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The Best Way to Levitate Things Using Sound

Acoustic levitation is poised to become an everyday technology.
Historically speaking, levitation is the domain of metaphysics: magic, spirits, divine acts. ​Photo via trialsanderro​rs/Flickr

A team of Brazilian physicists has developed a new method of acoustic levitation that may finally turn what was once only a concept into a viable, practical technology—levitation as a part of everyday life.

Historically speaking, levitation is the domain of metaphysics: magic, spirits, divine acts. IRL, however, it's not an especially exotic phenomenon—at least technically. Levitation is achievable through a number of means, such as magnetism, electric fields, fluid buoyancy, lasers, and even the strange negative pressure of vacuum (dark) energy.

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Nevertheless, levitation is mostly utilized in MagLev technology, which itself is pretty limited. Researchers are also rather fond of levitating animals: mice, grasshoppers, frogs. A 2009 experim​ent found that, after several hours of being held aloft in a magnetic field, mice were able to more or less adjust to their new conditions.

Levitation isn't commonplace because it also remains pretty difficult—or, rather, remains difficult to achieve in a way that might have some real-world utility. Levitated objects tend to be positionally unstable; if an object held aloft by, say, a jet of air "wanders" in some direction, that wandering must be matched by a force in the opposing direction. It's like trying to rest an object on top of a melting cube of ice. In terms of effort, levitation becomes rather "expensive" in cases like these, an active mechanism rather than a passive one.

An alternative is to levitate the object in a kind of dip or hole. This requires being able to "shape" the upward force, which is not easy or simple either.

The Brazilian team, which hails from the University of São Paulo, came up with a variation on an almost-too-simple approach to the stability problem. Acoustic waves are produced above the object to be levitated and then fired downwards, where they reflect off of a concave surface below the object. The returning waves retain the surface's shape: a similarly convex dish or cradle that prevents the levitated object from "rolling" away.

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This has been attempted before, but, as is the trouble with a lot of levitation, it's required some very closely controlled conditions. Specifically, the convex surface and the acoustic source must remain at a fixed "resonant" distance from each other. The goal is to get the reflected waves and the incoming waves to interact with each other in such a way to produce standing waves, which are waves suspended in mid-flight. The phases of the different waves, how they're shifted forward and backward in time, align just so, with the result being an odd sort of sound-ladder suspended in space.

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This requires resonance, or a specific resonance where the distance between the acoustic source and the concave surface is a multiple of half the wavelength of the sound waves. If that distance is off, standing waves once again become moving waves—which, if you're trying to use those waves to levitate an object, isn't desirable.

That's a severe limitation. The São Paulo physicists came up with a way to do the same thing non-resonantly—in other words, without the limitation of fixed dimensions. In the typical resonant setup, the standing waves are created through the interactions between many produced and reflected waves, giving something more like the appearance of motionless waves.

In the new setup, levitation is achieved using only two waves, one reflected and one from the source, the first one, with the result being a wave superposition, where two waves essentially function as one. If those waves happened to be moving in opposite directions, we'd find the superposed wave moving in no direction—a platform for levitation.

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"The interference between these two waves creates a pressure node near the reflector surface," the researchers write in the Applied Physics Letters, "and a small particle can be levitated near the reflector. The small size of the reflector minimizes the multiple reflections, and the acoustic radiation force that acts on the levitated particles is almost independent of the distance between the transducer and the reflector."

In terms of applications, consider the transport of dangerous chemicals or highly-sensitive materials along a factory assembly line. "Modern factories have hundreds of robots to move parts from one place to another," noted Marco Aurélio Brizzotti Andrade, the team's leader, in an ​accompanying statement. "Why not try to do the same without touching the parts to be transported?"

Of course, why not stop there? Imagine a future without hands and grody fingers—where have they been?! what have they touched?!—where we manipulate the world with sound, eating our eggs and toast with help from a high-frequency whine. Sure, that's not quite promised by this method, but, c'mon, touching things is gross.