June 28, 2011

Acoustic cloak, now you can make a working cone of silence and sonar invisible submarines

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

The cone of Silence would enshroud its users within a transparent sound-proof shield. (Comedy gag from Get Smart)

The real life acoustic cloak 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.

Physics Review Letters - Experimental Acoustic Ground Cloak in Air

We present the design, fabrication, and performance analysis for a class of two-dimensional acoustic cloaking coatings in air. Our approach takes advantage of transformation acoustics and linear coordinate transformations that result in shells which are homogeneous, broadband, and compact. The required material parameters are highly anisotropic; however, we show that they are easily achievable in practice in metamaterials made of perforated plastic plates. The good performance of the fabricated design is assessed from measurements of the sound field produced around the cloak by a broadband source. The remarkably low complexity of the device made of perforated plastic plates shows that sound in air can be fully and effectively manipulated using realizable transformation acoustics devices.
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.

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.

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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