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ACOUSTICS 101
sound wave is retarded by the proper amount it may happen that the compression of the directly transmitted sound meets the rarefaction of the reflected sound at the ear at the same time. They thus neutralize each other's effect and produce a diminution in intensity. This is called a "dead spot." If a reflected sound is retarded a little more it may happen that two compressions coincide, producing an unusually loud sound. This is a sound focus. The most usual cause of such sound foci is a curved wall or ceiling which concentrates the sound to a focus.
Since dead spots and sound foci arise from the same cause as echo their removal may be brought about by the same treatment. Some care and experience are necessary in order to locate the particular portion of the room which is responsible for the production of a dead spot. Often this can be found only by trial-and-error experiment, since it is not possible to predict the path of reflected sound with the same accuracy as that of light. The offending portion, once found, must be treated in such a way as to decrease its power of regular reflection.
Professor Sabine mentions the case of a theatre, the ceiling of which contained a flat oval panel, to which the trouble was ultimately traced. In this case an irregular canopy, oval in plan and slightly larger than the panel, was hung just below it, with good effect.
Since a sound produced in a room is reflected back and forth from walls, floor, and ceiling, and since a portion is absorbed at each reflection, its intensity is finally so reduced that it becomes inaudible. Owing to the high speed of sound there may be many such reflections in the course of a single second in a room of ordinary size; and the greater the dimensions of the hall, the more prolonged will be the reverberation. If the walls of the room are covered with some highly sound-absorbent material, such as hair felt, two or three reflections may suffice to destroy the