Transmitting beam patterns of an echolocating bottlenose dolphin

Abstract

Measurements on a free-swimming subject, echlocating under a simple go paradigm, were conducted at SSC San Diego, with a linear array of seven simultaneously and individually processed wideband hydrophones (response to 400 kHz), arraigned either vertically or horizontally, and centered on a small underwater video camera, with the video output synchronized with the recording of acoustic data. The measurement apparatus itself served as the test target. Lowering it into the water provided the cue for the blindfolded subject to locate its position in the test pen, swim to the apparatus, and touch it with the tip of its rostrum; whereupon the trainer provided a bridge signal, indicating reward due. During this process, acoustic beampattern measurements were made on echolocation clicks, as a function of frequency, which support previous, non-simultaneous, statistical beampattern measurements. Early in the experiment, the subject emitted echolocation clicks that peaked at 78 kHz, with −3 dB beamwidths of 8 to 10 degrees; but also containing high frequency components (above 135 kHz), with beamwidths that narrowed with increasing frequency, ranging to only a few degrees around 300 kHz, far beyond the subjects hearing range. After several days, the task appeared to become easier and there was much less high frequency content, but the click repetition frequency increased. Wideband noise was then introduced into the test pen, at frequencies below 135 kHz; and as the task became more difficult, the subject resumed transmitting clicks with high frequency components, although the peak frequency remained at 78 kHz, and click rates again increased, to much higher values.