Introduction

Dolphins are able to investigate their environment through echolocation in conditions where vision is limited (eg turbid water or at night). Experiments conducted at the Kewalo Basin Marine Mammal Laboratory in Hawaii and Ocean Park in Hong Kong have shown that object shape can directly perceived through echolocation in a holistic manner, similar to the direct shape perception of shape through vision. No man-made sonar of comparable bandwidth can perform this feat. Dolphins produce echolocation clicks with a frequency range of up to 160 kHz. These clicks are broadband clicks with durations of 40-70 microseconds. When a dolphin inspects an object through its echolocation sense it send out a rapid train of clicks with inter-click-intervals as low as 1.2 milliseconds. The object in questions is ensonified and the reflections are perceived by the animal.

A typical click train as the animal is investigating an object

At frequencies of 100-150 kHz the acoustic wavelength in water is between 15 to 10 mm, and thus comparable to the diameter of the pipe used to create the objects. The results of the cross-modal experiments demonstrate that the information contained in the reflections must be sufficient to reconstruct the shape of the object in question and this project is trying to investigate the underlying acoustic properties that enable the dolphin to do so. Eight different clicks recorded from the echolocating dolphin are used as a signal in a playback experiment. Here, the signal, reproduced by a signal generator, is played through a transducer that is mounted in the focal point of a parabolic dish, thus creating a plain wave. The object is ensonified and the reflections of the object are recorded with a four-hydrophone array.

Research

This research at ARL investigates the dolphin's ability to recognize shape through echolocation. We perform this research with a 2-pronged approach:

• Experiments with Dolphins

  Experimental work is currently carried out in collaboration with Ocean Park in Hong Kong to investigate the acoustics of Dolphin echolocation. A dolphin has been trained to perform a cross-modal matching task, where the sample was exposed to only once sense (either to the visual or the echolocation sense) and the alternatives (between two and four) were presented to the alternate sensory modality. To expose an object only to the visual sense object were held in air by an experimenter (in air the dolphins echolocation system in functionally absent due to the impedance mismatch between the dolphins head and air). To expose objects to the echolocation sense only objects were suspended inside an anechoic box made out of redwood to reduce reflections from the enclosure and the tank wall. The front was covered with an opaque black Plexiglas, which allowed the echolocation signal of the dolphin to pass through but not light. Thus the only way for the dolphin to inspect the object was to use its echolocation sense.

  
  
  
• Numerical Simulations

  Hypothesis of how the dolphin is able to achieve its echolocation performance can be tested numerically via simulation. To do this, we numerically ensonify an object similar to the one presented to the dolphin and compute its backscattered field. Using this field, we can then compute the signals received at different points and times. These signals can be passed through the hypothesized signal processing techniques and the results can be compared against the dolphin's performance.

More information on this work can be found in our publications section.