Growth of forward masking for sinusoidal and noise maskers as a function of signal delay; implications for suppression in noise

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Abstract
The first 2 experiments were designed to determine whether mutual suppression in broadband noise increases in strength with increasing overall level. In experiment 1, masking functions (signal threshold vs. masker level) were measured [in humans] in forward masking as a function of the delay time of a 10-ms signal, both for a broadband noise masker (low-pass filtered at 8 kHz) and for sinusoidal maskers, 1, 2 and 4 kHz. In the latter case the signal frequency equaled the masker frequency. For short signal delays the masking functions were steeper for the sinusoidal masker than for the noise masker. At longer delays the slopes for both masker types decreased and the slopes for the 2 masker types became more nearly equal. In experiment 2, the effect of gating a low-level noise cue was investigated with the sinusoidal masker. At the longer signal delays the masking functions had equal slopes for the broadband noise masker and the sinusoidal masker with cue. At short signal delays the masking functions for sinusoidal maskers may be artificially steepened, since the subject lacks an effective cue to distinguish the signal from the masker. The equal slopes at longer delays indicate that mutual suppression of the components within a broadband noise does not increase in strength with increasing overall level. In experiment 3, the magnitude of mutual suppression in a broadband noise was estimated by comparing masking functions for a broadband noise and for a noise whose bandwidth was 20% of the center frequency. The suppression was estimated to be .apprx. 2 dB at 4 kHz and 8 dB at 2 kHz. A simple mathematical expression, suggested by Jesteadt et al., (1982) gave an accurate description of the amount of masking produced by the broadband masker as a function of masker level and signal delay.

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