Downward Spread of Masking

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Abstract
Pulsed pure-tone audiograms were obtained in quiet and in the presence of narrow bands of noise centered at 500, 1000, 2000, and 4000 cps and at several levels. The masking produced at lower frequencies was of particular interest: the bands of noise at about 75 db/cy produced from 12 to 24 db of masking at 100 cps and from 25 to 40 db of masking at frequencies between about 200 cps and lower effective limit of the band of noise. This phenomenon contrasts with the fact that a single pure tone of comparable frequency and intensity does not produce appreciable masking at these lower frequencies. It is hypothesized that a band of noise is functionally somewhat equivalent to a bunch of pure tones which when passed through a nonlinear transducer (the ear), produce aural difference tones which elevate the threshold for signals of lower frequency, while a single tone masker would not have this effect. It was found that two pure tones at a spectrum level and at frequencies comparable with the bands of noise produce aural difference tones (as measured by best beats) of a level which correspond well with the level of lower frequency masking effects in question.

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