Levitation is no longer just the realm of magicians, with engineers dabbling in ways to suspend objects in midair using magnets, heat flow or sound waves. Unfortunately, it usually only works with particles or tiny objects, but scientists from the University of Bristol have developed a new "acoustic tractor beam" that can trap larger objects, possibly paving the way for contactless production lines or even human levitation.

In previous acoustic tractor beam designs, two ultrasonic speakers face each other across an open space. The ultrasound waves emitted by both speakers meet in the middle, and the perfectly aligned pressure from both directions can keep particles and other very light objects hovering in that meeting point (called the standing wave).

But there are a couple of problems with this technique that have limited the size of objects that can be levitated. If an object isn't positioned perfectly, the difference in pressure can cause it to whizz off out of the tractor beam, and if an object is larger than the wavelength of the sound waves, it will spin faster and faster until it, again, flies off.

The Bristol device was designed to counteract those issues by arranging a set of speakers in a bowl shape. These speakers produce sound in a rapidly spinning pattern, creating a kind of "sound tornado" with louder waves spinning around a silent core. That core acts like a 3D version of the standing waves produced in earlier acoustic tractor beams, keeping objects levitating motionless in the eye of the storm.

To keep the tornado from transferring its spin to the object, the speakers quickly twist their direction back and forth, creating alternating vortices of sound that work to stabilize the tractor beam. After enlarging the silent core, the researchers successfully levitated a polysytrene ball measuring 2 cm (0.8 in), which is twice the wavelength of the 40 kHz ultrasonic waves used in the test. That makes it the largest object ever held in a tractor beam, and the researchers say it bodes well for the future of large-scale levitation.

"Acoustic researchers had been frustrated by the size limit for years, so it's satisfying to find a way to overcome it," says Asier Marzo, lead author of the study. "I think it opens the door to many new applications."

Previously, human levitation was believed to be impossible because larger objects require lower-pitch sound, which are audible and can be dangerous to us. But this study worked with a frequency of 40 kHz, which is higher than humans can detect and into the range of a bat's hearing. That makes it safe for the magician's lovely levitating assistant and everyone in the audience.

"Acoustic tractor beams have huge potential in many applications," says Bruce Drinkwater, supervisor on the project. "I'm particularly excited by the idea of contactless production lines where delicate objects are assembled without touching them."

The study was published in the journal Physical Review Letters and the researchers demonstrate their acoustic tractor beam in the video below.

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