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330 CINEMATOGRAPHIC ANNUAL
value of "k" for this stage is about .049. Hence, equation (2)
becomes :
.049x280,000 t" a ,
t = .,0 0 .A = 1.18 seconds,
12,840
that is, the time of reverberation for a tone of 512 d. v. is 1.18 seconds. Actual measurements of the time of reverberation in such a stage as the one we have here considered are in good agreement with this calculated time of 1.18 seconds. The usual sets and other equipment in the stage ordinarily will reduce the reverberation to slightly less than one second, a condition which experience has shown to be satisfactory, especially for the recording of speech.
The above calculations of reverberation have been limited to a frequency of 512 d. v. It is of course necessary to calculate and control the reverberation at other frequencies. If the reverberation be calculated at 128 d. v., 512 d. v., and 2048 d. v. it will be possible to determine the type of acoustic treatment which will give the best acoustic conditions. In general, the reverberation should be nearly uniform at all frequencies, with slightly more reverberation for the low frequencies than for the high frequencies. There are two reasons which suggest this type of reverberation characteristics for a room: (1) the low frequency components of speech and music are not so loud, judged by the ear, as the high frequency components, and therefore if all frequency components are to die away at the same rate and reach inaudibility at the same time, the time of reverberation should be somewhat longer for the low frequencies than it is for the high frequencies; (2) people are accustomed to hearing speech and music in rooms treated with materials which make the room more reverberant for the low frequencies than for the high frequencies. An audience, for example, is about two times more absorptive for the high frequency components than it is for the low frequency components. In general, the reverberation will be highly satisfactory if the room has a time of reverberation at 128 d. v. about fifty per cent in excess of the time of reverberation at 512 d. v. This subject, however, requires further experimentation in order to determine just what is the best type of absorptive material for the control of sound in interiors.