Acoustic cloak from metamaterials

We've heard a lot about how someday metamaterials -- composites that bend electromagnetic waves in bizarre ways -- could lead to invisibility cloaks. Also in the lab are acoustic cloaks that bend sound waves to hide objects underneath. Researchers from Duke University have just reported on their success making a small acoustic cloak that works in air, not just water as has already been demonstrated. Eventually, the metamaterial could be used to tweak acoustics in concert halls at a very high resolution or hide submarines from sonar. From Science News:

To manipulate sound waves in air, (electric engineer Steven) Cummer’s team designed and built a cloak that sits atop an object like a piece of draped carpet. By layering simple metamaterial building blocks – ordinary strips of perforated plastic – the researchers hid a triangular wooden block a couple of inches high and more than a foot long at its base.

Sound waves over a range of high but audible frequencies slowed and changed direction cleanly after striking the holey plastic. Most reemerged appearing to have traveled all the way down to the flat surface beneath the block.

The prototype is two-dimensional – both the speaker generating the sound and the microphone recording it must be in the same plane above the object. But Cummer believes he could make a 3-D version that would cover an entire bump on a log, not just a slice.

“We are still a long way from a full acoustic cloak, but this carpet cloak is a step in the right direction,” says Andrew Norris, a physicist at Rutgers University in Piscataway, N.J.

"You haven't heard it all"

• [js7a]

  You can tweak room acoustics with all different kinds of materials in any position you can imagine, but simulation modeling software isn’t good enough yet so it all has to be done by trial and error or a full scale model of the space. Architects can make some educated guesses, but it’s all prayer until the last strip of carpet and upholstered chair is installed. And then, the parade of aggregate subjective score judges begins, with people positioned in seats scattered all over the room at different occupancy levels. If it’s not good enough you guess at something from the simulations with the benefit of knowing where the scores need to improve the most; for example something like a few strips of vinyl or glass across one side of the ceiling, and then you run the subjective scores again. Repeat until the scores are satisfactory or you run out of time, money, or patience. Room acoustics simulation won’t be reliable for a decade, I’d bet, because it’s just too computationally intensive and nobody has accurate enough acoustic model of particular kinds of carpet etc. (and when they do, the carpet manufacturer will arbitrarily change materials or methods enough to make the formerly good models worthless.)

• [Anonymous]

  Bats everywhere are trembling at the thought of smashing into invisible bumps on logs.

• [awjtawjt]

  “Cummer”?

• [NuOrder72]

  Can one of these be used to cancel out “road noise?” Our backyard backs up to a rather busy road and I would love to bring it down a few decibels.

• [Vnend]

  So we are finally getting smart enough to build a Cone of Silence was only 50 something years later? The future is running late.

• [Anonymous]

  the thing about metamaterials is that they only work in extremely narrow bands… that is they can make a specific color disappear, or perhaps a single note, but all other frequencies (EM or sound, whatevs) are reflected quite a bit.

  so… cloaking a submarine in this stuff would just force sonar onto a slightly different frequency, where the submarine would then appear nice and bright on the scope.

  • [knoxblox]

    Despite the narrowness of the band, I wouldn’t be surprised if the military isn’t already seeing possibilities for further suppression of noise from guns used in sniper situations.