On the Rebound

Imagine descending into an Alice-in-Wonderland canyon where the echo that returns when you shout “hello” sounds like “olleh.” No such sound-reversal canyons exist in nature. However, physicists in Europe and the United States have recently been creating environments in the laboratory and underwater that exhibit reversed echoing. Instead of actual walls, from which only run-of-the-mill echoes would

reflect, the researchers direct their sounds to computerized microphone-loudspeaker units that return them in a time-reversed order–the last sound component to arrive is the first to be sent back.

Such setups refocus sound with remarkable precision. When a person sings out “hello” in such an environment, the sound not only comes back reversed but it also beams specifically to the vocalist’s head–and nowhere else.

These traits make time-reversed acoustics, as this technology is known, promising for a wide range of uses, including shattering kidney stones, tracking submarines, and broadcasting different translations of a speaker to listeners sitting side by side.

Mirror, mirror

Acoustics specialists build a time-reversal mirror using an array of piezoelectric transducers. Those devices can act both as microphones, which convert sounds’ pressure fluctuations into electric signals, and as loudspeakers, which convert electric signals into vibrations that people hear as sounds.

Time-reversing sound requires yet more from these transducers–they also must be able to capture, process, and store sounds. Some of the gridlike arrays used for medical applications are the size of dinner plates; some underwater versions can be taller than a 20-story building.

When an acoustic mirror picks up a sound pulse and its reverberations, each transducer converts vibrations into electric signals and loads them into its memory in time-reversed order. To create the echo, all the transducers reissue signals as sounds aimed back in the direction from which they came.

That’s akin to undoing the wave action that follows when you drop a pebble into a pond. It’s as if “the ripples in the pond collapse back onto the pebble,” says David M. Pepper of HRL Laboratories in Malibu, Calif., who has studied similar refocusing behavior in light.

Many environments bend and bounce signals around so much that they become severely distorted. The acoustic signals arriving at the array of transducers carry all the modifications they underwent on their way from the sound’s source, but those modifications are undone as the signals travel back to the source.

That’s an attractive feature of the time-reversal technique, its developers say. Other signal-conditioning techniques also can diminish distortion, but these require complex computations that time-reversal methods can forego.

In fact, an environment with many sound-bending and sound-reflecting obstacles actually improves a time-reversal mirror’s projection of sounds back to a specific location. The more reflecting surfaces there are in the environment, the wider the spatial spread of the components of sound the mirror gathers. And that provides the mirror with more precise guidance for targeting the echo.

“The best application is usually where the medium is really ugly and inhomogeneous,” says time-reversal pioneer Mathias Fink of the University of Denis Diderot in Paris, France.

Fink and some other scientists also expect that the same techniques may work in the electromagnetic realm, perhaps increasing the signal-carrying capability of wireless communication systems (see “Beyond Sound,” below).

Search and destroy

For many years, scientists have been investigating the use of inaudible, high-intensity ultrasound for incision-free surgery (SN: 1/6/01, p. 12: Available to subscribers at Beyond Imaging), but the hodgepodge of tissues in the human body distorts acoustic signals, making it difficult to precisely aim the acoustic energy.

That’s where time-reversal mirrors can come in. For more than a decade, Fink’s group has been developing versions of ultrasonic devices, known as lithotripters, for destroying mineral stones that can form in the kidney and gall bladder. First, the lithotripter’s time-reversal mirror broadcasts unfocused sound into an organ to create reflections that reveal a stone’s location. Then, the device sends a time-reversed form of the reflections, but greatly amplified, to shatter the target.

Even if a stone is moving during the procedure, the time-reversed system works so rapidly that the target is still within the focal spot when the echoed signal returns. Then the next broad signal determines the target’s location anew, since many shots are usually needed to destroy a stone.

By the early 1990s, Fink’s group and an industrial partner had developed a lithotripter with a time reversal mirror composed of 128 transducers. In clinical trials, the device proved it could track kidney stones, even as the patients breathed and fidgeted. However, the project folded because the instrument was too expensive for commercialization.

Fink’s team didn’t give up. In the past few years, the group has come up with a way to simplify the device and is working with another company on a 16-transducer mirror.

Emad S. Ebbini and his colleagues at the University of Minnesota in Minneapolis are also showing how such mirrors might be applied in medicine. They are investigating time-reversed ultrasound for two procedures: knocking out heart tissue that misfires electrically and destroying tumors. The group is creating a system that uses ultrasound signals to both generate images of the affected region and to burn away the troublesome tissue.

Among human tissues, the skull may create the greatest distortions of sound waves. Fink and his colleagues are developing time-reversal mirrors that would beam tissue-killing ultrasound into brain tumors. However, they have to introduce mathematical methods to alter features of the sound signals to counteract the effect of the skull. The researchers describe some of their work on brain-tumor instruments in the January Journal of the Acoustical Society of America.

While medical applications of time-reversal mirrors remain on the horizon, an instrument that finds defects in titanium aircraft engine parts will probably be in operation sooner, perhaps within 2 years, Fink says. Time reversal can aid engine makers by pinpointing flaws within the irregular microstructure of titanium that other scanning methods miss. Fink’s lab has been collaborating with the French aerospace giant Snecma, headquartered in Paris, to develop a detector.

Beneath the waves

Another acoustically noisy environment where time-reversal methods are drawing attention is the ocean. In coastal waters, sounds ping-pong between the sea floor and surface, creating echoes that foil undersea communications and interfere with acoustic detection of submarines and mines, says William A. Kuperman of Scripps Institution of Oceanography in La Jolla, Calif.

Kuperman became interested in time-reversed acoustics in the mid-1990s while working on a Navy-funded ocean-modeling project. His challenge was to devise computer models of ocean acoustics that would make it possible to take signals received at underwater listening posts and trace them back to their sources–for instance, enemy submarines. The advantage of doing it all on computers, he notes, is that no sonar pings are generated, so there would be nothing to tip off a target that it’s being stalked.

When Kuperman learned about Fink’s work in which read signals were driven back to their sources, the Scripps researcher began similar experiments on a much grander scale.

Over the past 8 years, Kuperman and William S. Hodgkiss of Scripps, Tuncay Akal of the NATO SACLANT Undersea Research Center in La Spezia, Italy, and their colleagues have conducted various tests in the Mediterranean Sea using transducer arrays some 80 meters long. Such arrays are fine for research, but they would have to be shrunk down dramatically to be of wider practical use, Kuperman notes.

The test results have been “spectacular,” Kuperman says. “Time reversal is undoing the complexity of the ocean.” From a distance of 10 kilometers, for example, the team refocused a sound pulse into a spot only a few meters in depth.

Now that Kuperman and his colleagues have established how well ocean-based time reversal can work, they’re investigating how to exploit time-reversed acoustics for detecting submarines and mines, developing a better understanding of the ocean environment, and communicating between undersea vehicles.

One of the nifty features of this communication scheme is that the information is covert, Kuperman says. “Off the focus, you get [only] rumbling.”

The researchers have scheduled their next round of undersea experiments for the end of this month.

Still auditioning

As a step toward more household possibilities, Fink and his colleagues have assembled a time-reversal mirror made of conventional audio equipment that actually does change people’s “hellos” into “ollehs.”

Getting more practical, scientists are considering such uses as auditorium systems that could simultaneously transmit completely different sound streams to people sitting right next to each other. This concept could be applied, for example, at the United Nations, where speeches must be simultaneously translated into many languages for different listeners. Instead of donning headphones, however, the UN delegates would simply hear the appropriate translation where they sit.

Similarly, family members traveling in a car might simultaneously listen to different radio stations.

Until recently, however, no one has actually determined what audio applications of time-reversal techniques are possible under everyday circumstances. In a new experiment, Fink and his collaborators Sylvain Yon and Mickael Tanter set up an audio time-reversal mirror in their busy laboratory. Researchers had been concerned that sound absorption by people moving around might undermine the system. In this month’s Journal of the Acoustical Society of America, the researchers report that the time-reversal setup outperformed a standard sound-focusing method and was scarcely perturbed by the room’s occupants.

As researchers push time-reversal techniques toward implementation in hospitals, oceans, and homes, their results are likely to have plenty of unexpected reverberations.

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Beyond Sound

Frequency boost may push echo technique into cell-phone realm

Now that time-reversal mirrors are proving themselves in acoustics applications, researchers aim to usher the technology into the electromagnetic realm of telecommunications. In the past few years, engineers have recognized that high-rise clutter in cities and other complicated environments where radio signals bounce around a lot–long considered a curse–actually increase the capacity of wireless information channels for cell phones and other devices (SN: 1/20/01, p. 37: Available to subscribers at Technique puts more data into airwaves). By reflecting off many obstacles, a signal takes multiple pathways, thereby increasing the effective number of communications channels.

In a new experiment using ultrasound in water to model an electromagnetic environment, Mathias Fink of the University of Denis Diderot in Paris and his team have demonstrated that time-reversal arrays simultaneously can send different data streams through a cluttered environment to multiple receivers. The transmission was successful even when the receivers were in a tightly spaced cluster, as cell-phone users might be in a train car. The team reported its results in the Jan. 10 Physical Review Letters.

The findings suggest that time-reversal techniques could be better at increasing signaling capacity than the methods devised so far by wireless specialists, says Fink. A time-reversal approach could also bring other benefits to wireless communications, predicts Jean-Pierre Fouque of North Carolina State University in Raleigh. He has developed mathematical descriptions for the temporal focusing of time-reversed signals.

The tendency of those signals to target specific receivers would make such communications more secure than conventional cell-phone links, Fouque suggests.

Also, because time-reversed echoes aren’t broadcast in all directions like ordinary wireless messages, communications systems using them would be more energy efficient.

“Time reversal has the potential to change a lot of things in communications,” comments theorist George C. Papanicolaou of Stanford University.

However, the oscillations of typical wireless signals are thousands of times faster than those of the ultrasound transmissions in Fink’s laboratory. Moreover, as Papanicolaou and his colleagues have demonstrated, time-reversed signals require transducers that operate across an extraordinarily wide band of frequencies.

Because of such technical challenges, upgrading time reversal to the wireless realm seems far-fetched, some telecommunications specialists say. Equipment that can precisely handle such a broad frequency range is extremely expensive to build, notes Aris L. Moustakas of Bell Labs’ Lucent Technologies in Murray Hill, N.J.

As another practical concern, the cost of rights to use such a large chunk of the electromagnetic spectrum for a wireless system would be high. “It’s a neat idea, but I don’t know how important it will be in the telecommunications industry soon or ever,” Moustakas comments.

Fink’s group is forging ahead nonetheless. It has built a one-transducer time-reversal mirror that Fink says is intended for microwaves at the cell-phone frequency of 2 billion cycles per second (gigahertz). In the coming months, Fink’s team expects to test whether the device performs well enough to make a significant difference in wireless networks.

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