Acoustics: Seeing with Sound

To the human submariner, the built-in sonar system of the porpoise is an object of particular envy. How does a series of clicks and squeaks enable the graceful swimmer to “see” so well through the murkiest water? Scientists from the Lockheed-California Co. are still searching for the answer. But their research is already pointing toward an extra, nonaquatic dividend—a practical aid for blind people walking on land.

Pulsed Beam. Working in an all but echoless 10-ft. by 13-ft. room lined with sound-absorbing wedges of glass fiber, Lockheed’s scientists have set up a sort of searchlight with a sound generator in its throat. The researcher sits in a chair, covers his eyes with a blindfold and presses a button with his right hand. Out of the searchlight comes a beam of noise, 50 pulses per second, which sounds like a distant chorus of crickets and spring peepers. The mixed frequencies are higher than human ears normally hear, but the researchers have found that they can quickly adjust to the higher pitch.

In his left hand the blindfolded subject holds a tiller by which he can swing the sound beam, searching for test objects—small wires, lengths of pipe, pieces of cloth—hung at random from the chamber’s roof. When the beam hits a target, an echo comes back, and from the character of that echo an experienced listener can tell an amazing amount about the target.

Telltale Echoes. The secret is the mix of frequencies in the sound pulses, a formula that Lockheed copied from the porpoises. Small objects such as wires do not reflect the longer sound waves of the lower frequencies. The echoes that they send back are predominantly high-pitched, and a listener quickly learns to judge target size by the tone of the echo. Once he knows the size of an object, he can tell its distance by the loudness of the echo. Judging a target’s material is a more subtle job, but in general, such hard materials as metal and glass send back a clear buzz, while cloth and other soft surfaces send a mushy return.

Lockheed’s echo-location system may some day have an important impact on antisubmarine warfare, for it shows that the human ear, when working with the proper mixed frequencies, can determine the composition of an unseen object. This could correct a major failing in present sonar systems in which whales are sometimes mistaken for ene my submarines. It may also put the Seeing-Eye dog out of business. Lockheed scientists hope to reduce the sound generator to the size of a flashlight; then the blind may learn to “see” with their skilled ears.