______________________________________________________________________________ | File Name : TIMEREV1.ASC | Online Date : 11/20/94 | | Contributed by : Jerry Decker | Dir Category : KEELY | | From : KeelyNet BBS | DataLine : (214) 324-3501 | | KeelyNet * PO BOX 870716 * Mesquite, Texas * USA * 75187 | | A FREE Alternative Sciences BBS sponsored by Vanguard Sciences | |----------------------------------------------------------------------------| The following is from the July 22, 1994 issue of Science, Volume 265. ------------------------------------------------------------------------------ Time-Reversed Sound Waves Resonate among Physicists by James Glanz Carelessly chosen words often prompt people to wish they could take back what they have just said. Now, thanks to a team of French acoustics researchers, that may no longer be an empty wish: They have developed a device that can take a sound, flip it around, and send it back along its original path in time-reversed form - almost as if time were going backwards. Team leader Mathias Fink, director of the Waves and Acoustics Laboratory at the University of Paris, faces the device and says "bonjour" to it. An instant later, the phonemic reversal "ruojnob" arrives back at his mouth INAUDIBLE to anybody else - the reconstruction is crisp, despite strong echoes in the room. But the experiment is not as frivolous as it sounds. The French team has already applied it to medicine and materials analysis. They have "made heavy inroads in medical ultrasonic imaging," says John Gilmore, a physicist at General Electric's Research Center in Schenectady, New York, who has seen Fink's setup. Other researchers are now beginning to apply it to undersea communications. The key to this trick is the acoustic wave equation, which is no respecter of time. The equation looks the same whether time is moving backward or forward, and this means that for every burst of sound or ultrasound DIVERGING FROM a source - and possibly being reflected, and fragmented by multiple barriers and propagation media - there exists in theory a set of waves that PRECISELY RETRACES all of these complex paths and CONVERGES, in SYNCHRONY, at the original source, as if time were going backwards. It is not a simple task, however, to take a sound wave and produce its time- reversed twin. Following similar work with microwaves and light, acousticians have succeeded in solving this problem in limited cases using phase-conjugate mirrors (PCMs), materials that can spontaneously reverse an approximately continous and monotone signal of any spatial shape. But acoustic systems usually have to deal with pulsed mixtures of many tones, and in such cases, says Fink, a simple PCM "isn't sufficient to bring the waves back to a good focus." Instead, he says, "you have to completely time-reverse the signal." Fink realized in the late 1970's that a computer and an array of transducers might be able to work fast enough to store digitized versions of the peaks and troughs of incoming acoustic waves, process them, and then, shoot them out backward. Unlike a passive reflector, such a device should also be able to convert a diverging wave into a converging one. At the time, however, the cost of the necessary memory and analog-to-digital converters was "crazy," Fink recalls. But by 1990, prices had dropped sufficiently and computers had gotten faster. Fink and his collaborators in Paris have now built several devices, the first time-reversal mirrors (TRMs) for acoustics, which rely on fast computer processing and a novel class of materials called piezocomposites arranged in an array. The piezocomposite transducers turned out to be a key element. A normal piezoelectric material will produce an electrical signal when an acoustic wave passes through it, and conversely will emit sound if excited with an electrical signal. "On paper, it isn't difficult to make one channel," says Fink. The problems start when you put the crystals in an array, because the signals coming and going from one transducer will interfere with its neighbors. Fink's team got around this difficulty by using piezocomposites: rodlike piezoelectric elements embedded in a polymer matrix. These are engineered to respond in one direction only, along the axis of the rods, and so do not affect nearby transducers. Once these technical problems were overcome, Fink and his team were quick to show the practical applications of their device. Two years ago they began to tackle the problem of medical imaging, scattering ultrasonic pulses off a kidney stone in order to track it - through layers of fat and connective tissue - in real time while a patient breathes. The team starts by sending in an ultrasonic seed pulse, some of which scatters off the stone and some off small, random inhomogeneities in its surroundings. The TRM initially sees a reflected signal from the stone buried in the noise. The entire scattered signal is reversed by the TRM, rerouted back through the body, scattered again, gets reversed again, and so on. During these iterations, a pulse ping-pongs steadily between the stone and the mirror - but signals from fine-scale features of the tissue are occasionally missed by the TRM on its return shots, and these scatterers gradually lose all their "ping-pong balls" of ultrasound, leaving only the stone's clear signal. Once the stone has been reliably located, intermittent amplified pulses can be applied to shatter it. As the stone moves, the process is repeated to locate it again and again. This location method, which has been tried successfully on two patients in France without the final treatment, should improve on the current method of steering the ultrasound using x-ray measurements. Similar location methods underlie Fink's approach to an important engineering task - finding defects in titanium alloys, whose randomly oriented, crystalline grains also create noise. Such a defect was blamed for the crash of a DC-10 airliner in Sioux City, Iowa, in 1989. The TRM could find defects "as small as half the size of what we can currently detect," a critical difference for safety, says Gilmore. Although Jan Achenbach, director of the Center for Quality Engineering and Failure Prevention at Northwestern University in Evanston, Illinois, thinks that TRMs may still be too expensive for this application - he has his own array to steer the beam without actually time-reversing it - he agrees there is a "tremendous simplification in doing the scanning ELECTRONICALLY" as opposed to the current, mechanical methods. Potential uses of TRM technology are not just up in the air. Independently, researchers at the University of Washington in Seattle have proposed building an underwater TRM, consisting of an array of hydrophones, to overcome the problem of "multipath distortion" in underwater communications. The problem arises because acoustic transmissions bounce off the ocean's surface and floor, and bend in temperature and pressure gradients, so that "the pulse comes staggering in several times," says Darrell Jackson of the University of Washington. Along with colleague James Ritcey, Jackson proposes a solution based on time- reversal with a twist: One submarine sends out a "probe" pulse, and multiple copies of it arrive at another sub's TRM. This turns all the signals around and sends them back, but this time with digital information encoded into them. All of these encoded signals should then arrive back at the original sub simultaneously, so that one path can't confuse the data stream arriving along another. For all the practical uses of TRMs, Fink is not neglecting basic science. He has recently put together experiments to test the limits of acoustic time- reversal. He is totally scrambling sound pulses by passing them through a "forest" of thousands of scatterers, then reassembling the pulses with a TRM and sending them back through the forest to see how well the original signal survives. So far, none of his pulses has lost its reversibility in the forest. But Fink has still more severe tests planned for his acoustic pilgrims. If those pan out, growing numbers of physicists are likely to be saying "bonjour" to this technology. ------------------------------------------------------------------------------ Vanguard Note There are consumer devices available from two sources for personal noise cancellation devices. I found them in an airline magazine but don't have the information at hand for this file. Next time you fly, check out the advertisements. Also, the seed technology companies for this in the US are listed in the file NOISECNC.ASC on KeelyNet. ------------------------------------------------------------------------------