Spread of excitation produced by maskers with damped and ramped envelopes

Abstract

Three experiments compared the ability of pairs of modulated narrow-band sounds with identical power spectra to mask signals above, below, and at their center frequency. In the first experiment, forward-masked psychophysical tuning curves (PTCs) were measured using a 1600-Hz 10-ms signal and 400-ms ‘‘damped’’ and ‘‘ramped’’ sinusoidal maskers. The damped maskers were produced by repeating a 25-ms sinusoid which had been shaped by an exponentially decaying function with a half-life of 4 ms. The ramped maskers were simply time reversals of the damped maskers. When the masker frequency was below that of the signal, ramped maskers were more effective than damped maskers, whereas the reverse was true for maskers above the signal. Experiment 2 measured thresholds for a 400-ms 1000-Hz damped or ramped sinusoidal signal, in the presence of a simultaneous masker with the same envelope shape, as a function of the masker frequency. Again, ramped maskers were more effective below the signal frequency, and damped maskers more effective above it. Experiment 3 was similar to experiment 2 except that the masker was a damped or ramped narrow-band noise, and the signal was an unmodulated sinusoid. The pattern of results was generally similar to that obtained in experiments 1 and 2. The results of all three experiments are inconsistent with the ‘‘traditional’’ method of calculating excitation patterns, which is based on the power spectrum of the stimulus, and also differ from the predictions of a recent model of auditory processing [R. D. Patterson, J. Acoust. Soc. Am. 96, 1409–1418 (1994)]. It is suggested that changes to the peripheral filtering stage of Patterson’s model may provide a useful first step toward accounting for the data.