Researchers claim cloak can render subs invisible to sonar

PORTLAND, Ore. — Duke University engineers will reveal Friday (Jan. 11) details of an acoustic cloak fabricated from metamaterials that they claim can render objects invisible to sonar.

Although the acoustic cloak has only been mathematically simulated, the engineers claim that devices based on their blueprint can make submarines invisible to sonar. They work by redirecting sound waves around the hull so that they emerge on the other side without distortion.

"We have shown that acoustic cloaks theoretically do exist," said Duke University professor Steven Cummer. "Our recipe shows how to make an acoustic material that essentially opens up a hole in space--making whatever is inside that hole 'disappear'."

Further, the engineers claim that the technique proves that waves can be redirected around objects in different media, opening up the possibility of improving the acoustics in concert halls by cloaking structural beams from sound waves in air. It may even be possible to redirect seismic waves around buildings, or ocean waves around ships.

Over a year ago, Duke University physicist David Schurig, working with professors David Smith at Duke and John Pendry at the Imperial College in London, demonstrated an invisibility cloak that could shield a seven-cubic-inch area from microwave detection. The microwave cloak used metamaterials to redirect electromagnetic waves around objects, making them invisible to radar.

Now Cummer, of Duke's Pratt School of Engineering, has created a blueprint for performing the same feat with sound waves. Cummer collaborated with Schurig (now a professor at North Carolina State University) Smith and Pendry, as well as professor Marco Rahm at Duke and Anthony Starr, an engineer at SensorMetrix Inc. (San Diego).

Metamaterial array resonators in free space, which enable properties not possible with natural materials.

Like the microwave cloak, the acoustic cloak depends on metamaterials--composite materials that enjoy properties impossible for natural materials, by virtue of an internal structure that resonates in the presence of waves. For instance, the microwave cloak used arrays of split-ring resonators in free-space. When microwaves passed through them, the resonators behaved as if they were a dielectric whose magnetic permeability and electrical permittivity could be fashioned in any combination--even ones impossible for natural materials.

Natural materials possess only positive values for their index of refraction, but metamaterials can have both positive or negative indices. By tailoring the index of refraction of each split-ring resonator in the cloak--bending waves in one direction at the front of an object and then in the opposite direction at the back of an object--microwaves were bent around the cloaked area, essentially rendering it invisible to radar.

Several groups have since verified that microwave cloaking does work as reported, but at least one group has claimed that acoustic cloaks in free space are impossible. In contrast, Cummer claims that acoustic cloaks are possible, and he is disclosing details of his design so that engineers can verify his group's results.

Besides fabricating an acoustic cloak based on the blueprint, the researchers also claim that the technique will make it possible to use metamaterials to bend and concentrate sound waves in new ways.