Engineering: Tread lightly

 

A new cloak could provide invisibility from sound waves.

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Engineers in the US have developed an ‘invisibility cloak’ that could prove imperceptible to sound detection.  This technology could have many uses, such as for hiding a submarine from sonar detection, or hiding objects that compromise the acoustics in concert halls.
 
"We've devised a recipe for an acoustic material that would essentially open up a hole in space and make something inside that hole disappear from sound waves," said Steven Cummer, Jeffrey N. Vinik Associate Professor of Electrical and Computer Engineering at Duke University's Pratt School of Engineering.
 
Currently existing as only as a theory, the acoustic cloak would work along the same principles as a ‘microwave cloak,’ as designed by researchers at Duke and the Imperial College London in 2006.  The team started with a shell, like the microwave cloak, and attempted to derive the mathematical specifications required to prevent such a shell from reflecting sound waves, a key characteristic for achieving invisibility. Theoretically, it worked.

Acoustic cloaks

"We've now shown that both 2-D and 3-D acoustic cloaks theoretically do exist," Cummer said. Although not as general as the one used to devise the microwave cloak, "it opens up the door to make the physical shape of an object different from its acoustic shape," he said.
 
The existence of an acoustic cloaking solution also indicates that cloaks might possibly be built for other wave systems, Cummer said, including seismic waves that travel through the earth and the waves at the surface of the ocean.
 
A report on the acoustically invisible cloak was published in Physical Review Letters last Friday.

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