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Researchers create a sonic tractor beam

Using a simple set of loudspeakers, scientists have figured out a way to levitate and rotate objects in midair. If perfected, this “sonic tractor beam” could find uses ranging from treating kidney stones to creating artificial gravity on the International Space Station.

Scientists have used sound to levitate objects before. That feat isn't surprising, as sound is a wave of pressure strong enough to move your eardrum. However, instead of audible sound, sonic levitation utilizes higher ultrasonic frequencies that are beyond the range of human hearing. When blared from loudspeakers in the right configuration, these sound waves can combine to form a sonic scaffolding called an interference pattern—a sort of a force field that can hold a small object aloft.

Previously, to lift an object, scientists had to put banks of speakers on opposite sides of it, or a bank on one side and a sound reflector on the other. In the new study, reported online today in Nature Communications, physicists achieve levitation using just one block of speakers on one side. To do that, they employ a special algorithm that calculates the exact interference patterns needed to levitate an object using this “single-sided emitter.” The new technique enables scientists to move the floating object vertically and laterally, but it also allows them to rotate the object, something that previous techniques couldn’t do. Moreover, the one-sided speaker setup allows for easier access and more precise control of the floating object.

“It’s hard to get across how many times we tried and failed,” says lead author Bruce Drinkwater, a mechanical engineer at University of Bristol in the United Kingdom. “You’ve got this array of loudspeakers and you’re continually popping particles where you think they should levitate, and then watching them continually drop down.” But with the algorithm’s help, Drinkwater and his colleagues were able to dictate the bead’s motion, whether it hung in the air, spun on its own axis, or danced from side to side. “After we got the [algorithm] working, we put [the bead] in and it just stayed there—it was absolutely amazing”

The algorithm works by constructing the best possible interference patterns, one that not only keeps the bead floating, but lets it twist and move with some freedom. The interference pattern comes about by adjusting the precise synchronization, or “phases,” of the waves leaving the various speakers. By setting the phase differences just right, the researchers make the waves combine to reinforce one another in some places and cancel out one another in other places. In that way they create a complex 3D pattern of high and low pressure regions, which the authors call an "acoustic hologram," that can support the bead against the pull of gravity. As the algorithm tunes the phases, the interference pattern and resulting hologram change, enabling researchers to move the bead around.

The algorithm can fashion acoustic holograms of various spatial configurations, but Drinkwater and his team focused on three: the “twin trap,” the “vortex,” and the “bottle.” The twin trap pinches the object like a pair of tweezers and allows for rotation and movement of the object. The vortex, which spins and entraps the bead in the center of a tornadolike flurry, rotates the bead on its own axis. Lastly, the bottle traps the bead as if in a sonic container, keeping it stable.

Tony Jun Huang, a mechanical engineer at Pennsylvania State University, University Park, says he hopes this brings acoustic manipulation into the spotlight. “Not many people work in this field, and not many people recognize the importance of it,” he says. He hopes that in the future, Drinkwater and his colleagues will pair with biologists and doctors to demonstrate applications that until now, were impossible to explore.

And that’s exactly what Drinkwater and Asier Marzo, first author and computational engineer at the University of Bristol, hope to do next. “My main target for the future is in vivo levitation,” Marzo says. Ultrasonic waves can penetrate the body relatively gently, he notes, so the sonic tractor beam might be used to remove kidney stones and clots, deliver drug-laden capsules to various parts of the body, or control microsurgical instruments. “This isn’t just theoretical anymore," Marzo says. "Now we have proof that we can do one-sided levitation and that paves the way for lots of other research.”