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ACOUSTIC POWER LEVELS
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sumably a loudness balance throughout the audible range is desirable. Evidently in any instance in the chain from studio to theatre where there is a limit to the power possible to handle, the low notes will probably be the first to give trouble as they require greater power for equal loudness. However, it must be kept in mind that the presence of many partial tones in most signals make impossible the application of these curves to do more than to indicate tendencies.
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FREQUENCY
Fig. 1 . Variation with Frequency of Loudness and Sensation Levels for Flux of 4X10 2 Microwatts per Square Centimeter.
Characteristics of the Electrical System — Amplifiers
The amount of power possible from sound reproducing equipment will depend on the amplifiers incorporated into the apparatus and the efficiency of the loud speakers. We must determine the maximum output power without distortion in quality. The distortion in which we are interested in this discussion is caused by requiring amplifiers to work at overload conditions and is detectable as a certain fuzziness and rattle in the reproduced sounds. Overload involves a non-linear amplification of signals and has the effect of introducing frequencies which may not be contained in the input.6 The effect is to be avoided as it reduces articulation and naturalness of speech and tends to destroy quality of music, the degradation in quality being dependent upon the frequency distribution of the extraneous signal.7 We conclude that amplifying equipment should be designed to supply the necessary power without encroaching on the non-linear part of the amplification curve.
There are several practiced methods of rating amplifiers. Those made by the Western Electric Company are rated by their electric
'Willis. F.C. and Melhuish. L. E.: "Load Carrying Capacity of Amplifiers. " Bell System Technical Journal, 5 (1926). No. 4. p. 5 73. 7Steinberg. loc. cit.