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76
BRITISH KINEMATOGRAPHY
Vol. 16, No. 3
Reverberation Characteristic
The recordist accustomed to working in a particular studio should have exact knowledge of the reverberation characteristics of the studio, and whilst a very great deal can be done by calculation from published absorption co-efficients of the various surfaces exposed to the impact of sound, by far the most satisfactory method is to record the decay of sound at various bands of frequencies. Fig. 3 illustrates an instrument which it would be convenient to use for such an investigation.
The instrument employs an input potentiometer and a high gain amplifier followed by a balanced rectifier. The D.C. output operates a slipping clutch mechanism which picks up forward or reverse driving movement from the periphery of a motor-driven disc and so drives the input potentiometer and recording stylus.
With no input signal there is an initial unbalance in the rectifier circuit which causes the clutch to drive the potentiometer and pen to the end of the scale where the attenuation is zero. Raising the signal level causes the clutch to operate in the reverse direction until there is a balance of current in the rectifier. The distance the potentiometer moves in restoring balance is recorded by the scriber on a moving wax paper strip and thus a measurement is made and recorded of the amount of change on the input signal.
Fig. 2. Oscillograms indicating
irregularities in rate of decay due
to interference effects.
The acoustical properties of a room cannot be given by reverberation time alone, as the study of the oscillograms reproduced in Fig. 2 clearly indicate. These curves were obtained by connecting the output of a transmitter to an oscillograph and obtaining an actual graph of the decaying sound.
A careful examination of such graphs, obtained under various conditions, was shown by Eyring to prove that for diffuse conditions there is a steady rate of decay of sound level, but when resonators are present the decibel-time decay curve may be composed of two or more straight lines. The resonant or " live " portion of a room acts as a decaying source to a " dead " portion, hence the rate of decay of sound is changed by resonance.
The measured reverberation time only yields the " average " decay rate and takes no account of the variations in the rate within the time of decay. Too much emphasis cannot, therefore, be laid upon the importance of audiographic methods of measurement, as the results obtained therefrom afford a ready means of examining the quality of the sound in various parts of the auditorium. This is shown clearly by the oscillograph records.
The air pressure is recorded on a linear scale as vertical ordinates and the time base operated by a tuning fork on a moving strip of film. Note the echoes and the irregular rates of decay.