One of the greatest perils of wearing an invisibility cloak has always been that someone could hear you tiptoeing about, even if they couldn't see you.
Well, scientists have now solved that problem by creating a "cloak" that makes objects invisible to sound waves.
Okay, so it's not quite a wearable cloak yet. However, the material could be used to conceal a ship from sonar or to improve the soundproofing in concert halls.
The theory behind this "acoustic cloaking" was proposed in 2008, but only now has it been successfully demonstrated to work in air with sounds detectable by the human ear. (Earlier this year, other researchers showed it could work under water at frequencies inaudible to humans.)
How it works
The acoustic cloak and the invisibility cloak are both made of the same stuff -- metamaterials, which are are man-made materials that have properties not found in nature.
In an invisibility cloak, metamaterials force light waves to travel around an object, making it look as though the object were not there.
Theoretically, the acoustic cloak works the same way, just with sound waves. The BBC reports:
"Fundamentally, in terms of hiding objects, it's the same - how anything is sensed is with some kind of wave and you either hear or see the effect of it," said Steven Cummer of Duke University [who published his findings in Physical Review Letters].
However, because sound and light waves behave differently, the acoustic and invisibility cloaks are structured very differently.
Dr. Cummer and his colleagues, who first described the theory of acoustic cloaking in an article in Physical Review Letters in 2008, cloaked audible frequencies between one and four kilohertz, which, on a piano, cover two octaves in the upper half of notes.
They accomplished this by stacking plastic sheets that had a regular pattern of holes in them. The holes were sized and arranged in a specific relationship to each other so that incoming sound waves would be reflected as though the stack were not there.
When the researchers tested the ability of these stacked sheets to cloak sound, they placed the cloak on a flat surface with a four-inch block of wood underneath. No sound waves passed through the block and it was impossible to detect the wood block from outside the stacked sheets using sound waves.
So far, the cloak has only been demonstrated to work for very directed sound waves in two dimensions. However, Ortwin Hess, a director of Imperial College London's Centre for Plasmonics and Metamaterials, told the BBC that the approach is notable for its simplicity.
"It's almost like someone could take a pencil and poke holes in a particular way in the plastic."
He believes that with more work, it will be possible to do this with three dimensions.
Still, this demonstration shows not only how a ship could be concealed from sonar but also how it could be protected from incoming sound. The acoustic cloak could also be used to soundproof a studio or refine the acoustics of a concert hall.
And who knows, perhaps someday it could be used to erase the sounds you make as you glide around in your invisibility cloak.
Photos: Top: U.S. Navy photo by Photographer's Mate Airman Doug Pearlman. (Wikimedia Commons); Bottom: The acoustic cloak. (Courtesy of Steve Cummer)