New cloak provides best protection yet.
According to an article by Alexander Movchan published in January 2012 by the Department of Mathematical Sciences, University of Liverpool, Liverpool, U.K., “Cloaking devices made of a composite of soft and hard materials can divert elastic vibrational waves around an object as though it wasn’t there.”
Movchan was offering comment on work done by Martin Wegener, Nicolas Stenger and Manfred Wilhelm of the Karlsruhe Institute of Technology in Germany. “[They] have designed and tested a cloak that makes an object in a flexible medium invisible to elastic vibrational waves; that is, the waves pass by the object as though it wasn’t there. The work, which is presented in Physical Review Letters, describes a cloaking device that is both more efficient and covers a wider bandwidth than any other existing cloak,” Movchan says.
How they did it
“[The scientists made] the cloak from a 15 cm diameter, 1 mm thick disc of polyvinyl chloride (PVC), into which they etched holes arranged in 20 concentric rings. A circular region was left in the centre of the disc just slightly larger than a two-euro coin. This region constituted the item to be hidden. By filling the holes with polydimethylsiloxane (PDMS) and varying the size and spacing between holes from ring to ring, the researchers were able to vary the Young’s modulus across the disc in such a way that elastic (sound) waves within a certain frequency range approaching the disc are bent round the central region and then re-form as if there had been no obstacle in the way.
“The performance of the acoustic ‘invisibility cloak’ exceeds that of existing electromagnetic devices and could open up new ways of manipulating waves, including the development of shields against seismic waves.”
The experts are impressed
John Pendry, Imperial College London, the first designer of an electromagnetic invisibility cloak, called the latest results “an impressive achievement.” A number of research groups are now building acoustic cloaks.
Ulf Leonhardt of St Andrews University in the U.K., who has also designed invisibility cloaks, was impressed with its simplicity. “I am always amazed how much one can do just by drilling some holes in plastic,” he says.
Called by Ross McPhedran of the University of Sydney in Australia and Alexander Movchan of Liverpool University in the U.K. “the clearest demonstration of effective cloaking in the literature to this point,” they also point out that the device leaves the passing wave pattern more intact than any other cloak built to date, and it covers the greatest bandwidth—one octave, which in terms of electromagnetic waves is more than would be needed to span the whole visible spectrum.
Steven Cummer, Duke University in the U.S. notes its practical application: “I can imagine that using this one might be able to engineer structures that can reduce vibration in critical locations, and thus might be made lighter or stronger than otherwise,” he says. “And that’s the sort of thing that would be valuable in a lot of different applications.”