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420 RICHARDSON'S BLUEBOOK OF PROJECTION
atre can sometimes be located by timing the interval between the direct and the reflected sound, remembering that both travel at approximately 1,000 feet per second.
Reverberation
(20) Just as a plaster wall presents a rough surface to the waves of light, and distorts them in reflecting them, so a wall broken up by doors, columns or other large irregularities will distort the sound it reflects until it is no longer recognizable as an image of the original sound. Music or words cannot be distinguished. This form of reflected sound is called reverberation. Moreover, repeated reflections from even the smoothest walls and ceilings will also cause reverberation. It is a very common and very troublesome fault in theatres. Echo, as we can now see, is merely one type of reverberation — that type in which the sound is not distorted by reflection.
(21) The easiest remedy for reverberation is to use the lowest possible volume. Most theatre sound is much too loud. (See Page 639.) The next easiest is to point the speakers away from the reflecting surface and toward absorbing surfaces — such as the audience itself. The most troublesome and expensive remedy, but the surest, is to install drapes or acoustic plaster over the reflecting surface. Re-plastering should always be directed by an acoustic consultant; it is too expensive an operation to experiment with.
(22) Since echo is merely a special form of reverberation (that form in which the reflected sound is not distorted) it is treated by any of the methods just mentioned. In addition, it can be attacked by breaking up the reflecting surface through use of coffering (artificial recesses in the wall or ceiling). This last will cause distortion of the reflected sound and convert the echo into reverberation, which is the lesser of the two evils.