Phase effects in masking related to dispersion in the inner ear. II. Masking period patterns of short targets

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Abstract
This article investigates how the amplitude and phase characteristics of the inner ear influence the spectrotemporal representation of harmonic complex sounds. Five experiments are reported, in each of which three sets of maskers are compared that differ only in their phase spectra. The amplitude spectra of the complexes were flat and the phase choices were (a) zero phase, (b) Schroeder phases with a positive sign, and (c) Schroeder phases with a negative sign. In the first four experiments, the spectra contained all harmonics between 200 and 2000 Hz. In experiments 1 and 2, the signal frequency was fixed at 1100 Hz and the fundamental frequency of the maskers was varied. In experiments 3 and 4, the fundamental frequency of the maskers was fixed and the signal frequency varied between 200 and 2000 Hz. In experiments 1 and 3, the signal duration was long compared to the period of the maskers. In experiments 2 and 4, the signal duration was only 5 ms and thresholds were determined for different time points within the masker’s period. The results show a strong correlation between the minima of the short signal’s thresholds and the threshold of the long signal. In experiment 5, the spectral extent of the masker was shifted to values one octave lower (100 to 1000 Hz) or one or two octaves higher (400 to 4000 Hz and 800 to 8000 Hz, respectively). For each spectral region, masked thresholds of a long signal were obtained for three values of the fundamental frequency. In all five experiments the thresholds depended very much on the specific phase choices with differences of up to 25 dB. The masker with a negative Schroeder phase always led to the highest thresholds. The thresholds of the masker with a positive Schroeder phase, on the other hand, were for a wide range of parameters lower than the thresholds for the zero-phase masker. These phase effects are most likely caused by the phase characteristic of the basilar-membrane filter, which affects the flat envelopes of the two Schroeder-phase maskers in a very different way. For an appropriate choice of parameters, one of the two becomes even more strongly modulated than the zero-phase complex. This latter observation imposes some restrictions on the second derivative (curvature) of the phase-versus-frequency relation for the auditory filters.

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