Cylinder wall thickness difference discrimination by an echolocating Atlantic bottlenose dolphin

Summary The capability of an Atlantic bottlenose dolphin Tursiops truncatus to discriminate wall thickness differences of hollow cylinders by echolocation was studied. A standard cylinder of 6.35 mm wall thickness was compared with cylinders having wall thicknesses that differed from the standard by ± 0.2, ± 0.3, ± 0.4, and ± 0.8 mm. All cylinders had an O.D. of 37.85 mm, and a length of 12.7 cm. The dolphin was required to station in a hoop while the standard and comparison targets, separated by an angle of ± 11° from a center line, were simultaneously presented at a range of 8 m. The dolphin was required to echolocate and indicate the side of the standard target. Target location on each trial was randomized. Interpolation of the dolphin performance data indicated a wall thickness discrimination threshold (at the 75% correct response level) of –0.23 mm and +0.27 mm. Backscatter measurements suggest that if the dolphin used time domain echo cues, it may be able to detect time differences between two echo highlights to within approximately ± 500 ns. If frequency domain cues were used, the dolphin may be able to detect frequency shifts as small as 3 kHz in a broadband echo having a center frequency of approximately 110 kHz. Finally, if the dolphin used time-separation pitch (TSP) cues, it may be able to detect TSP differences of approximately 450 Hz.Discrimination tests with the thinner comparison targets were also conducted in the presence of broadband masking noise. For an echo energy-to-noise ratio of 19 dB the dolphin's performance was comparable to its noise-free performance. At an energy-to-noise ratio of 14 dB the dolphin was unable to achieve the 75% correct threshold with any of the comparison targets.Abbreviations c sound velocity - DI_R receive directivity index - difference between highlight intervals of two targets - th wall thickness difference between standard and comparison targets; - E energy flux density - E_e echo energy flux density - E_e/N_L echo energy to noise ratio - E(f) frequency spectrum of artificial echo - e(t) artificial echo - N_J ambient noise spectral density - N_L received noise spectral density - O.D. outer diameter - p instantaneous acoustic pressure - R target range - SE source energy flux density in dB - s(t) dolphin sonar signal - time between first and second echo highlights - TS_E target strength based on energy - TSP time-separation pitch