Making 3-D Pictures with Sound

Before long, the operations officer on a U.S. Navy ship may be able to tune in a device that can reproduce a three-dimensional image of an enemy submarine scores of fathoms below the surface. Or a brain surgeon may have at his fingertips the means to see, in 3D, a deep, tiny tumor that even modern X-ray techniques could not detect. Such far-out capabilities are now within reach thanks to Scientists Alexander Metherell, John Dreher, Lewis Laramore and Hussein El-Sum, of the McDonnell Douglas Corp.'s Advanced Research Laboratories at Huntington Beach, Calif. Last week, writing in the Journal of the Acoustical Society of America, Metherell and his collaborators described the method--called acoustical holography.

The method uses "pure" sound waves of a single frequency that are bounced off the subject and picked up by one or more scanning microphones. At the same time, a sound signal of the same frequency is transmitted directly to the microphones. The two tones--reflected and direct--interfere with each other in a complex sound pattern that is, in effect, an acoustical "picture" of the object being scanned. The mixed pattern of sound is transmitted as electrical en rgy from the microphones to an oscilloscope--similar to a television picture tube. The oscilloscope then converts the electrical energy into light patterns. A special polaroid camera records a time exposure negative of the converted sound pattern.

Rapid Read-Out. From this point on, Metherell's technique closely parallels that of optical holography (TIME, March 18, 1966). The filmed pattern is illuminated from one side by light from a helium-neon laser device. The light is diffracted by the converted sound pattern into an image of the original object. Viewers standing on the opposite side of the film can then see a measurable, three-dimensional representation of the object that has been scanned. By reconstructing three such photographs taken with sounds of different frequencies, the scientists believe that they wil soon be able to make multicolored sound pictures of even greater accuracy. The oscilloscope pattern can also be fed into a computer rather than being filmed, for rapid readout that shows a two-dimensional image for quick identification.

Thus far, Metherell has successfully taken black and white sound pictures of a model submarine, a model aircraft carrier, an airplane silhouette, the letter R and various geometric shapes. Using low frequencies, acoustical holography could explore for oil and mineral deposits at depths of several miles. Archaeologists could use higher frequencies to search for buried cities. Oceanographers may well map the ocean floor in the same way. And at frequencies between 1 and 10 megacycles, diagnostic holograms may some day chart not only tumors, but soft areas of the body--such as muscles, blood vessels and brain tissue--that can not be usually seen with standard X-ray techniques.

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