Additivity of simultaneous masking, revisited

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Abstract
Lutfi compared simultaneous masking functions (signal threshold vs. masker level) for individual sinusoidal and narrow-band noise maskers, and for those maskers presented in pairs. Lutfi found that the pairs of maskers produced 10-17 dB excess masking over that predicted from the linear sum of their individual masking and explained the results in terms of a model in which the effects of the maskers are summed after undergoing independent compressive transformations. Lutfi''s results may have been influenced by 2 factors: combination-product detection, and the use of different detection cues for single maskers and for pairs of maskers. When the stimulus conditions were chosen so as to minimize the likelihood of combination-product detection, excess masking was only 3-5 dB. For a single narrow-band noise masker, subjects make use of the relatively slow envelope fluctuations to enhance performence. When 2 independent narrow-band noise maskers are added, the effectiveness of this cue is reduced, and between 3 and 9 dB of excess masking occurs. When the 2 noises are derived from the same source, and have correlated envelope fluctations, no excess masking occurs. Lufti''s compressive-nonlinearity model clearly fails in some situations.

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