On the upper cutoff frequency of the auditory critical-band envelope detectors in the context of speech perception

Studies in neurophysiology and in psychophysics provide evidence for the existence of temporal integration mechanisms in the auditory system. These auditory mechanisms may be viewed as "detectors," parametrized by their cutoff frequencies. There is an interest in quantifying those cutoff frequencies by direct psychophysical measurement, in particular for tasks that are related to speech perception. In this study, the inherent difficulties in synthesizing speech signals with prescribed temporal envelope bandwidth at the output of the listener's cochlea have been identified. In order to circumvent these difficulties, a dichotic synthesis technique is suggested with interleaving critical-band envelopes. This technique is capable of producing signals which generate cochlear temporal envelopes with prescribed bandwidth. Moreover, for unsmoothed envelopes, the synthetic signal is perceptually indistinguishable from the original. With this technique established, psychophysical experiments have been conducted to quantify the upper cutoff frequency of the auditory critical-band envelope detectors at threshold, using high-quality, wideband speech signals (bandwidth of 7 kHz) as test stimuli. These experiments show that in order to preserve speech quality (i.e., for inaudible distortions), the minimum bandwidth of the envelope information for a given auditory channel is considerably smaller than a critical-band bandwidth (roughly one-half of one critical band). Difficulties encountered in using the dichotic synthesis technique to measure the cutoff frequencies relevant to intelligibility of speech signals with fair quality levels (e.g., above MOS level 3) are also discussed.