Auditory filter shapes derived in simultaneous and forward masking

Abstract

In this paper we describe a method for comparing frequency selectivity in simultaneous and forward masking. The method is designed to eliminate off-frequency listening, which may have had a confounding influence in earlier studies. Thresholds for 1-kHz sinusoidal signals were measured as a function of the width of a spectral notch, centered at 1 kHz, a noise masker. In experiment I thresholds were measured in forward masking for signal durations of 5, 15, and 45 ms, with a noise spectrum level of 40 dB SPL/Hz. Thresholds decreased with increasing notch width for all signal durations. However, the change was more gradual at longer signal durations. This is consistent with a model in which the process of decay of masking follows the auditory filter. For each signal duration, threshold was also measured as a function of the level of a noise without a spectral notch. Each signal threshold for the notch-noise condition was then expressed as the level of a flat-spectrum noise which would give the same masking. When transformed in this way, the data for the three signal durations coincide, and can be interpreted in terms of the same auditory filter. In experiment II the 5-ms signals, whose level was fixed, followed the masker with delays of 5, 15, or 25 ms. The noise spectrum level was varied to find threshold. The change in threshold with notch width was independent of signal delay, confirming that the process of decay of masking follows the auditory filter. Experiment III showed that short-term spectral changes produced by differences in the shapes of the envelopes of signal and masker did not influence the results. In experiment IV thresholds for a 5-ms signal were measured in simultaneous and forward masking, both for a fixed noise level and for a fixed signal level. The derived auditory filters are similar for the two types of measure for each type of masking. However, the auditory filters derived in forward masking have bandwidths 17% smaller and slopes 48% greater than those in simultaneous masking. The differences between simultaneous and forward masking are interpreted in terms of suppression.