Detection of amplitude modulation and discrimination of interaural differences in modulation phase for high-frequency carriers

Abstract

Using a 4-kHz pure-tone carrier and modulation frequencies (fm) of 2–512 Hz, thresholds for monaural detection of sinusoidal amplitude modulation (SAM) were measured, as were threshold modulation depths for the discrimination of interaurally in-phase and out-of-phase SAM envelopes. For one listener, interaural differences in envelope phase for fm up to 256 Hz were discriminable at modulation depths (m) for which the monaural modulation itself was just detectable, and all monaural and binaural thresholds were within a range of 5 dB (20 log m) across fm. For a second listener, binaural thresholds increased more rapidly with increasing fm than monaural thresholds. Another experiment examined the notion that interaurally delayed SAM envelopes are processed as time-varying interaural level differences. In this task, listeners discriminated interaurally in-phase SAM envelopes from SAM envelopes carrying a fixed interaural phase difference (IPD). Threshold m was measured for several fm and IPD combinations. For a given IPD, thresholds were either constant or decreased with increasing fm. The data suggest that dynamically varying interaural level differences are the cue to discrimination at low fm, while the fixed interaural time difference is the likely cue at high fm. [Work supported by NIDCD DC00683.]