Frequency selectivity in amplitude-modulation processing

Abstract

Effects of frequency selectivity in the audiofrequency domain are well established and associated with the existence of independent frequency channels (critical bands). Several studies suggested a comparable spectral decomposition in the modulation-frequency domain. However, in these studies the bandwidth and the shape of the postulated channels were derived only indirectly and for a restricted range of low-frequency modulations. A principal problem in modulation masking experiments is the restricted dynamic range of the modulation depth available to detect the test modulation. Particularly, in conditions with a stochastic carrier and a stochastic masker-modulation, two basic factors limit resolution: The amplitude statistics of the bandlimited masker-modulation noise and the intrinsic envelope fluctuations of the carrier noise. The present study offers an optimized experimental condition for the derivation of modulation-filter shapes. A direct approach to obtain the modulation-filter shape is presented and modulation-tuning curves are derived up to modulation rates of 100–160 Hz for different spectral positions of the carrier. In addition, across-frequency effects in modulation masking with the probe and the masker separated by about two octaves are examined. The experimental results are compared with threshold estimates based on the integrated modulation power in a hypothetical modulation filter.