The threshold for a sinusoidal signal masked by a band of noise is often times lower when the masking noise is modulated than when it is unmodulated. The difference in masked thresholds is referred to as the modulated-unmodulated difference, or MUD. These present experiments examined the effects of masker bandwidth, masker rate, and masker level on the MUD at several different signal frequencies. The MUD generally increased with increasing masker bandwidth; for masker bandwidths wider than a critical band (or an equivalent rectangular bandwidth-ERB), the results may be influenced by across-channel processes underlying comodulation masking release. The MUD for an ERB masker (MUDERB) was influenced less by masker rate than was the MUD for a broadband (BB) masker (MUDBB). The MUDERB and especially the MUDBB increased significantly with increasing masker level when the modulated masker was modulated at a depth (m) of 1.0, but not when it was modulated at a depth of 0.75. These results have significant implications for extending the MUD paradigm to hearing-impaired subjects. Finally, the MUDERB and the MUDBB increased with increasing signal frequency. This effect for the ERB masker is largely (if not completely) due to the wider absolute bandwidths at higher frequencies. The effect with the BB masker may be influenced by differences in the magnitude of suppression across frequency.
It is possible to estimate temporal resolution at discrete spectral locations by subtracting the masked threshold produced by a modulated masker from that produced by an unmodulated masker (the difference is referred to as the modulated-unmodulated difference, or MUD). This paradigm may be especially useful for measuring temporal resolution in subjects with hearing loss, provided that the MUD is independent of level. The purpose of the present study was to examine the MUD as a function of masker level at several signal frequencies. In the first experiment, the sinusoidally amplitude-modulated masker had a depth (m) of 1.0. The MUD increased by as much as 15 dB as the spectrum level of the masker increased from 0 to 40 dB SPL. In the second experiment, the modulated masker had a depth of 0.75 or 1.0. When the masker depth was 1.0, the MUD increased with increasing masker level, as in experiment one. When it was 0.75, however, the MUD — though reduced — was essentially independent of masker level. These results suggest that a masker depth of 0.75 may be used to compare temporal resolution between normal-hearing and hearing-impaired subjects without being complicated by effects of masker level. [Work supported by NIDCD.]
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