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1950-51 Theatre Catalog, 9th Edition, Page 33 (13)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 33
Page 33

1950-51 Theatre Catalog, 9th Edition, Page 33

Prefabricated Types

Prefabricated acoustic materials are sold in the form of flat tiles or boards that are applied to the walls of a room. They may be cemented directly to the wall or ceiling .by means of uacoustic cements,' made especially for the purpose, or they may be nailed to furring, or supported on suspension systems. Frequently the edges of tiles are kerfed for the reception of splines, which assist in the alignment of the tiles to a true flat surface.

Oftentimes, however, the architect requires that the acoustic treatment be applied in broad areas, unbroken by visible tile or board joints. Acoustic plasters and sprayed-on materials lend themselves admirably to this purpose.

There are various types of the prefabricated kind. A common type is the homogeneous porous absorbent consisting of wood fibers, or glass fibers, or granulated material, held together with a suitable binder. Another common type is the porous material having a hard, nonporous surface which is perforated (Fig. 1) so that the sound waves can

FIGURE 2-Impedance tube for measurement of normal-incidence absorption coefficient.

pass into the porous region and be absorbed. The perforations might be regularly spaced slots, circular holes, or irregular fissures.

The perforations range from 1/16 to 1/4 inch in diameter, or width, and extend deeply into the material. As long as the total area of the perforations is greater than 10 per cent of the total surface, the surface of the material behaves as if it were transparent to sound. The effect of the perforations is the same as if a much

greater surface area of the material were exposed to the sound.

Another type is the porous material installed in blanket form, such as glass Wool or rock wool, and protected by a perforated surfacing of wood, metal, or asbestos cement board. The principal advantage of the prefabricated acoustic material lies in the uniformity of the product. The manufacture can be carefully controlled, and, in general, there are relatively small variations in the absorption coefficients for a particular type.

Acoustic Plosters

Several different types of acoustic plasters are available. Some consist of granulated inorganic substances which are mixed with a foaming agent and a suitable binder, and are applied with a trowel. Sometimes the plaster is stippled (in order to improve the absorption of sound) before it has set hard. Other types consist of fibrous material, usually rock wool or asbestos, which is mixed with a binder and sprayed directly on to the wall by means of special equipment. Acoustic plasters are generally difficult to handle, and careful control must be

FIGURE 3-National Bureau of Standards reverberation room. Used for measurement at random-incidence absorption caeHicient.


1950-51 Theatre Catalog, 9th Edition, Page 33