Masked pure-tone thresholds of the bottlenosed dolphin (Tursiops truncatus) at extended frequencies

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Abstract
Masked underwater pure-tone thresholds were obtained for a bottlenosed dolphin (Tursiops truncatus) using an “up-down staircase” method of stimulus presentation and a “go/no-go” response procedure. Thresholds were determined at two continuous broadband masking noise levels at test frequencies of 30, 60, 90, 100, 110, 120, and 140 kHz. The Critical Ratio (CR), defined as the ratio of signal power to noise spectrum level at masked threshold, was calculated for both noise levels. A psychometric function relating CR's to frequency showed parallel functions with CR's previously reported for the dolphin [Johnson, J. Acoust. Soc. Am. 44, 965–967 (1968)] for frequencies up to 100 kHz. Results for 30, 60, 90, and 100 kHz were 31, 32, 45, and 40 dB, respectively, supporting previous findings for the dolphin. Results beyond 100 kHz, where no other values had previously been obtained, show a sharp increase at 110 kHz (51 dB) followed by a slight decline at 120 kHz (46 dB). Generally, the CR function suggests that the frequency resolving ability of this echolocator is not optimal at these higher frequencies even though peak energies of the outgoing echolocation pulse have been recorded in this higher range.

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