It is shown that a distinctive characteristic is that, because their dimensions are comparable with the wavelength of low-frequency sound, the sound field is characterized by strong simple standing-wave patterns which cannot be eliminated without eliminating the reverberation itself.
The paper gives the current views of the author and his colleagues in the Engineering Department of the British Broadcasting Corporation on the design and construction of talks studios and listening rooms or control cubicles, which are considered together on account of their similarity with respect to acoustic behaviour. For 20, 12, and 9 modes in 1‐, 1 2 ‐, or 1 3 ‐oct bands, the constants are 1150, 1280, and 1355 cps, respectively. The formula turns out to be a constant divided by the cube root of the volume, where the constant is a function of the measuring bandwidth and the number of normal modes required therein. For rooms having satisfactory mode distribution, an approximate formula has been developed for determining the lowest midband frequency for which a room may be used for measurements of continuous spectrum sounds. When both the frequency and angular criteria are combined, only a few small regions of dimension ratios appear to be good.
The angular‐distribution index is more regularly behaved with rather definite stratification apparent as a function of the room height/length ratio, when the height direction is taken as the angular reference. No clearly defined optimum room dimension, as predicted by Bolt, emerges from this study. Considerable variation in the frequency‐spacing criterion exists not only for changes in room dimensions but also from one half‐octave band to the next.
The criteria adopted were computed for each half‐octave band over the first 4 octaves of normalized frequency for rooms with dimension ratios ranging from 3 1 3 :1 to 1:1. This paper describes the mathematical model used for computing both the frequency and angular distribution of the normal modes in rectangular rooms.
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