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Duke Scientists Find Recipe For 'Sound Cloaking'

Just like an electromagnetic cloak can make an object invisible to radar, a sound cloak can make an object invisible to sonar.
Scientists at Duke University's Pratt School of Engineering this week said they found a recipe for artificial materials that potentially could be used to create a 3-D "sound cloak," an idea that refers to changing the shape of sound.

The sound cloak behaves similarly to the "invisibility cloak" idea previously demonstrated by the university, where the cloak deflects microwave beams so they flow around an object with little distortion, making it appear like the object isn't there.

In the case of the sound cloak, it's as if there was a hole opened up in space where the object inside would disappear from sound waves, said Steven Cummer, Duke associate professor of electrical and computer engineering, who made the discovery with his team.

"When we first demonstrated the electromagnetic invisibility cloak, we questioned if that idea could be extended to other kinds of waves. The early ideas suggested that it was not possible. But now we know it's possible because we found the recipe," Cummer told InformationWeek.

The recipe Cummer referred to includes the materials needed to build the sound cloak. Since the sound cloak's properties are not found in nature, the materials would have to be developed artificially.

"You can't just go to Home Depot and buy these materials. You can build artificial materials using composite metamaterials that do have the property to make sound waves behave differently," Cummer said.

The sound cloak is just a theory at this point, but it provides a path for building something that works. It's difficult to predict if the sound cloak will evolve into a practical application down the road. Cummer said he wouldn't be surprised, however, to see some kind of experimental demonstration of the concept in a year or two.

One possible implementation of the sound cloak could be in the military, where sound waves are used routinely to detect objects in the ocean. Just like an electromagnetic cloak can make an object invisible to radar, a sound cloak can make an object invisible to sonar, said Cummer.

The idea also can be used to design the acoustics of a building or a room by shaping sound differently. Imagine being able to eliminate the sound that reflects off of objects. Auditoriums and studios could be designed in a whole new way.

With the new discovery, it's possible that this idea could be applied to other types of waves as well, including waves on the surface of water and seismic waves. Cummer sounded hopeful: "If it works for one, it's likely to work for most."

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