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1950-51 Theatre Catalog, 9th Edition, Page 32 (12)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 32
Page 32

1950-51 Theatre Catalog, 9th Edition, Page 32

Behavior of Acoustic Materials*

A Review of Popular Sound-Absorbent Substances And Factors To Be Considered in Their Selection

BRIEF: Theatre architects . . . and engineers . . . need accurate data on the performance of acoustic materials . . . which are used to control the acoustics of theatres . . . Descriptions of prefabricated materials . . . and acoustic plasters . . . are given in the following article . . . and the mechanism of the sound-absorption process in porous materials is briefly described below.

There are two commonly used absorption coeficientsethe iGandorrg-incidencei, coeji'icient . . . and the unormal-incidenceii coefficient . . . The experimental mthods used for measuring the two coe/jicients are outlined . . . The significance . . . and limitations . . . of these coehicients in theatre design are pointed out . . . and it is concluded that . . . at the present time . . . the random-incidence coehicient is more useful in auditorium design.

Recommendations for painting acoustic materials are also made . . . and illustrations of the results of painting are included.

The control of the acoustics of auditoriums and theatres, and the quieting of noisy rooms, both require the installation of acoustic materials. By an acoustic material is usually meant a sound-absorbent substance which is fastened in flat patches to the walls and ceiling. Recently, however, some absorbers have been


Chief. Sound Section. National Bureau of Standards

fashioned in the form of cylinders, cones, and spheres, and have been suspended at a distance from the walls and ceiling of the room.


Modern building materials and techniques lead to hard, smooth interior walls that are notoriously poor sound absorbers. After a source of sound, for instance a speakeris voice, is suddenly stopped in a room with hard plaster walls, the sound does not stop immediately. Rather, it is reiiected from wall to wall, only a small fraction of its energy being absorbed at each redection, until eventually the energy is so far reduced that the sound becomes inaudible. This persistence of sound after the source is stopped is called reverberation, and the time required for the energy of the original sound to decay to one millionth of its original value is called "the reverberation time, of the room.


Excessive reverberation is common and a serious defect in many churches, theatres, offices, and factory rooms. Conversation in reverberant rooms is annoying. Preceding speech syllables overlap suc FIGURE l-Some types 01 acoustic materials.

ceeding syllables and, if the reverberation time is very long, conversation becomes extremely diiiicult.


Reverberation, however, is not withOut its benefits. The reinforcing effect of reverberant enclosures can actually serve to enhance speech and music, provided the reverberation time is held within certain limits. The acceptable limits for speech are from 1a second for small rooms to approximately one second for large rooms.


The principal function of acoustic materials is to absorb sound energy which originates within the room. It is possible to predict in advance of construction, or remodeling, the amount of such material that must be placed in a given room to obtain the optimal reverberation time.

Absorbent Qualities

Acoustic materials have the ability to absorb much greater fractions of the energy of the sounds incident upon them than do ordinary plaster or masonry walls. In a room with walls treated with such materials, a sound wave undergoes many fewer reiiections before it becomes inaudible.

Choice of a Type

Architects and engineers are faced with the problems of deciding what material should be used in an auditorium to secure the proper amount of sound absorption, and deciding what is the most economical absorbent to quiet a noisy location. It is clear that they must have accurate data on how acoustic materials absorb sound. Such data have been available for many years.1 However, the special uses to which a material is put will in general require more detailed information than can be obtained from the results of routine laboratory tests. Nevertheless, the results of such tests are of great importance to the theatre engineer, and he should understand their signifitance and limitations.


There are two main kinds of acoustic materials. One kind is prefabricated, a familiar example being the one-squarefoot tiles which are commonly used in many public places. The other is the kind which is manufactured, so to speak, at the moment of application, and includes acoustic plasters and sprayed-on fibrous materials.

*fl rrlrilml i't'nimi of (1 (HI/m- original/y [Wynn/ml ()i'lobm' 34. 1947, a! I/yr ,nrlrly u/ Mal/ml PHI/IVE ling/mum Conrail/[rm in Nru' York and Mild/lbw! in (I): Jomvml of Mn 5.M.F.l:., Arman, 1948.

1950-51 Theatre Catalog, 9th Edition, Page 32