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'Invisibility cloak' against sound waves and sonars

19 Mar 2014  | Julien Happich

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Researchers from the Department of Electrical and Computer Engineering at Duke University (North Carolina) have engineered a hollow structure that could conceal any object placed inside from sound. This was made possible by using design rules and crunching numbers through computational simulations.

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 the sound wave had encountered no obstacle.

Acoustic Invisibility Cloak

PhD student Bogdan Popa showing the 3D acoustic cloak

"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, the cloaked object would absorb any incident sound 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 in these cloaks, and could that be used in sonic installations to boost certain sound characteristics or to create new effects at room level?

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