Abstract
The manner in which the auditory system encodes the “shape” of the amplitude envelope of AM signals was examined. On the basis of modulation frequency tuned channels previously demonstrated for FM signals [R. H. Kay and D. R. Matthews, J. Physiol. (Lond.) 225, 657–677 (1972)] it was thought the auditory system might be responding to the power spectrum of the amplitude envelope. A 500‐Hz carrier was amplitude modulated at 2, 4, or 8‐Hz with either a sine wave or square‐wave signal. The Method of Adjustment was used to determine the “modulation thresholds” for the 6 frequency by waveform conditions. At each of the modulation frequencies, subjects' thresholds were lower for square wave than for sine wave modulation. The mean ratio between the thresholds for the two wave forms was 1.32 with a SD of 0.22, which approximates 4/π (1.27) as would be predicted on the assumption that the auditory system performs a Fourier analysis on the amplitude envelope of the modulated 500‐Hz tone. Additional data suggest that the two waveforms are indiscriminable until the third harmonic of the square wave reaches its independent threshold. Some implications for speech perception are considered. [Work supported by Penn. State Department of Psychology for partial fulfillment of master's degree.]
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