Reconciling frequency selectivity and phase effects in masking

The effects of auditory frequency selectivity and phase response on masking were studied using harmonic tone complex maskers with a 100-Hz fundamental frequency. Positive and negative Schroeder-phase complexes (m(+) and m(-)) , were used as maskers and the signal was a long-duration sinusoid. In the first experiment, thresholds for signal frequencies of I and 4 kHz were mea sured as a function of masker bandwidth and number of components. A large d ifference in thresholds between the m+ and m- complexes was found only when masker components were presented ipsilateral to the signal over a frequenc y range wider than the traditional critical band, regardless of the absolut e number of components. In the second experiment, frequency selectivity was measured in harmonic tone complexes with fixed or random phases as well as in noise, using a variant of the notched-noise method with a fixed masker level. The data showed that frequency selectivity is not affected by masker type, indicating that the wide listening bandwidth suggested by the first experiment cannot be ascribed to broader effective filters in complex-tone maskers than in noise maskers. The third experiment employed a novel method of measuring frequency selectivity, which has the advantage that the overa ll level at the input and the output of the auditory filter remains roughly constant across all conditions. The auditory filter bandwidth measured usi ng this method was wider than that measured in the second experiment, but m ay still be an underestimate, due to the effects of off-frequency listening . The data were modeled using a single-channel model with various initial f ilters. The main findings from the simulations were: (1) the magnitude resp onse of the Gammatone filter is too narrow to account for the phase effects observed in the data; (2) none of the other filters currently used in audi tory models can account for both frequency selectivity and phase effects in masking; (3) the Gammachirp filter can be made to provide a good account o f the data by altering its phase response. The final conclusion suggests th at masker phase effects can be accounted for with a single-channel model, w hile still remaining consistent with measures of frequency selectivity: eff ects that appear to involve broadband processing do not necessarily require across-channel mechanisms. (C) 2001 Acoustical Society of America.