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DEVELOPMENTS IN SOUND TECHNIQUE AND EQUIPMENT
B. C. Sewell, M.B.K.S.*
Summary of paper delivered to a joint meeting of the British Kinematograph Society and the Association of Cine-technicians on January 22, 1947. The paper has been more fully reported in The Cine-Technician, March /April and May /June, 1947.
MR. SEWEIvIv commenced his paper by outlining the physical properties of sound waves, and discussing the principles of acoustics and reverberation. Definitions of the decibel and the phon led to an explanation of the need for equalisation, and the methods adopted to secure it.
The next point ^considered was the design of microphones. The directional properties * of the moving coil, ribbon, and cardioid types were contrasted ; the particular field of application of the directional types, in excluding extraneous sounds and modifying apparent reverberation, was stressed.
Mr. Sewell next turned to the requirements of volume range in film recording. The causes of background noise were discussed, a point mentioned being that in variable density recording a low but appreciable density produced a maximum background noise. This led to an explanation of the principles of noiseless recording and the advantages of fine grain emulsions. Distortions inherent in variable density and variable area recordings were outlined.
Arising out of the rectification effect, the speaker gave an explanation of the benefits of push-pull recording, differentiating between Class 'A and Class B. Flutter was defined, and the method of measurement explained ; a flutter meter consisted of a selective amplifier tuned to the frequency of the test tone employed, followed by a limiter and discriminator. A meter indicated the departure of the frequency from the discriminator mean. A cathode-ray oscilloscope might also be employed to indicate the waveform of the flutter.
Mr. Sewell briefly discussed disc and magnetic recording ; he suggested that for the latter to be applied to film work, means of synchronisation could be devised, although there would at present remain the serious disadvantage of the invisibility of the record.
A primary object in the recording of speech, said the speaker, must be intelligibility. Removal of the lower frequency components had little effect upon intelligibility, but of high frequency components, the reverse was true. The reverberation characteristics of kinemas frequently caused a bass component to be augmented and prolonged. Consequently, it was often felt desirable to give dialogue recordings a low bass constant. Electronic compression might assist in maintaining intelligibility ; by far the greater part of the energy in speech lay in the region below 1000 c.p.s., and it was sometimes an advantage to employ pre and de-emphasis of high frequencies in studio processes, although this might introduce distortion of sibilants.
Finally, the speaker referred to acoustic perspective, and concluded by discussing the possibilities of stereophonic recording. It was possible by the use of two or more microphones, recording and reproducing channels, and loudspeakers, to simulate normal hearing so that lateral discrimination was restored. Twin stereophonic tracks had been recorded on the German Magnetophone.
* Gainsborough Pictures, Ltd.
Property of U. S. Army
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