Cloak of silence

Scientists have shown off the blueprint for an “acoustic cloak”, which could make objects impervious to sound waves.

Being woken in the dead of night by noisy neighbours blasting out music could soon be a thing of the past. The technology, outlined in the New Journal of Physics, could be used to build sound-proof homes, advanced concert halls or stealth warships.

Scientists have previously demonstrated devices that cloak objects from microwaves, making them “invisible”. “The mathematics behind cloaking has been known for several years,” said Professor John Pendry of Imperial College London, UK, an expert in cloaking. “What hasn’t been available for sound is the sort of materials you need to build a cloak out of.” The Spanish team who conducted the new work believe the key to a practical device are so-called “sonic crystals”. These artificial composites, also known as “metamaterials”, can be engineered to produce specific acoustical effects.

“Unlike ordinary materials, their acoustic properties are determined by their internal structure,” explained Professor Pendry.  These would be used to channel any sound around an object, like water flowing around a rock in a stream. “The idea of acoustic cloaking is to deviate the sounds waves around the object that has to be cloaked,” said Jose Sanchez-Dehesa of the Polytechnic University of Valencia, one of the researchers behind the new work. He believes a material that consists of arrays of tiny cylinders would achieve this effect. Simulations showed that 200 layers of this metamaterial could effectively shield an object from noise. Thinner stacks would shield an object from certain frequencies. “The thickness depends on the wavelength you want to screen,”

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