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ACOUSTIC POWER LEVELS IN SOUND PICTURE REPRODUCTION
S. K. Wolf and W. J. Sette*
THE advent of sound pictures made especially prominent the question of acoustic power necessary to produce satisfactory hearing in auditoriums. The large scale employment of electrical sound reproducing apparatus made imperative a detailed investigation into this problem which had attracted some attention in the past. At the same time the perfection of electrical devices for recording, amplifying, and reproducing sounds gave us a degree of control over sounds which allows such fundamental measurements as those reported in Dr. Fletcher's book, "Speech and Hearing." Thus the question of acoustic power is most urgently raised when the means for regulating and measuring it had reached an advanced state of development.
It is the purpose of this paper, which is an amplification with some modification of a previous paper by the authors,1 to consider the power levels dealt with in sound picture reproduction, with reference to the ear, the auditorium, and the electrical system. We undertake a consideration of what constitute desirable loudness levels in auditoriums, a study of the bearing of auditorium acoustics on the maintenance of adequate sound levels, and a determination of the power output of the reproducing equipment. Equations and curves relating required acoustic power and theatre volume are derived. Experimental data appear to substantiate our theoretical conclusions. We have attempted to follow the terminology developed in "Speech and Hearing."
Listening Levels
The optimum sensation levels for listening to speech and music will be first discussed. Ordinary conversation is maintained at a level of about 70 decibels above threshold of audibility for the strongest speech sounds, with about 50 decibels, more or less, for the consonants." In other words, vowel sounds thus spoken are ten million times as powerful as when just audible. The so-called mean power fluctuates about an average closer to a level of 65 decibels. This intensity is such that words are heard at maximum intelligibility, greater or less intensity both tending to induce intelligibility. Accordingly, in the theatre this customary level may be considered comfortable from the standpoint of articulation and naturalness. Our observations in practice check this conclusion.
•Electrical Research Products Inc., New York. N. Y.
'Wolf. S. K. and Sette, W. J.: "Factors Governing Power Capacity of Sound Reproducing Equipment in Theatres," Journal Society Motion Picture Engineers, 15 (1930), No. 4. p. 4 15.
2Fletcher. H.: "Speech and Hearing," 1st Ed., D. Van Nostrand, New York (1929). p. 272.
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