Does acoustic cloaking herald the end of sonar detection?

Taking the old adage “hear no evil” to a whole new level, the newly developed 3D acoustic cloak will make objects completely undetectable by rerouting sound waves.

Growing up with invisibility cloaks and Star Trek style cloaking devices might have given modern engineers lofty ideals to aim towards, though an unassuming pyramid of perforated plastic sheets doesn’t exactly look like the cloaking technology of the future by anyone’s estimation. Despite this, no one can argue with the ingenuity and skill that engineers at Duke University have employed to develop the acoustic cloaking device.

The device works by rerouting sound waves, which in turn makes it appear as if the cloak and anything beneath it are simply not there. The 3D nature of the cloaking device means that no matter what direction the sound comes from, or where the observer is located, it will be completely undetectable. The benefits and future ramifications for sonar avoidance are huge, and there is also scope for the technology to expand into architectural acoustics.

“The particular trick we’re performing is hiding an object from sound waves,” said Steven Cummer, Professor of Electrical and Computer Engineering and 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.”

The relatively unexplored field of metamaterials helped to develop and expand on the initial idea and turn the concept into reality. Metamaterials cover the combination of natural materials in repeating patterns to achieve unnatural properties, in this case plastic and air effecting sound waves.  The completed device looks like a rough pyramid of stacked plastic sheets, each with a repeating pattern of holes throughout.

This arrangement of perforations and the shape of the stack allows the sound waves to mimic what they’d be had they simply bounced off a flat surface. As well as changing the trajectory of the waves, the engineers also took the speed variable into consideration. Since the sound waves are travelling a shorter distance, the speed must be adapted by slowing it to compensate. Without this crucial and detailed orientated step, it would be impossible to get a truly accurate cloaking effect.

"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."

Initial tests were performed by covering a small sphere with the cloak and pinging it with short bursts of sound from various angles. They went on to compare the mapped wave responses with those of a flat surface. The results confirmed that the cloaking device worked.

While the experiment is in the early stages, Cummer is already thinking about long-term applications of the technology. "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."

One of the joys of engineering is taking big ideas and making them realistic and tangible realities. Here’s hoping a teleportation device isn’t far behind.

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