EFFECT OF MASKER LEVEL ON OVERSHOOT IN RUNNING-NOISE AND FROZEN-NOISEMASKERS

Masked thresholds were measured with running- and frozen-noise maskers . The 5-kHz signal was 2 ms in duration. The masker was low-pass noise (20 Hz-10 kHz); its total duration was 300 ms. The overall level of t he masker was 30, 50, or 70 dB SPL. The onset of the signal was delaye d by 0, 3, 8, 18, 198, or 278 ms relative to the onset of the masker. In all frozen-noise measurements, the signal was added to the same fin e structure of the noise. Overshoot in frozen noise was measured for t wo starting phases of the signal that led to a 10-dB difference for la rge signal-onset delays. In all three configurations (running noise an d frozen noise with two different signal phases) masker level had a si milar influence on overshoot. At the intermediate masker level (50 dB SPL), a significant amount of overshoot (up to 15 dB) was observed in all three conditions. At the low and the high masker levels, overshoot was very much reduced, and even became negative in most conditions fo r the 30-dB-SPL masker. For the 50-dB frozen-noise masker, the total v ariation of thresholds with signal phase was 8 to 11 dB for long signa l-onset delays, but only 3 to 6 dB for short delays. For the low- and high-level maskers, where only a small overshoot was observed, the thr eshold variation with phase for a signal at masker onset was the same as that for the long-delay condition. An explanation for the variation of signal detectability with masker level is proposed that refers exp licitly to the compressive input-output characteristic of the basilar membrane at intermediate levels.