Peak sound pressure level and spectral frequency distributions in echolocation pulses of Atlantic bottlenose dolphins, Tursiops truncatus

We investigated the relationship between peak sound pressure level (PPL) and the shape of the spectral frequency distribution of echolocation pulses emitted by Atlantic bottlenose dolphins (Tursiops truncatus) in a target detection task. Two dolphins were trained to station on a bite plate and tail rest, echolocate, and report target condition (present or absent). A microprocessor data acquisition system [M. G. Ceruti and W. W. L. Au, J. Acoust. Soc. Am. 73, 1390–1392 (1983)], was used to collect amplitude and frequency data for each pulse and monitor subjects' responses. Ten sessions of 50 trials each (25 target present and 25 target absent, presented randomly) were chosen for each subject from sessions conducted during Nov. 1982. Pulses were classified as “bimodal” or “unimodal,” depending on the number of maxima in the frequency spectrum between 30 and 135 kHz. For bimodal pulses, the degree of bimodality BI, also was measured by summing the maximum intensities using the intervening minimum as a baseline. The results collapsed across subjects yielded 15 670 bimodal and 12 383 unimodel pulses. The mean PPL for bimodal pulses was 207 dB re:1 μPa and for unimodal pulses, 197 dB. The Pearson product moment correlation coefficient between BI and PPL was 0.69. Peaks in the averaged bimodal spectrum occurred at 60 and 135 kHz or beyond, while the averaged unimodal spectrum was peaked at 120 kHz. These findings suggest that bimodal frequency distributions during high-pressure echolocation represents a physiological process in the pulse emission system.