Modulation masking produced by beating modulators

This study examined whether "modulation masking" could be produced by tempo ral similarity of the probe and masker envelopes, even when the masker enve lope did not contain a spectral component close to the probe frequency. Bot h masker and probe amplitude modulation were applied to a single 4-kHz sinu soidal or narrow-band noise carrier with a level of 70 dB SPL. The threshol d for detecting 5-Hz probe modulation was affected by the presence of a pai r of masker modulators beating at a 5-Hz rate (40 and 45 Hz, 50 and 55 Hz, or 60 and 65 Hz). The threshold was dependent on the phase of the probe mod ulation relative to the beat cycle of the masker modulators; the threshold elevation was greatest (12-15 dB for the sinusoidal carrier and 9-11 dB for the noise carrier, expressed as 20 log m) when the peak amplitude of the p robe modulation coincided with a peak in the beat cycle. The maximum thresh old elevation of the 5-Hz probe produced by the beating masker modulators w as 7-12 dB greater than that produced by the individual components of the m asker modulators. The threshold elevation produced by the beating masker mo dulators was 2-10 dB greater for 5-Hz probe modulation than for 3- or 7-Hz probe modulation. These results cannot be explained in terms of the spectra of the envelopes of the stimuli, as the beating masker modulators did not produce a 5-Hz component in the spectra of the envelopes. The threshold for detecting 5-Hz probe modulation in the presence of 5-Hz masker modulation varied with the relative phase of the probe and masker modulation. The patt ern of results was similar to that found with the beating two-component mod ulators, except that thresholds were highest when the masker and probe were 180 degrees out of phase. The results are consistent with the idea that no nlinearities within the auditory system introduce distortion in the interna l representation of the envelopes of the stimuli. In the case of two-compon ent beating modulators, a weak component is introduced at the beat rate, an d it has an amplitude minimum when the beat cycle is at its maximum. The re sults could be fitted well using two models, one based on the concept of a sliding temporal integrator and one based on the concept of a modulation fi lter bank. (C) 1999 Acoustical Society of America. [S0001-4966(99)03408-6].