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Engineering researchers in the US have demonstrated the world’s first three-dimensional acoustic cloak, offering the potential for future applications such as sonar avoidance.
The device reroutes sound waves to create the impression that both the cloak and anything beneath it are not there. The acoustic cloak works in all three dimensions, no matter which direction the sound is coming from or where the observer is located.
Steven Cummer, professor of electrical and computer engineering at Duke University in North Carolina, said: “The particular trick we’re performing is hiding an object from sound waves. By placing this cloak around an object, the sound waves behave like there is nothing more than a flat surface in their path.”
To achieve this trick, Cummer and his colleagues turned to the developing field of metamaterials - the combination of natural materials in repeating patterns to achieve unnatural properties. In the case of the acoustic cloak, the materials manipulating the behaviour of sound waves are simply plastic and air. Once constructed, the device looks like several plastic plates with a repeating pattern of holes poked through them stacked on top of one another to form a sort of pyramid.
To give the illusion that it is not there, the cloak must alter the waves’ trajectory to match what they would look like had they had reflected off a flat surface. Because the sound is not reaching the surface beneath, it is traveling a shorter distance and its speed must be slowed to compensate.
“The structure that we built might look really simple,” said Cummer. “But 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.”
To test the cloaking device, researchers covered a small sphere with the cloak and “pinged” it with short bursts of sound from various angles. Using a microphone, they mapped how the waves responded and produced videos of them traveling through the air.
Cummer and his team then compared the videos to those created with both an unobstructed flat surface and an uncloaked sphere blocking the way. The results clearly show that the cloaking device makes it appear as though the sound waves reflected off an empty surface.
Although the experiment is a simple demonstration showing that the technology is possible, Cummer believes that the technique has several potential commercial applications.
“We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance,” said Cummer. “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.”
A demonstration of acoustic cloaking can be viewed here.