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1950-51 Theatre Catalog, 9th Edition, Page 34 (14)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 34
Page 34

1950-51 Theatre Catalog, 9th Edition, Page 34

FIGUEE kFissured surface of a porous acoustic material.

FIGURE 6-Same material as in Figure 5 other application of tour coats oI brush-applied paint.

FIGURE 7-Granular surface of a porous acoustic material.

FIGURE 8-Same material as in Figure 'I after applicau'on of live coats of brush-applied paint.


exercised when they are applied. Occasionally, however, there are economic advantages in favor of plaster and sprayed-on materials.

A third kind of acoustic material has been used in Europe, and consists of sponge rubber having a low modulus of elasticity, and covered with a thin impervious skin. Such absorbents do not seem to be commercially available in this country.

Other Absorption Methods

It should be pointed out here that there are additional ways of securing sound absorption in a room, besides the use of acoustic materials. For example, thin panels of plywood or similar material, not exceeding approximately onequarter inch in thickness, mounted on frames with an air space between the wall and the panels, can have a considerable amount of absorption at low frequencies. Details of this and other types of sound-absorbent constructions as well can be found in the recent book, iiAcoustical Designing in Architecture," by Vern O. Knudsen and Cyril M. Harris (John Wiley & Sons, Inc., New York, 1950).

Mechanism of Absorption

Acoustic materials can also be classified by the way in which they absorb sound (Fig. 1). A knowledge of the mechanism of absorption is important, especially if one is going to be faced with the problem of painting the material, or keeping it decorated. In almost all cases, the absorber is porous, and the absorption of sound is due largely to the Viscous damping of the motion (caused by the incident sound) of molecules of air in the pores. In the absence of any other absorption process, a porous material can absorb sound only if its interior is permeated by interconnecting pores and voids, and if the surface is open to permit direct access to the interior. Some cellular materials like foamed glass do not absorb sound readily because the voids are completely isolated from one

another by the walls of the cells so that air cannot pass from one void to another.

Sometimes the absorption process is. aided by vibration of the porous material itself. In addition, the propagation Within a porous material is inHuenced by the tortuousity of the channels (see the sketch to the right in Fig. 1) and by thermal effects. In fact, it is difficult to say which absorption mechanism predominates in a given porous absorbent without conducting a complicated investigation as to how sound is propagated through a material. The sponge rubber described earlier absorbs sound by frictional damping in the rubber. Oddly enough, some very soft and porous

lfibrous materials, after being covered

with an impervious layer of paint, behave like sponge rubber. As a general rule,however, it is Very important to preserve the porosity of an absorbent. Painting is, therefore, diflicult. This point will be further discussed later.


A number of elements are important in the choice of an acoustic material. The user is interested not only in sound absorption, but is also concerned with light reflection, fire resistance, appearance, strength, and paintability, all of which are important. However, we shall discuss only the measurement of soundabsorption coefiicients, and the influence of painting on acoustic properties.

Important Factors

Suppose a beam of sound waves is incident upon an acoustic material. The beam carries poWer. The absorption coeiiicient is the fraction of the power which is absorbed. For example, if a material absorbs 65 per cent of the sound power incident on it at 512 cycles per second, the absorption coefficient at this frequency is 0.65.

Measurements of absorption coefficients are usually made at frequencies of 128

FIGURE 4-Simplified geometrical acoustics of an auditorium having a sound-absorbent ceiling.

1950-51 Theatre Catalog, 9th Edition, Page 34