Engineers at Duke University have succeeded in building the world's first 3D acoustic cloak that can hide itself and any objects beneath it by rerouting sound waves.

Built using a few perforated sheets of plastic, the cloak has the ability to hide itself no matter which direction the sound is coming from, which could be a significant development for sonar avoidance to hide naval ships, or in improving architectural acoustics.

"The particular trick we're performing is hiding an object from sound waves," said Steven Cummer, professor of electrical and computer engineering at Duke University.

"By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path."

Plastic pyramid

In order to make the cloak work, the engineers constructed a device that looks like several plastic plates with a repeating pattern of holes poked through them, stacked on top of one another to form a pyramid.

The shape of the pyramid alters the sound waves' trajectory so that they look exactly the way sound waves would look if they had reflected off a flat surface.

"The structure that we built might look really simple," said Cummer.

"But I promise you that it's a lot more difficult and interesting than it looks. We put a lot of energy into calculating how sound waves would interact with it. We didn't come up with this overnight."

Works underwater too

The team tested out their invention by covering a small sphere with the cloak and sent short bursts of sounds towards it from various angles, then mapped how the sound waves responded.

"We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance," Cummer 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."