By Laura Shin
Posting in Design
Scientists have demonstrated a "cloak" that makes objects invisible to sound waves, which could someday be used to conceal ships from sonar.
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)
Jun 27, 2011
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there is scientific evidence invisible cloak that can trigger the sea animals to create tsunamis, that what project harp told you about, it seems 1940 invention they develop the devastating man made disaster, base of my concerned there are many strategic event that conspiracy may arise...
I don't think this article explains the uses to which the 'cloak' could be put. Sonar works as in radar, by sending out a signal, then using the echo from an object, [in this case a ship], to: [a], detect the presence of, [b], the direction of, and [c], the distance travelled by the signal. In this case, it seems, the signal would travel through the 'cloak', bounce off the object, back through the cloak, and be detected. If sound were to be generated by the object of the search, then it would pass through the 'cloak' and be detected as normally. As far as the sea animals' safety is concerned, if this system worked, and came into general use, then sonar would be used less, and possibly the fears for the orientational confusion of cetaceans would happily be addressed. Ian Colley.
Submarines are routinely built with rubber-like sound absorbent coverings. There are pictures of boats with square depressions where tiles of these materials have dropped off. US Navy (and, presumably, our allies) have used a device code named Prairie Masker since the Cold War. It generates bubbles that reflect the noise made by a ship back toward itself, concealing the ship from submarines.
This is no new technology, that is pushing the envelope, just ask any man who has been married for 10 years or more. I have been cloaking my wife's voice for over 20 years (been married 30, but it took me ten to perfect the technology) :-)
This sounds great. As most technology benefits (or intended to) humankind. However I hope that they will consider the possible danger to sea life such as the porpoise. Would they not "see" the object disguised and collide with fatal results?
... but real invisibility involves reconstructing the incoming wave pattern on the other side. This seems more of a 'black hole' which can be 'seen' as a feature obscuring the background.
A real Invisibility Cloak from Michigan Technological University: Now You See It, Now You Don't--An Invisibility Cloak Made of Glass http://www.mtu.edu/news/stories/2010/july/story29391.html
It would be quite a stretch but perhaps someday this could help make supersonic transport more practical (i.e. quiet)