'In one ear, out the other': New cloaking device makes hidden objects completely inaudible
The simple toy, a little bigger in diameter than an iPad, and which looks like lightweight sheets of plastic stacked together to form a sort of Mayan pyramid with tiny holes, acts as if sound passes right through it and whatever it hides inside. It works with sound coming from any angle, and does its magic by rerouting the sound waves to avoid collision with hard matter.
Professor of electrical and computer engineering at Duke, Steven Cummer, explains on Duke’s website that “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,” Cummer said of the results of the result of the research, whose findings appear in the journal Nature Materials.
Accomplishing this nifty trick would be difficult without harnessing the power of metamaterials, which result from combining existing materials in ways that force them to alter their properties and behave unnaturally. The acoustic cloak is essentially just plastic and air cleverly combined, according to Cummer.
Once the sound hits the structure, the result of painstaking mathematical calculations by engineers comes into play. It goes as far as fooling the sound as to how far it travels, making it compensate for its surroundings. The deceptively simple design is anything but, the professor explains.
“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,” Cummer said.
The project was clearly a success for Duke, who tested it on a small cloaked sphere and bounced sounds off of it with a microphone, then doing the same without the device.
Prospects for development are, understandably, endless the cloak’s creators believe. The cloak could in future be used to fool enemy sonar, as well as for things like architectural acoustics, to compensate for the conflict between sound and the structural requirements of a building.
But this is not the first time the military has thought of something like this. Of course, the ability to cheaply 3D-print it is a huge bonus, but the concept of using metamaterial, as well as the principle of making sound pass around an object go back some time.
America’s Defense Advanced Research Projects Agency (DARPA) has been at it for ages, announcing in 2007 that they’d like to be able to make a whole military vehicle inaudible. In their 2008 defense budget they write that the “effort will use directed ultrasound technology to enable the capability to significantly reduce sound emissions from large scale tactical military hardware. Theory predicts that nonlinear effects of high-power acoustic radiation on the atmosphere can cause acoustic energy to dissipate, rather than radiate."
However, DARPA’s dream only went as far as reducing noise by 30dB, which, while allowing US forces to operate much closer to enemy positions, would still not make them completely ‘disappear.’ Duke’s project seems to be the beginning of a whole different game.