Backward Ran the Sound

By Tim Folger|Wednesday, February 01, 1995
Mathias Fink has invented a time machine of sorts. Although Fink’s device--he calls it a time-reversal mirror--won’t hurl you into the past or future, it does play tricks with time, or more precisely, with sound. If you were to stand in front of Fink’s mirror at the Waves and Acoustics Laboratory at the University of Paris and speak into it, you would hear a peculiar echo: anything you said would come back to you exactly reversed. Your hello would echo--almost instantaneously--as olleh. Fink is quick to point out that his invention, quirky as it might seem, is no mere toy. It may in the near future lead to a vastly improved ultrasound imaging device that could diagnose problems as diverse as kidney stones and faulty aircraft parts.

Ultrasound frequencies lie above the audible range, and like X- rays they penetrate solids. Unlike X-rays, though, ultrasound waves reflect off soft-tissue boundaries, allowing doctors to view internal organs. But ultrasound has its drawbacks. When you put an echographic system on someone’s abdomen, says Fink, two-thirds of people give good images, and one-third give really bad ones. The quality of the image is especially poor for people with a lot of body fat, which tends to distort the ultrasound waves and weaken the echo. Similar problems arise when ultrasound is used to probe metal or stone structures for internal cracks: the signal often gets distorted by tiny imperfections in the crystals or by the overall shape of the structure.

Fink and his colleagues have found an unusual way to get around these problems. Instead of just sending out an ultrasound beam and constructing an image from the echo, the French physicists’ time-reversal mirror--really a single device that is both microphone and speaker-- transmits an ultrasound pulse, picks up the echo, and feeds the echo to a computer. The computer digitizes the echo, takes it apart, and reassembles it in reverse order--the last piece of the echo to hit the microphone becomes the first part of a new transmitted beam. All this happens in a few thousandths of a second.

Why go through all this trouble? Because by reversing the ultrasound pulse in time, Fink’s device also reverses it in space: the pulse retraces precisely the path that the original echo took as it traveled from the target back toward the microphone. The reversed echo automatically closes in on the target--a kidney stone, say--that the original echo diverged from; there’s no need to aim the beam. And since the strongest part of the original echo always comes from some discontinuity--a kidney stone in an otherwise fleshy body, or a crack in a jet engine--by repeating the echo-reversal process over and over, the ultrasound image becomes ever sharper.

Here’s how the process works. The time-reversal mirror consists of around a hundred thin, tenth-of-an-inch-long slivers of piezoelectric ceramic--a material that converts sound to electricity and vice versa. When an echo hits this array, the piezoelectric rods start vibrating, and they send electric signals to a computer. (The rods are embedded in epoxy to keep them from disturbing one another.) The computer keeps track of when and from which piezoelectric rod each electric pulse comes.

To construct a reversed echo, the computer sends signals back to the rods in the reverse order of the signals it received. The vibrating rods thereupon produce a reversed ultrasound pulse, almost as if time were flowing backward. And because the direction of the pulse is determined by the interaction of the sound waves coming off each rod, the mirror precisely reverses that direction as it’s reversing the sound’s time sequence.

Fink’s team has been working on the device for some eight years, but only recently, thanks mostly to faster computers to process the echoes, has the device reached the threshold of practicality. A large French aeronautics firm, SNECMA, is already testing the time-reversal mirror and has succeeded in detecting internal defects smaller than a fiftieth of an inch long in a piece of forged titanium. French hospitals are keenly interested in the mirror as well; it’s thought that intense bursts of time- reversed sound may be used not only to image kidney stones but also to destroy them.
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