On the Pitch of Periodic Pulses

Subjects adjusted the frequency of one periodic pulse train to match the pitch of another train fixed in frequency. Two modes of pitch perception are found. In the first mode, for pulse rates less than 100 pps, the pulse trains are ascribed a pitch equal to the number of pulses per second, regardless of the polarity pattern of the pulses. In the second mode, for fundamental frequencies in excess of 200 cps, the sounds are assigned a pitch equal to the fundamental frequency. Between these frequency regions a mode transition occurs in which the pitch judgments generally fall between the pulse-rate and fundamental-frequency values. Amplitude and phase spectra are computed for the stimuli. The stimuli are studied on an electrical analog of the basilar membrane. Waveforms of membrane displacement and first spatial derivative of displacement are obtained from the analog. An effort is made to correlate the psychophysical results with the displacement and derivative patterns observed on the analog membrane. The two pitch modes are found to be manifested in the mechanical operation of the cochlea.