Modulation masking within and across carriers for subjects with normal and impaired hearing.

The detection of amplitude modulation (AM) of a carrier can be impaired by additional (masker) AM applied to the same carrier (within-carrier modulation masking, MM) or to a different carrier (across-carrier MM). These two types of MM were compared for young normal-hearing and older hearing-impaired subjects. The signal was 4- or 16-Hz sinusoidal AM of a 4000-Hz carrier. Masker AM with depth 0.4 was applied either to the same carrier or to a carrier at 3179 or 2518 Hz. The masker AM rate was 0.25, 0.5, 1, 2, or 4 times the signal rate. The signal AM depth was varied adaptively to determine the threshold. Both within-carrier and across-carrier MM patterns were similar for the two groups, suggesting that the hypothetical modulation filters are not affected by hearing loss or age. The signal AM detection thresholds were also similar for the two groups. Thresholds in the absence of masker AM were lower (better) for the older hearing-impaired than for the young normal-hearing subjects. Since the masked modulation thresholds were similar for the two groups, it seems unlikely that abnormal MM contributes to the difficulties experienced by older hearing-impaired people in understanding speech in background sounds.