the

surfaces of the room. This sound is associated with the reverberant sound

field.

The surface absorption coefficient _ is defined as the ratio of the acoustic

energy absorbed by the surface to the acoustic energy striking the surface:

_ ј

Wabs

Win

(7-1)

The energy absorbed at the surface may be transmitted through the material

or may be dissipated within the material.

The surface absorption coefficient is generally a function of the

frequency of the incident sound wave. Some representative values of the

surface absorption coefficient for various interior surfaces are given in

Appendix D.

The effect of people and furniture in a space is given by the product of

the surface absorption coefficient and the area р_SЮ, because the surface area

of a person or chair is not always easy to determine. Values for this quantity

are also given in Appendix D.

7.1.2 Noise Reduction Coe⁄cient

In some cases, it is desirable to have a single-number rating to use in comparing

the acoustic absorbing qualities of different materials. It is generally

better to have available the spectrum of values (surface absorption coefficient

as a function of frequency) for design purposes, however. The noise

reduction coefficient (NRC) may be used for rough comparison of acoustic

absorbing characteristics.

The noise reduction coefficient is defined as the average of the surface

absorption coefficients in the 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz octave

bands. By convention, the NRC values are always rounded off to the nearest

0.05. For example, let us determine the NRC value for 1-inch thick fiberglass

formboard. Using the values from Appendix D, we find the following:

NRC ј р1=4Юр0:34 ю 0:79 ю 0:99 ю 0:93Ю ј 0:7625

After rounding this value off to the nearest 0.05, we find the noise reduction

coefficient for the formboard:

NRC ј 0:75

The value of NRC-0.75 gives a rough measure of the effectiveness of

the insulation in abosrbing sound; however, there is no indication that the

material is actually more effective for high-frequency sound than for lowfrequency

sound.

270 Chapter 7

Copyright © 2003 Marcel Dekker, Inc.

7.1.3 Mechanism of Acoustic Absorption

When selecting acoustic materials for noise reduction, it is important for the

designer to be aware of the mechanisms involved in absorption of the

acoustic energy in the material. In addition, it is important to know the

frequency distribution of the sound in the room, in order that the appropriate

sound absorbing material may be matched with the acoustic field. A

different material would be selected to absorb low-frequency sound than

would be chosen to absorb high-frequency sound, generally. Let us consider

three cases.

Porous felt-like sound absorbing materials are commercially available

as mats, boards, or preformed components. Materials of manufacture

include glass fibers, mineral or organic fibers, textiles or open cell foams,

usually polyurethane foams. Representative curves for a porous felt-like

material are shown in Fig. 7-1. It is noted that the absorption coefficient

is smaller at low frequencies (250 Hz or lower), but is near unity at high

Room Acoustics 271

FIGURE 7-1 Surface absorption coefficient _ for formboard, a porous felt-like

material.

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frequencies (1000Hz or higher). The porous felt-like material is more effective

in absorbing sound at higher frequencies than at lower frequencies.

The mechanismfor absorption of acoustic energy for the porous materials

is the fluid frictional energy dissipation between the air and the solid

fibers. At high frequencies, the energy dissipation is larger because the particle

velocity is larger than at low frequencies. The expansion and contraction

of the air within the irregular spaces of the material also result in

momentum losses for the air. The data presented in Fig. 7-1 also illustrate

that the absorption coefficients are larger for the thicker material, which has

more surface area for energy dissipation.

Unperforated panel absorbers involve one or more layers of metal or

plywood with an air space behind the panel. Representative curves for a

plywood panel are shown in Fig. 7-2. It is observed that the absorption

coefficient is larger at the lower frequencies (500Hz and below). The absorption

coefficient is also larger for the thinner panel. The absorption coefficient

may be increased by placing a porous acoustic absorbing material in

the air space behind the panel.

The absorption for the unperforated panel is related to the transmission

loss for the panel. As discussed in Chapter 4, the transmission loss for a

272 Chapter 7

FIGURE 7-2 Surface absorption coefficient _ for plywood panel with a 2-inch air

space behind the panel.

Copyright © 2003 Marcel Dekker, Inc.

panel in the stiffness-controlled region of acoustic behavior, encountered at

low frequencies, decreases as the frequency is increased.

The absorption characteristics of perforated panels backed by an air

space are illustrated in Fig. 7-3. The graph is shown for 1/2-inch (12.7 mm)

thick perforated plywood panels with 3/16-inch (4.8 mm) diameter holes.

The panel is backed with a 2.25 inch (57 mm) air space filled with a porous

acoustic material. The absorption coefficient is largest in the mid-range of

frequencies (250–500 Hz).

When the thickness of the panel is small, the absorption is primarily

due to dissipation of the acoustic energy within the acoustic material behind

the panel. For larger panel thickness, the perforated panel acts as a resonant

cavity-type absorber (Helmholtz resonator). In this case, the absorption is

greatest around the resonant frequency for the cavity. For example, the

resonant frequency for the perforations in the panel with 11% open area

is approximately 300 Hz.

In summary, we see that if we wish to absorb noise mainly in the highfrequency

range (above about 1000 Hz), we should use a porous acoustic

material on the surface. If we wish to absorb sound mainly in the lowfrequency

range (below about 250 Hz), we could use an unperforated

panel with an air space behind the panel. For absorption of sound mainly

Room Acoustics 273

FIGURE 7-3 Surface absorption coefficient _ for perforated plywood panel backed

by a porous absorbent material. Curve A, 11% open area; curve B, 16.5% open area.

Copyright © 2003 Marcel Dekker, Inc.

in the intermediate frequency range (250 –1000 Hz), we would select a perforated

panel with an absorbent material behind the panel. We would need

to use combinations of these materials for effective absorption of sound over

all frequencies.

7.1.4 Average Absorption Coe⁄cient

In general, the various surfaces in a room will not have the same value of

surface absorption coefficient at the same frequency. In this case, we need to

determine an average value for the absorption coefficient for use in acoustic

design. The appropriate average surface absorption coefficient is the

surface-weighted average:

__ ј

__jSj

So

(7-2)

The quantity So is the total surface area of the room. When people or

furniture are present in the room, the absorption will be increased. To

take this effect into account, the total absorption capacity р_SЮ for the

people and furniture is added in the numerator of Eq. (7-2), but the surface

area of the people and furniture is not included in the total surface area of

the room So in the denominator.

The subjective acoustic characteristics of a room may be estimated

from the value of the average surface absorption coefficient. The subjective

perceptions are listed in Table 7-1 (Beranek, 1954).

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