Rectangularly pulsed, 800-MHz microwaves were coupled via waveguide from a 500-W source to the parietal area of the head of normal human observers (Os).
Pulse widths from 5 to 150 μS and pulse-repetition rates (PRRs) from 50 to 20,000 pulses per second (pps) were employed.
Sine-wave audio-frequency (AF) signals could be presented alternately to or concurrently with microwave pulses (RF signal) under conditions in which O could adjust the amplitude, frequency and phase of the AF signal.
By matching timbre and loudness of the perceived RF and AF signals during a succession of psychophysical measures—some while O’s head was being immersed in water—the Os yielded the following results:
- (1) Both loudness and perceptual thresholds of the RF signal were biphasic functions of pulse width and of PRR;
- (2) When pulse widths increased toward 100 μs, some subjects perceived a different sound that was lower in pitch and was referred externally to the head;
- (3) By appropriate phasing of AF and RF signals after matching for pitch and timbre, loudness of the RF signal could be reduced below the threshold of perception; and
- (4) Extent of immersion of the head in water was correlated with reduced loudness of the RF signal. Some of the data are interpreted as posing explanatory difficulties for an exclusively ther-moelastic mechanism of RF hearing.