FROM the early Greek days down to a comparatively recent time the problem of theatre acoustics has been a perplexing one. Whether or not the Greeks were familiar with the theory and laws of acoustics is a much disputed question. That the Greek theatres had excellent practical acoustics there can be no doubt, but this may have been due to habitual adherence to primitive conditions logically developed into grand form, rather than to any studied application of acoustic laws in their building operations. Certain it is that the Greek theatre had no walls to reflect sound, although many authorities claim that inverted vases of varying size were used instead to intensify sound and increase its volume.
Centuries later, Charles Garnier, the architect who built the famous Paris Opera House, when questioned as to the manner in which he obtained such perfect acoustics, replied: "I just trusted to luck." Today, one need not trust to luck. Experts understand certain well-defined rules of acoustics, which, when properly applied, produce uniformly good results.
The audibility of sounds depends upon the loudness, distinctness, and the quality of the effect produced by them. There is always plenty of sound in any auditorium, but the difficulty is to regulate it. Sound waves radiate from their source in all directions in the manner of a constantly expanding sphere. The tones strike the ceiling and walls solidly and rebound to the auditor within a small fraction of a second. If the auditorium be properly proportioned the reflected sound waves will be received almost simultaneously and in audible unison with the direct sound. As a result, the audience, in what are usually considered the worst seats, hear quite as distinctly as those in the best seats.
Given the size and shape of an auditorium and the materials of which its walls are composed it is now possible to determine accurately beforehand its acoustic value. The acoustic requirements for an opera house and for a regular theatre differ greatly, because of their different formation. The auditorium of an opera house is larger and more open than an ordinary theatre, and has. shallow' tiers of boxes instead of one or two deep balconies.
The problem to be considered here is that of the theatre. As the human voice is capable of projecting distinguishable words only about seventy-five feet without expansion, it is decidedly important that the auditoriumm be confined within that area, and as the downward waves are largely absorbed instead of being reflected, it follows that the height of the ceiling should be about half that distance. Sound naturally loses in volume with each reflection, diminishing in its intensity until it crosses what scientists term the "threshold of audibility." The rapidity with which sound dies away depends upon the size of the room, its shape, and the materials employed for furnishings, walls and ceiling. In theatres the magnitude and distribution of the audience are also great factors in the propagation or absorption of sound.
The side walls of an auditorium should gradually curve inward toward the proscenium opening, the rear wall following the curved line of the seats. The side walls should be coved at the top to meet the ceiling, and all walls should be made reflective and not absorbent in their quality. Science teaches that sound waves are reflected in exactly the same manner as light rays, the angle of incidence being equal to the angle of reflection, a fact that argues for an avoidance of deep recesses and curves. Irregular lines, sharp turns or abrupt curves, like the deep recesses usually provided for stage boxes, should be avoided. High ceilings, too, are bad, as sound waves carry farther if not hampered by vacant space far above the audience.
Experiments covering a number of years, made by Professor Wallace C. Sabine of Harvard University, noted authority on acoustics, demonstrated that walls have either constructive, absorbent or reflective power. Professor Sabine states that plastered brick or hollow tile walls have proven the best, and are powerful reflectors of sound with very slight absorbing power. In this connection other authorities recommend a plaster composition of hydrated lime, slaked and prepared at the mills. Professor Sabine in discussing the various conditions that offer natural obstruction to the projection of the human voice, gives a comprehensive analysis of the problem in the following manner:
"The dissonant (interference) are those places in which sound first uttered is carried up, strikes against the solid bodies above and, reflected, checks as it falls the rise of the succeeding sound. The circumsonant (reverberation) are those in which the voice spreading in all directions is reflected into the middle, where it dissolves, confusing the case endings, and dies away in sounds of indistinct meaning. The resonant (echo) are those in which the voice comes in contact with some solid substance and is reflected, producing an echo and making the case terminations double. The consonant are those in which the voice is supported and strengthened, and reaches the ear in words which are clear and distinct."
Naturally one must lessen or obviate, so far as possible, all of the obstructions described in the first three explanatory phrases above quoted, and strive for the attainment of the conditions enumerated in the last one. The difficulties resulting from interference and reverberation never entered into the acoustic problem in the open Greek theatre, with its large unobstructed area, nor was echo a serious consideration in these edifices, as there was but one doubling of the case endings. In modern theatres there may be many echoes, each arriving after the direct sound at a different interval of time, and less distinguishable and therefore more disturbing.