June 25, 2011

Scientists Develop Acoustic Cloak

Scientists have developed a new device that makes objects invisible to sound waves.

The approach borrows ideas from attempts to "cloak" objects from light.

The scientists said it uses simple plastic sheets with arrays of holes, and could be put to use in making ships invisible to sonar or in an acoustic design for concert halls.

Researchers have put forth effort to try and create "invisibility cloaks" since the feasibility of the idea was theorized in 2006.

Those approaches are based on metamaterials, man-made materials with property that do not occur in nature.  The metamaterials are designed so they force light waves to travel around an object to an observer.

However, researchers quickly found that the mathematics behind these light waves could be applied to sound waves.

"Fundamentally, in terms of hiding objects, it's the same - how anything is sensed is with some kind of wave and you either hear or see the effect of it," Steven Cummer of Duke University told BBC News.

"But when it comes to building the materials, things are very different between acoustics and electromagnetics. The thing you need to engineer into the materials is very different behavior in different directions that the wave travels through it," he said.
Cummer described the theory of acoustic cloaking in Physical Review Letters in 2008.  That work showed acoustic invisibility in a shallow layer of water.

Now, Cummer and his team have shown off an acoustic cloaking technique that works in air, for audible frequencies between one and four kilohertz.

The team uses stacked sheets of plastic with regular arrays of holes through them.

When the sheet is placed on a flat surface, the stack redirects the waves so reflected waves are exactly as they would be if the stack were not there at all.

"How the sound reflects off this reflecting surface with this composite object on it - which is pretty big and has a cloaking shell on it - really reflects... just like a flat surface does," Cummer told BBC.

Ortwin Hess, a director of Imperial College London's Center for Plasmonics and Metamaterials, said the work was "a really remarkable experimental demonstration".

"It shows very nicely that although acoustic and electromagnetic waves are very different in nature, the powers of transformation optics and transformation acoustics are [similar] - I'm quite pleased that there's activity on both ends," Hess told BBC.

Hess said the demonstration was for very directed sound waves, and only in two dimensions, but the most notable aspect of the approach was its simplicity.

"It's almost like someone could take a pencil and poke holes in a particular way in the plastic," he added.

"It's a bit more challenging for three dimensions. I don't see any reason why it shouldn't be possible but it won't be just an afternoon's work."


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