Dolphin echolocation signals measured at extreme off-axis angles: Insights to sound propagation in the head.

Abstract

Echolocation signals radiated along the beam axis of an Atlantic bottlenose dolphin resemble single transient-like oscillations. As the azimuth of the measuring hydrophones in the horizontal plane progressively increases with respect to the beam axis the signals become progressively distorted. At approximately ± 45 deg, the signals begin to divide into two components with the time difference between the components increasing with increasing angles out ± 90 deg. The time difference between the two pulses measured by the hydrophone on the right side of the dolphin’s head is on the average approximately 10 μs larger than the time differences observed by the hydrophone on the left side of the dolphin’s head. The center frequency of the first pulse is generally lower by 33 to 47 kHz than the center frequency of the second pulse. When considering the relative locations of the two phonic lips, the data suggest that the signals are being produced by one of the phonic lips and the second pulse resulting from a reflection within the head of the animal. The data also indicate that the process of generating echolocation signals is a complex one and the exact mechanisms are not yet known.