The loudness of four‐tone complexes centered at 250, 2000, and 4000 cps was measured as a function of the over‐all spacing, ΔF, of the components, both in the quiet and against various levels of a uniform masking noise. When the masking noise was held at a constant level, the loudness of the complex increased more with ΔF at moderate sensation levels—between about 30 and 60 db—than at either higher or lower levels. Near the masked as well as the absolute threshold, the loudness decreased as ΔF was increased beyond the critical bandwidth. Only when ΔF was less than a critical band, was loudness independent of ΔF and was the amount of loudness summation invariant with level. These results support the hypothesis that the amount of loudness summation depends upon the slope of the loudness functions for the individual critical bands that form the complex.

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