The dynamics of the bottlenose dolphin sonar beam.

Abstract

Much research on dolphin echolocation has focused on animals that have been trained to remain stationary or to carry a device that allows the animals to move but restricts the location of the sonar beam. In such cases, a small number of hydrophones measures sonar characteristics while dolphins solve echolocation tasks. As a result, much is known about the beam axis but relatively little is known about other parts of the beam. One reason for this disparity is that it has been difficult to interpret the results from off axis measurements using a small number of hydrophones (that may or may not sample simultaneously). In this paper, we report results from a system of 47 hydrophones in a 0.75 × 0.75 m2 matrix that allowed measurements to be made at multiple locations in the beam simultaneously, with a sample rate of 1 Msample/s. The system both visualizes and records echolocation clicks in real time across the whole cross section of the beam, hence allowing the full dynamics of the sonar beam to be revealed. As a demonstration of the system's utility, we present results obtained with the system to assess the acoustic properties of clicks produced spontaneously by free-swimming dolphins.