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reverberation times as defined above are allowable for large than for small rooms.
We thus have a very usable criterion of excellence for rooms that are for audience purposes. Thus for example, if the volume of a proposed room is 200,000 cubic feet, then the absorbing power, including that of the audience, should be 6667 units in order to give a reverberation time of 1.5 seconds, the value which according to the graph is desirable in a room of this size. If the calculated total absorbing power of the room surface and the audience is less than this, then sufficient area of absorbent treatment should be introduced to bring the total up to the desired
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Log. of Volume Fig. i — Reverberation Times of acoustically good rooms of different volumes
value. It should be said that the departure from the values shown without materially affecting hearing conditions is fairly large, certainly as great as 5 % and possibly as great as 10 %, so that the graph serves simply as a working basis for designing rooms with comfortable acoustics.
Coming now to the question of the proper reverberation time for sound recording rooms, I have to confess the lack of any very precise data. As you know, the early practice in phonograph recording and in radio broadcasting was to cut down sound reflection to the limit, by the use of heavy drapes hung well out from the wall and over every foot of available surface. While this practice serves to eliminate any trace of room effects, at the same time it produces a dull lifeless quality to music particuarly, that is extremely trying to artists and is easily perceptible to discriminating auditors. Gradually the tendency toward less deadening and longer reverberation times has grown up. In 1926, I conducted a series of tests for station WLS in Chicago to find out whether differences in room conditions Were perceptible to radio listeners, and what condition was preferred. It was arranged to vary the absorbing power of the studio in three steps by the removal of drapes, thus increasing the computed reverberation time from 0.25 to 0.64 seconds. In this test, the same short program was broadcast under each of the three conditions, and the preference of the radio listeners was asked for. Of the 121 replies received
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