Scientists create the sound of silence

Subject: Scientists create the sound of silence Sat Feb 16, 2008 6:50 am

Scientists have created the sound of silence, an "acoustic cloak" that could one day block out the din of noisy neighbours.

Two teams have come up with a design of special materials that could cloak an object from sound and a third has already devised a scheme to create them, so sound waves travel seamlessly around them.

The work follows recent research on "invisibility cloaks", where a number of teams have shown that synthetic materials, called metamaterials - which are designed down to the microscopic level - can make beams of light flow around an object to make it invisible.

Physicists had suspected that it should be possible to create similar "acoustic cloaks". But there were technical problems in creating one that worked in three dimensions, with some even believing that it would prove impossible.

Now one team, including Sir John Pendry of Imperial College London and Dr Steven Cummer at Duke University in North Carolina, reports in Physical Review Letters that it has come up with a way to do it, "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," says Dr Cummer.

Drs Huanyang Chen and Che Ting Chan at Hong Kong University of Science and Technology have done it too, describing in the journal Applied Physics Letters the mathematics to demonstrate the feasibility of a three-dimensional acoustic cloaking device, if a "metamaterial" with very demanding properties could be engineered, since nothing like it exists in nature.

"Recently, it was proved that an "invisbility cloak" can be achieved for electromagnetic waves," says Dr Chan. "Our work shows that the same can be done for acoustic waves, making an object invisible to probing with sound sources (for example, sonar)."

Now an effort to make an acoustic cloak is under way by Dr Jose Sanchez-Dehesa Moreno and colleagues at the Polytechnic University of Valencia, Spain.

The cloak would be made up of cylindrical rods of the right size and mechanical properties to interact with sound and he tells The Daily Telegraph that his new work represents "an important advance in order to make feasible the acoustic cloaking. Our results are still theoretical predictions and numerical simulations but, at least, we think we have a first proposal that could be engineered."

In principle this would allow any object, from a submarine to a boisterious parry, to become inaudible by bending sound waves Other applications could be found in improving the acoustics of concert halls, for example Dr Chan says that acoustic cloaking would probably work best for certain frequencies.

"No one can design a cloak that works for all frequencies." However, Dr Moreno says he believes "a cloaking device working in a wide range of wavelengths will be possible, following our proposal."

http://www.sott.net/articles/show/149039-Scientists-create-the-sound-of-silence

Scientists create the sound of silence

Scientists create the sound of silence

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