Acoustic 'cloaking device' shields objects from sound

By Jason Palmer
Science and technology reporter, BBC News

Graphs showing acoustic 'cloaking'
Image caption,
Reflections of sound off a surface (top), off an object on it (middle) and off a cloaked object (bottom)

Scientists 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.

Much research has been undertaken toward creating Harry Potter-style "invisibility cloaks" since the feasibility of the idea was first put forward in 2006.

Those approaches are mostly based on so-called metamaterials, man-made materials with properties that do not occur in nature. The metamaterials are designed such that they force light waves to travel around an object; to an observer, it is as if the object were not there.

But 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.

"The thing you need to engineer into the materials is very different behaviour in different directions that the wave travels through it," he told BBC News.

In 2008, Dr Cummer first described the theory of acoustic cloaking in an article in Physical Review Letters, and earlier this year a group from the University of Illinois Urbana-Champaign demonstrated the first practical use of the theory in an article in the same journal.

That work showed acoustic invisibility in a shallow layer of water, at ultrasound frequencies above those we can hear.

Now, Dr Cummer and his colleagues have shown off an acoustic cloaking technique that works in air, for audible frequencies between one and four kilohertz - corresponding to two octaves on the higher half of a piano.

Image caption,
The cloaking shell is made of easily-manufactured sheets of plastic with holes through them

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.

Hole poking

Ortwin Hess, a director of Imperial College London's Centre for Plasmonics and Metamaterials, called the work "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."

Professor Hess pointed out that 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 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.

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