Critical ratio and bandwidth of the Atlantic bottlenose dolphin ( Tursiops truncatus )

Masked underwater pure-tone thresholds were obtained for an Atlantic bottlenose dolphin using an “up-down staircase” method of stimulus presentation and a “go/no-go” response procedure. Broadband noise at two levels was used to measure the animal's critical ratio at test frequencies of 30, 60, 90, 100, 110, 120, and 140 kHz. For frequencies of 100 kHz and below, the critical ratios were similar to those measured by Johnson [J. Acoust. Soc. Am. 44, 965–967 (1968)]. The results indicate a sharp increase in the critical ratio to 51 dB at 110 kHz followed by a slight decline to 46 dB at 120 kHz. Different bandwidths of noise with very sharp cutoffs were used to measure the dolphin's critical bandwidth at test frequencies of 30, 60, and 120 kHz. The critical bandwidth was approximately 10 times (10 dB) wider than the critical ratio at 30 kHz and 8 times wider (9 dB) at 60 kHz. At 120 kHz, the critical bandwidth was approximately the same as the critical ratio.