Receiving beam patterns and directivity indices of the Atlantic bottlenose dolphin T u r s i o p s t r u n c a t u s

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Abstract
The receiving beam patterns of an Atlantic bottlenose dolphin was measured in both the vertical and horizontal planes for frequencies of 30, 60, and 120 kHz. Measurements in the vertical plane were performed by training the dolphin to rotate on its side and station on a vertically oriented biteplate device. A signal source was positioned 3.5 m directly in front of the animal on an arc while the position of a broadband noise source was varied in azimuth along the arc, with the animal stationed at the origin of the arc. Using a go/no‐go response procedure, the masked threshold of the dolphin was determined by varying the intensity of the noise source via a tracking method of stimulus presentation, for different azimuth’s of the noise source. Measurements in the horizontal plane were obtained with the dolphin stationing on a horizontal biteplate. Two masking noise sources, projecting equal levels of uncorrelated noise, were located at ±20° on either side of the arc midpoint. The levels of the noise source were held constant and the masked thresholds determined by varying the level of the signal source while positioned at different azimuths along the arc. Results indicated that maximum sensitivity in the vertical plane (the major axis of the beam) occurred between 5° and 10° above the midline of the animal’s mouth and that the beam patterns were not symmetrical about the major axis. The sensitivity dropped off more quickly with increasing angle above the head than below. The beam patterns in the horizontal plane were directed forward and were fairly symmetrical about the midline of the animal’s body. The beam patterns in both planes became narrower as the frequency increased. Using the beam patterns measured in both planes, the receiving directivity index was calculated to be approximately 10, 15, and 21 dB for frequencies of 30, 60, and 120 kHz, respectively. A two‐element rectangular array was used to model the dolphin’s directivity index with each aperture having a cross‐sectional area of 8.1 cm2. The frequency dependency of the dolphin’s directivity index was found to be very similar to that of the model. Results also indicate that the receiving beam pattern at 120 kHz was slightly broader than the transmitting beam pattern for echolocation clicks with peak frequencies between 110 and 120 kHz.

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