The acoustic field on the forehead of echolocating Atlantic bottlenose dolphins (Tursiops truncatus)

Author Posting. © Acoustical Society of America, 2010. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 128 (2010): 1426-1434, doi:10.1121/1.3372643.Arrays of up to six broadband suction cup hydrophones were placed on the forehead of two bottlenose dolphins to determine the location where the beam axis emerges and to examine how signals in the acoustic near-field relate to signals in the far-field. Four different array geometries were used; a linear one with hydrophones arranged along the midline of the forehead, and two around the front of the melon at 1.4 and 4.2 cm above the rostrum insertion, and one across the melon in certain locations not measured by other configurations. The beam axis was found to be close to the midline of the melon, approximately 5.4 cm above the rostrum insert for both animals. The signal path coincided with the low-density, low-velocity core of the melon; however, the data suggest that the signals are focused mainly by the air sacs. Slight asymmetry in the signals were found with higher amplitudes on the right side of the forehead. Although the signal waveform measured on the melon appeared distorted, when they are mathematically summed in the far-field, taking into account the relative time of arrival of the signals, the resultant waveform matched that measured by the hydrophone located at 1 m.This work was supported by the U.S. Office of Naval Research

Frequency-dependent echolocation beam pattern of the bottlenose dolphin.

Moore and others (2008) previously showed that bottlenose dolphins are capable of beam steering and controlling the vertical and horizontal widths of the echolocation beam. A follow-on study was performed using the same methods as the previous study, but with a younger animal and a higher resolution diamond-shaped hydrophone array for characterizing the beam. The dolphin performed a target detection task while stationed on a biteplate with targets placed up to 34 deg to either the left or right of the dolphin's longitudinal axis. The dolphin was capable of beam steering more than 28 deg to either side, which is a greater capability than previously reported and which exceeded the geometric coverage of the array. Frequency band-limited beam patterns suggested the presence of two beams, spatially separate from one another and which corresponded to higher and lower frequency energies. The finding is consistent with prior anatomical and acoustic evidence of two echolocation click sound sources in the delphinids. In addition, at low frequencies, a local minimum was observed at the center of the beam. The functional significance of the local minimum, if any, is unknown