No. 120: Nov-Dec 1998
When we (and all eyed animals) see an object, we are detecting light reflected from that object. When underwater, though, our vision is limited because light does not travel far. Sound, however, does; and sound is reflected from objects just as light is. This is of course the basis of underwater sonar, in which a sound source replaces the sun or a diver's floodlight. But even without an active sound source, the ocean is full of sound. Waves, rain, and the sounds made by marine animals create a background of noise that "illuminates" objects, not directly, but from the environment in general. Using only this enveloping background sound, it is possible to create acoustical images of objects. "Vision" of this sort is equivalent to "facial vision" in blind humans, who can hear objects using the environmental sound reflected from them.
J. Potter and his colleagues at the National University of Singapore have constructed an array of underwater microphones that detects "slices" of the acoustical environment around it. When processed by a computer, images of objects emerge by virtue of the background noise reflected from them.
This group has also estimated the ability of dolphins to detect and process background noise. They suggest that dolphins should be able to "see" objects at least 25 feet away without even using their active sonar; that is, their clicks. This passive acoustical imaging would be a useful evolutionary development because dolphin clicks warn some prey and allow them to escape.
(Anonymous; "Cacophony of the Deep," Discover, 19:19, May 1998.)
Comments. Some insects can detect the sonar cries of pursuing bats and take evasive action. Perhaps some fish can detect pursuing dolphins, too.
Blind people can augment facial vision by tapping with a cane or using a mechanical clicker.