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1947-48 Theatre Catalog, 6th Edition, Page 260 (246)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 260
Page 260

1947-48 Theatre Catalog, 6th Edition, Page 260



ACTION OF SOUND on encountering a solid medium; in addition to being partially reflected, absorbed, or transmitted. sound is also refracted (changed in direction) by the more dense medium.

phenomena, and have used that knowledge, more often to explain why the theatre was not good than successfully to make it satisfactory. Most present theatres were built by some rule of somebodyis thumb.

Knowing that theatres usually turn out bad acoustically, there is a third architectural approach to the problem: build it first and fix it later. This means padding the walls or hanging up a drapery to stop an echo. No acoustically good theatres have resulted from this approach.

The final architectural triumph, however, is supposed to banish the demon of bad acoustics. It is born of despair and the public address system. If all surfaces in a theatre are made very soft and sound absorbent, the audience can hear practically nothing. There are no echoes. Then, if a powerful public address system is installed, powerful enough to frighten children blocks away, the audience can hear everything, including (and this causes much bewilderment) the echoes supposedly banished by the padding. Some of the largest theatres in America were thus designed. In them a strong human voice unamplifled can hardly be heard a hundred feet. It takes much amplification to make a mighty pipe organ audible. And in some thousands of seats, everything the audiences do hear, they hear twice.

Oddly enough, a number of acousti TWO METHODS of breaking up a curved rear wall of an auditorium to avoid focused sound reflections are shown here. (Drawings through courtesy Hale J. Sabine and Celotex Corporation.)

cally bad theatres were built wrong not because of any of the architectural approaches mentioned, but as a result of a sincere and outwardly intelligent effort to make them good. The miscalculations have come about through: (1) making an auditorium which is acoustically good while the asbestos is down, but bad when the play is on, through lack of provision for acoustic characteristics of stage, fiyloft, and scenery; (2) assuming that the seats will all be occupied all the time (in two existing theatres acoustic conditions are fair when the house is full, bad when it is half empty, and it is impossible to rehearse in the empty theatre with the curtain up); (3) assuming that building materials with the word nacousticii in their names will contribute to good acoustics no matter where or how used,

The theatre offers an acoustic problem replete with complexities which arise out of the nature of the show and the habits of the audience. The architect and the engineer must work under a severe handicap unless they know show business.

The architect who knows show business well enough to design a theatre will often feel competent to prepare plans and specifications which will result in adequate exclusion and elimination of noise. The more specialized task of getting the sound of the show to the audience usually calls for the services of a physicist or engineer trained in acoustics to undertake the requisite calculations and tests.

Action of Sound

In the theatre useful sound emanates from actors, orchestral instruments, organ, and loudspeakers; located on the stage, in the pit, and above and at the sides of the proscenium. The architects job is to get it to the paying customers no matter where they sit without distortion or appreciable loss of intensity. Getting the sound to all the customers is a problem of distribution. Getting it there at almost equal intensity everywhere and having it die away rapidly at a predetermined interval after it has ceased to emanate from the source, so as not to interfere with the next sound as it comes along, is a problem to be solved by achieving a certain reverberation time.

Part of the sound pressure wave goes straight from the source to auditor. Part of it is reflected from ceiling and walls. Part of it gets to the auditor after it has been refiected back and forth about the house many times. Part of it has been for a trip around the stage, or has been reflected from the scenery or cyclorama.


Ceilings are the principal distribution surfaces. When ceilings are laid out, they must be planned to reflect the sound back to the audience, either directly or via walls, but in such a man* ner that it will neither be concentrated in certain spots, nor refiect back and forth between parallel surfaces, nor get to the audience out of phase with the direct wave. Moreover, since sound travels only about 1150 feet per second in air, the length of the path of the first reflected wave must not exceed that

moo SEC.


MULTIPLE REFLECTIONS of a: single sound wave front in a closed room are shown here, at 1/200, 1/100. 1/50, and 1/17 second. (Drawings courtesy of Hale I. Sabine and the Celotex Corporation.)

of the direct wave by more than 50 feet (preferably less) or the audience will hear everything twice,

The position of the side walls is determined by sight lines. The depth of the house (curtain to back wall) is governed by visibility requisites. Sound distribution requirements govern the shapes of the side walls, ceiling, ceiling under the balcony, and rear wall.

The fact that the angle of reflection of a sound wave is equal to the angle of incidence (neglecting diffraction phe-, nomena at low frequencies) makes it possible to lay out tentative shapes on paper. But since even the most unorthoe dox appearing shapes sometimes result in good theatres and vice versa, tests are necessary to establish the workability of any design.

Plan and section of the theatre may be tested for distribution in ripple tanks, three-dimensional models by spark photographs and by tracing reflections of a small beam of light. The technique of testing is fairly complex. It can only be pointed out here that rough approximations are possible with single gravity

1947-48 Theatre Catalog, 6th Edition, Page 260