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March, 1950
GLOVER AND MCLAREN [ ACOUSTICS
79
At the time when these experiments were carried out, the technique of measurement was not sufficiently advanced to detect appreciable variations of the decay curves at each frequency, but it is felt that with modern methods, which include pulse measurement which gives a critical analysis of the decay of sound in a room, valuable information could have been obtained.
Having established the two factors which form the basis of good acoustical design, the next point is to determine how these effects are to be obtained. As is well known, the reverberation time is controlled by the amount of absorption, either inherent in the structure or deliberately added, that is present in the studio ; the reverberative effect is obtained by the position in which the added sound absorbing material is placed in the room, by the shape of the room and by profile of the surfaces.
Reverberation Frequency Characteristic
Let us first of all consider the problem of obtaining the correct reverberation time in the studio ; what should be the optimum reverberation time for a given
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VOLUME OF STUDIO IN CUBIC FEET
Fig. 5. Maximum number of performers as a function of studio volume.
size, and what should be the shape of the reverberation time-frequency characteristic ? F. R. Watson, who was a contemporary of W. C. Sabine, calculated a curve of optimum reverberation time for various sizes of halls, taken from places which were considered to be good acoustically, and the B.B.C. have adopted and modified this curve to meet the requirements of broadcasting. This curve is shown in Fig. 4, and marked against it are the names of some studios and well-known concert halls.
In conjunction with this curve is another, Fig. 5, also based on the work of F. R. Watson, which indicates the optimum number of performers which should be used in a studio of given volume. Hence, by the use of these two curves, the required size of studio and its optimum reverberation time can be determined.
The next point that requires our attention is the shape of the reverberation time-frequency characteristic. About this subject there are several opinions, but generally speaking the opinion held in the B.B.C. is that the curve should be flat, but that a slight increase of reverberation time at the low frequency end of the scale is not objectionable in studios intended for music. At the