Acoustic Cloaking Device Lets Sound Travel Uninterrupted Around Objects

Sound Wave Propagation Along a Cylinder

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A new type of acoustic cloak would allow sound waves to travel around an object unimpeded, and could be used to build better concert halls, quiet spaces and noise-shielding head gear, researchers say.

Much like an optical invisibility cloak works to fool light waves, an acoustic cloak would allow sound waves to travel around an object like water flowing around a smooth rock in a stream. To the sound wave, whose wave form would propagate without obstruction, it would seem as though nothing is present. A noise-canceling system, on the other hand, would create an exact opposite wave form to cancel out the sound wave.

A wave-form-preserving cloak would be useful for objects trying to evade sonar, because it would appear there is nothing for sound waves to hit and bounce back. But it would also have applications in areas where you'd want sound to be preserved or channeled, like a concert hall.

A team of researchers from Valencia, Spain, propose a system of cylinders that work together to allow sound waves to travel uninterrupted. Their design involves 120 separate 15-millimeter cylinders arranged around an object to be cloaked, in this case another cylinder measuring 22.5 centimeters in diameter. A high-pitched sound wave, at 3061 hertz in this case, would maintain its original wave-front pattern as it moved around the object. To the sound waves, then, the 22.5-cm cylinder would be invisible (unhearable?)

The cloak only works in a narrow operating band, but a different frequency would just require a different cylinder setup, according to the researchers. It could conceivably be scaled up to a wide range of frequencies, to create better acoustics in concert halls, quieter public parks and even helmets that protect the ears from loud noises, according to the American Institute of Physics. The system is described in the AIP journal _ Applied Physics Letters_.