Sound waves can be used to keep an object hovering in the air, and a new technique works even in crowded spaces
17 June 2022
Ultrasonic sound waves have been used to levitate objects in crowded rooms to make hologram-like displays. Such acoustic levitation was previously only practical in empty spaces, but a new algorithm can quickly readjust the sound waves when they encounter an obstacle to keep the object in the air.
Sound waves are comprised of air particles moving together. If manipulated in the right way, they can pick up and move objects. However, if the sound waves run into some other object that reflects or scatters them, the levitating object can come tumbling down.
Ryuji Hirayama at University College London and his colleagues previously used sound to levitate glowing beads to create floating 3D shapes. Now, they have developed a computational technique that enables them to levitate and manipulate objects above bumpy surfaces and near objects.
Hirayama and his colleagues used 256 small loudspeakers arranged in a grid to levitate objects with precisely shaped ultrasound waves. When these sound waves encountered objects that would usually scatter them, like a wall or a houseplant, a computer algorithm quickly adjusted their shape to maintain levitation.
The researchers demonstrated their technique by 3D printing a small plastic rabbit, then levitating objects near it. In one experiment, they made illuminated beads fly around the rabbit in the shape of a butterfly whose “wings” could be controlled by the motion of a researcher’s fingers.
In another, they levitated a piece of nearly transparent fabric above the rabbit and made it spin while a projector cast images of the rabbit onto it. The result was a seemingly 3D rabbit hologram hovering above its plastic counterpart.
They also levitated a drop of paint over a glass of water. This showed that their algorithm works even when suspending objects that can change shape above a surface that can wiggle as it reflects sound.
Bruce Drinkwater at the University of Bristol in the UK says that the new technique could be used to project information with lots of “wow factor” in museum displays or advertising. It could also be employed in chemical engineering, using sound waves to mix materials without anyone having to touch them. The new method seems more robust than previous ones, so it could make acoustic levitation practical more broadly, he says.
Hirayama says that, so far, he and his colleagues have only considered acoustic levitation in spaces full of sound-scattering objects that don’t move at all or move only in a few predictable ways, such as a hand trying to touch a levitating hologram. Their next goal is to perfect their mid-air object manipulation using sound when everything in the room is moving in unexpected and unanticipated ways.
“We want to make this technology practical and have it react to objects in real time,” he says.
Journal reference: Science Advances, DOI: 10.1126/sciadv.abn7614
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