Acoustic ‘cloaking device’ shields objects from sound

By | June 24, 2011

Acoustic cloaking deviceScientists have shown off a “cloaking device” that makes objects invisible – to sound waves.Such acoustic cloaking was proposed theoretically in 2008 but has only this year been put into practice.Described in Physical Review Letters, the approach borrows many ideas from attempts to “cloak” objects from light.It uses simple plastic sheets with arrays of holes, and could be put to use in making ships invisible to sonar or in acoustic design of concert halls. …

researchers quickly found out that the mathematics behind bending these light waves, called transformation optics, could also 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,” said Steven Cummer of Duke University. “But when it comes to building the materials, things are very different between acoustics and electromagnetics. …

It works by using stacked sheets of plastic with regular arrays of holes through them. The exact size and placement of the holes on each sheet, and the spacing between the sheets, has a predictable effect on incoming sound waves.

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

That means that an object under the stack – in the team’s experiments, a block of wood about 10cm long – would not “hear” the sound, and any attempts to locate the object using sound waves would not find it.

“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,” Dr Cummer said.

…”… 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.” …

“It’s almost like someone could take a pencil and poke holes in a particular way in the plastic,” he told BBC News.

“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.”

The work shows that an object can be hidden from sonar, and protected from incoming sound, but the same principles could be applied in the other direction – that is, containing or directing the sound within a space, for instance in soundproofing a studio or fine-tuning the acoustics of a concert hall.  …

via BBC News – Acoustic ‘cloaking device’ shields objects from sound.

Neat. I’d like to make some of this for my home recording studio to hide my computer noise from the rest of the room. Where are the exact details on the hole placements?

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