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74
BRITISH KINnMATOGRAPHY
Vol. 16, No. 3
is analysed, and the modulation, etc., adjusted by the sound engineer so that the listener hearing the reproduced sound in the theatre receives the natural effect desired.
The acoustic condition of the monitor room (in which the sound being recorded is played over through an amplifier-loud-speaker system) should therefore approximate to that of the theatre, and a reverberation period of 1 \ sees, is recommended, with a distribution of reflectors, resonators and absorbents simulating the arrangement of the theatre auditorium.
In the sound film studio assembly a small theatre is used for review and editing purposes, and the period therein should be between 1 1 and 2 sees., reflecting, resonating and absorbing surfaces being disposed as recommended for the kinema.
In the play-back studio, used, as its name suggests, for synchronisation of sound to mute sequences or any of the many dubbing processes in connection with the adjustment of sound records in sound film work, adjustable absorbents will be necessary to enable the period to be changed from § sec. to 2 sees, according to requirements.
Optimum Reverberation Time as affected by Frequency
It is a mistake to provide a uniform reverberation time throughout the range of frequencies. In auditoria in which the acoustics are considered
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Fig. I. Graph Showing Variation of Optimum Reverberation with Frequency. Example : If optimum for music at C4 is 1.25 sees, optimum at C2 is 1 .25 x / .55= 1 .94 sees.
ideal the reverberation is invariably greater at the lower frequencies.
Walter A. MacNair, of the Bell Telephone Laboratories, argues that it is more reasonable to specify that the loudness of all pure tones shall decay at the same rate for all frequencies, as it is the loudness which takes into consideration the ultimate appreciation of the ear. For the frequencies between 700 and 4,000 c/s., loudness and sensation-level are approximately equal, but outside this band the loudness-sensation level curve is steeper. MacNair thence deduces an optimum-frequency curve, which is reproduced in modified form in Fig. 1.
Knudsen, on the other hand, recommends a reverberation characteristic such that on the average all frequency components reach inaudibility at the same instant. His curves for speech and music are also shown in Fig. 1. It must, however, be borne in mind that the frequency distribution of sound energy to which the average audience is accustomed is influenced principally by the selective absorption characteristics of the audience and the hall. The absorption of the audience has by far the greater influence, and " the characteristic of the audience " is therefore shown on the figure.