The acoustic cloaking device works in all three dimensions, no matter which direction the sound is coming from or where the observer is located, and holds potential for future applications such as sonar avoidance and architectural acoustics. The study appears online in Nature Materials.
‘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 in North Carolina. ‘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, Cummer and his colleagues turned to metamaterials, the combination of natural materials in repeating patterns to achieve unnatural properties. For the new acoustic cloak, the materials manipulating the behaviour of sound waves are 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 resemble a pyramid.
To give the illusion that it isn’t 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.
In a statement Cummer said: ‘The structure that we built might look really simple…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.’
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, with the results showing that the cloaking device makes it appear as though the sound waves reflected off an empty surface.
The experiment demonstrates the technology is possible and Cummer believes 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.’
This research was supported by Multidisciplinary University Research Initiative grants from the US Office of Naval Research and from the US Army Research Office.