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642 Transactions of S.M.P.E., Vol XI, No. 32, 1927
Action of Sound in a Room
What is desired for ideal acoustics is that the sound reaching an auditor in any part of a room shall be of suitable loudness and distinctness for comfortable hearing with an elimination or control of echoes, reverberation, "dead spots," and other faults. To a great extent, it is possible to secure such ideal conditions; and it is the purpose of this paper to explain some of the fundamental actions of sound and to show how motion picture rooms may be adjusted so as to have good acoustic properties.
Sound travels out in spherical waves from a speaker or a musical instrument with the great velocity of 1120 feet per second at ordinary temperature, about as fast as a rifle bullet. As a result, sound will be reflected back and forth about 30 times a second between walls of an auditorium 40 feet apart. Because of these rapid reflections, an auditorium of usual size is filled with sound in a small fraction of a second, thus-insuring a loudness in every part of the room.
A speech sound, such as any one of the words uttered by a speaker that requires about one-tenth of a second for its completion, thus travels 112 feet before the word is finished; so that, in the open air, a speaker would be at the center of a sphere of 112 feet radius that would be filled with the sound of the word. In an auditorium, the sound waves would be reflected several times in traveling 112 feet, so that instead of a sphere there would be overlapping bundles of sound traveling in every direction and completely filling the room with the sound of the word before the speaker finishes saying it. In the open air, the utterances of a speaker progress with practically no distortion, and perfect acoustics are obtained. In a room, however, the reflected sound joins with the direct sound and has large possibilities of distortion. A study of the action of the reflected sound is thus the most important consideration in obtaining good acoustics in a room.
When sound arrives at a wall or ceiling, it is reflected, absorbed, and transmitted in varying amounts depending on the nature of the reflecting surface. A hard plaster wall, for instance, reflects 95% or more of the incident sound and therefore absorbs but little; whereas a layer of hairfelt one inch thick may absorb 50% with a correspondingly smaller reflection. The following table gives the absorbing values of common materials.