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1947-48 Theatre Catalog, 6th Edition, Page 261 (247)

1947-48 Theatre Catalog, 6th Edition
1947-48 Theatre Catalog
1947-48 Theatre Catalog, 6th Edition, Page 261
Page 261

1947-48 Theatre Catalog, 6th Edition, Page 261

waves in water and mercury tanks and that, within some limitations, varying frequencies may be studied stroboscopically with capillary waves,

Photographs and visual observation reveal (1) desirable distribution; (2) undesirable standing waves (repeated reflections between two surfaces); (3) echoes (a number of reflected waves whose path of travel is long coming to the audience, or part of it, in phase); (4) dead spots, into which very'ilittle sound penetrates (as is sometimes the case under balconies or where direct and reflected waves arrive out of phase and cancel out); (5) focal points (such as the concentration of reflected waves on a small area of seats from a ceiling vault). When the wave pattern is rapidly broken up, and waves from all directions, of approximately equal size, cover the audience area, the distribution is probably satisfactory.

Spark photographs (actual pictures of the sound waves in air) have the advantage of greater clarity than most ripple tank pictures and are useful in the study of diffraction phenomena, and precise location of sources of trouble.

A small beam of light, reflected from surface to surface in a model, makes it possible to trace with reasonable exactness the path of the center of a sound wave.

There are pitfalls to be avoided when sound distribution tests of architectural shapes are made:

(1) Sound sources are at numerous locations and tests must be repeated for from 6 to 12 locations on the stage with various curtain trims as well as for all sound sources other than the stage.

(2) Absorbent surfaces must be made non-reflecting (open or beached) in the model.

(3) Stage and house must be tested together. They constitute a pair of coupled rooms. Plaster cycloramas must be designed as part of the acoustical planning of the house and tested with the house model. (A cyclorama can be tipped toward the back of the stage far enough to put its focal point out of harms way. Thus rigged it is also easier to light than if it is vertical.)

(4) Chandeliers, ceiling cuts for lights, and ventilation ducts must be included.

(5) If reverberation calculations show any absorbent wall material to be desirable, its location must be determined by test so as not to intefere with sound distribution.

Finally, a ceiling under a balcony sloped up toward the back of the house, or a back wall which follows the curve of the seats, will almost invariably render good hearing impossible in at least part; of the theatre.


Reverberation is easy to calculate. But what reverberation is desired is another thing. The scale of desirable reverberation times runs from long for Widorls HToccata" on the organ (it was composed to be played on the organ in Saint Sulpice which is highly reverberant), to short for speech, which requires a high percentage of definition. In legitimate production, reverberation will vary from scene to scene depending upon the settings.

The ideal theatre will have provision for controlling reverberation. At least


MAXIMUM REINFORCEMENT 01 direct sound is indicated by the section (above) and the floor plan of an auditorium. (Stage area at left is incomplete.) With splays properly placed, the entire auditorium can receive reinforced sound. (These and the other drawings credited to the Celotex Corporation are reproduced from "Less NoiseeBetter Hearing," by Hole I. Sabine, and published by the company.)

one attempt to do this has been made, using wall panels which can be changed, but an analysis of the results is not at hand. Unless some means of controlling reverberation is provided, the best that can be achieved is a compromise between the optimum times for the various types of productions.

Reverberation optima have been the subject of much investigation. Recent studies directed toward the determination of optimum reverberation time for various house volumes show slightly longer times than those considered optimum five years ago. Moreover, as clients demand more and more precise acoustic conditions, optimum reverberation times tend to increase slightly. In a theatre planned for more than one type of production, if means are not provided for reverberation control, it is well to choose a reverberation time which is an average between optima for speech and orchestral music. The better the first reflections are controlled the closer orchestral optima should be approached.

For the calculations, the house and the part of the stage, enclosed in a box set are considered. Tolerances, if any, are best taken on the long side. This is done for two reasons: (1) tolerance involves a margin of error. If the house is too dead, its correction is costly; if it is too live, correction is usually simple and cheap; (2) in calculation of reverbera tion time no account is taken of the stage house outside the set. The stage house has, however, some effect on the reverberation: when plein-air sets are used, the effective volume is greater than with a box set. In hung shows in which the files are full, there is more absorption than with a box set. Both these conditions in practice call for slightly longer reverberation than would be requisite in the house with the proscenium closed.

Optimum reverberation time, it must be noted, varies with frequency. In large houses (over 200,000 cu. ft.) a slightly longer reverberation in the low frequencies than in the highs is generally considered requisite. Recent investigations seem to indicate the desirability of reverberation times more nearly equal throughout the audible range than was standard some years ago. If the reverberation time is made equal throughout the audible frequency range, the sound absorption characteristics of the audience will slightly unbalance it in favor of the lows.

Unfortunately the theatre does not play to the same sized audience at every performance. And the most neglected feature of acoustic planning is the provision of means of compensation for audiences of varying size. Yet it is possible to have the reverberation time the same with no one present as with a
1947-48 Theatre Catalog, 6th Edition, Page 261