Abstract:

Frequency selectivity capabilities of an Atlantic bottlenose dolphin were examined by calculating critical ratios from masked hearing data. Absolute sensitivity to pure tones from 40--140 kHz was measured as a base line, and masked sensitivity was determined for the same signals masked by three levels of white noise (52, 42, and 32 dB re:1 (mu) Pa[sup 2]/Hz). Absolute and masked sensitivity were essentially constant between 40 and 120 kHz at each of the masking conditions. Sensitivity decreased approximately 100 dB per octave between 120 and 140 kHz. Critical ratios averaged across noise levels were constant between 40 and 120 kHz, averaging 26 dB. The critical ratio at 140 kHz was 42 dB. This flat trend in critical ratios between 40--120 kHz does not agree with the constant-Q filter-bank models used to account for earlier critical ratio and bandwidth measurements for the species [W. W. L. Au and P. W. B. Moore, J. Acoust. Soc. Am. 88, 1635--1638 (1990)], but comparison to other cetacean masked hearing work [Johnson et al., J. Acoust. Soc. Am. 85, 2651--2654 (1989)] indicates that the trend has been recorded before, but not explicitly reported.