Metamaterials are already being used to create invisibility cloaks and "temporal cloaks," but now engineers from Duke University have turned metamaterials to the task of creating a 3D acoustic cloak. In the same way that invisibility cloaks use metamaterials to reroute light around an object, the acoustic cloaking device interacts with sound waves to make it appear as if the device and anything hidden beneath it isn't there.

Steven Cummer, professor of electrical and computer engineering, and his colleagues at Duke University constructed their acoustic cloak using several sheets of plastic plates dotted with repeating patterns of holes. The plastic sheets, which were created using a 3D printer, were stacked on top of each other to form a device that resembles a pyramid in shape. The geometry of the sheets and the placement of the holes interact with sound waves to make it appear as if the device and anything sitting underneath it isn't there.

Despite its apparent simplicity, the device's construction was far from a haphazard affair, with a lot of time and research going into calculating how sound waves would interact with it. As Cummer puts it, "we didn't come up with this overnight."

Research scientist Bogdan Popa with a 3D acoustic cloaking device constructed with components created in a 3-D printer (Photo: Duke Photography)

To work effectively, the cloak needs to alter the trajectory of the sound waves so they behave as if they had reflected off a flat surface. To achieve this, the device needs to slow down the speed of the sound waves to compensate for the fact that they aren't traveling as far.

To test the effectiveness of the cloak, the researchers took a small sphere and covered it with the cloak. They then "pinged" the sphere with short bursts of sound from various angles and mapped how the sound waves responded using a microphone. The team then produced videos of the sound waves traveling through the air and compared them to videos produced with the cloak removed and another showing the sound wave behavior with an unobstructed flat surface.

The results showed that the acoustic cloak made it appear as if the sound waves were being reflected off a flat surface with no sign the sphere was there. Unlike the "silence cloak" developed at Germany's Karlsruhe Institute of Technology that worked only in two-dimensions, this held true no matter which direction the sound originated from or where the observer was located, prompting the team to call their creation the "world's first 3D acoustic cloaking device."

Cummer believes the technology has numerous potential commercial applications.

"We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance,” he said. "But there’s also the design of auditoriums or concert halls – any space where you need to control the acoustics. If you had to put a beam somewhere for structural reasons that was going to mess up the sound, perhaps you could fix the acoustics by cloaking it."

The team's acoustic cloaking device is detailed in a letter published in the journal Nature Materials.

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