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EETE APR 2014

Acoustic invisibility cloak tricks sound waves and sonars By Julien happich Using clever design rules and crunching numbers through computational simulations, Researchers from the Department of Electrical and Computer Engineering at Duke University (North Carolina), have managed to engineer a hollow structure that could cloak any object placed inside, from sound. The cloak is made of sheets of plastic, designed into a structure with precisely repeating perforation patterns and specifically engineered to process sound waves as they pass by, so as to interact with them in a way that conceals both the cloak and what’s inside. “To give the illusion that it isn’t there, the cloak must alter the waves’ trajectory to match what they would look like if they had not come across it”, explains professor Steven Cummer, leader of the project at Duke University. The cloak effectively works from all directions, it takes the sound wave “processes it mechanically” and restores it in its original wave front as if no obstacle had been encountered by the sound wave. “That mechanical processing involves diverting the wave energy around the object and through the cloaking shell, and then put it back together so that it leaves the shell just like nothing was there” clarified Cummer. “The incident sound energy is not reflected, and it is redirected (not absorbed) around the visible object by the cloaking shell so that it does not cast a shadow. And sound could bounce back off another object and pass through the cloak a second time”. This is very different from what you would get from any non-echoing material (like for example the spiky foam you find in anechoic chambers). Cummer highlights the difference: “Those spiky foams are designed to absorb incident sound and not reflect any back. If you tried to cloak an object with the same spiky foam, first you would get no reflection from it. Because a cloaked object should behave like nothing at all, that is good, because there are no reflections from “nothing”. However, any incident sound would be absorbed by the cloaked object as well. That is bad, because sound is not absorbed by “nothing”, it should pass straight through. Thus an object covered in absorbing foam would not reflect but would cast a big sound shadow, making it detectable.” So would it be conceivable to engineer deceitful echo signatures PhD student, Bogdan Popa showing the 3D acoustic cloak in these cloaks, and could that be used in sonic installations to boost certain sound characteristics or to create new effects at room level? Professor Cummer thinks this is the sort of tricks that can be done with this approach. “I am trying to engage as many audio design professionals as I can to explore these possibilities. It’s not really my area so I don’t have a good sense of what would or would not be useful, but I am fairly certain that there must be useful things that can be done this way.” Of course, another obvious application that one would think of is stealth submarines. “We conducted our tests in the air, but sound waves behave similarly underwater, so one obvious potential use is sonar avoidance” admits Cummer whose research was supported by Multidisciplinary University Research Initiative grants from the Office of Naval Research and from the Army Research Office. “There are major challenges with implementing the same approach in water. Air is much easier, which is why we did that first. But we are thinking about water.” But to what extent could this cloak be adapted to various shapes and does any specific shape of the cloak require specific computing with regard to the direction from which the sound comes? “Interestingly, the underlying theory says that you can design a cloak of any shape that works for sound coming from all possible directions”, answers Cummer. “A sphere has nice symmetry that makes it easier to design, but a cube is possible too. However, the parameters of the material you need for the cloaking shell (for acoustics, these are effective mass density and effective stiffness) depend critically on the shape. This cloaking concept could be adapted to other shapes, but they would have to be reengineered and re-fabricated”, he added. In fact, the cloaking shell has to have significant thickness to it in order to guide the acoustic wave energy. It could be designed thinner but then it becomes very hard to make if it is much more than about 10 times thinner than the size of the object it is hiding, according to Cummer. This limitation makes it impractical to use such metamaterials as a “skin”, for example to make stealth submarines. But one of the easiest shapes to craft for an acoustic cloak, the pyramid shape could be built as nuclear submarine hide-out, as long as it would be large enough for the sub to fit inside. Could these metamaterials be conceived as re-shapable re-configurable MEMS skins controlled by software to adapt the cloak to specific situations? “That is way, way down the road but there’s no fundamental reason that it couldn’t be done, provided access to computercontrolled reconfigurable-shape materials”, concluded Cummer. 12 Electronic Engineering Times Europe April 2014 www.electronics-eetimes.com


EETE APR 2014
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