Sound motion pictures : from the laboratory to their presentation (1929)

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ACOUSTICS 99 salvation brought about by the material used in construction. The design could not be worse from an auditory point of view. The shape of the ceiling, for example, is responsible for the unusual echo. The building, however, is made of wood, which, having the highest coefficient of sound exhaustion of any material, makes the acoustic properties quite satisfactory. There have been instances of attempts to overcome the deficiencies of a room by installing wires in certain positions. There is no scientific basis to justify their use; anyone familiar with the fundamentals of sound must readily realize that there can be no real improvement because of such installation. It has been claimed that an echo can be broken by stretching wires in front of the offending surfaces. Experiments have proved this not to be so. Sound waves pass through the wires with practically no change. Echo, in the usual sense of the term, means a definite or articulate repetition of a sound after an interval at least equal to the total duration of the sound that is being repeated. For the average ear the interval must be greater than one twelfth of a second if echo is to be noted. Thus it differs from reverberation, which is a confused or inarticulate but unbroken prolongation of the sound. Echo is always a bad feature. Reverberation, on the other hand, is desirable up to a certain point; only in excess is it an evil. Of the two echo is the more difficult to remove; the best procedure is prevention by foresight in construction, aided, if necessary, by expert advice. Echo arises by regular reflection of sound from smooth walls, ceilings, or proscenium arches, just as a mirror may reflect a beam of light without scattering it. If, however, the surface of the mirror be roughened, the reflected light will be diffused in all directions; and if the walls and ceiling of a room be similarly made irregular (on a sufficiently