Invisibility cloak that is also inaudible?

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Harry Potter fans may be keen to learn that the tantalising concept of an invisibility cloak becoming reality could be one step closer, according to German researchers from Karlsruhe Institute of Technology (KIT).

Progress of metamaterials in nanotechnologies enables light waves to be guided round an object to be hidden in such a way that the object appears to be non-existent. The researchers have applied this concept to electromagnetic light waves and it may also be transferred to other types of waves – such as sound waves. This technique has been demonstrated for the first time in an invisibility cloak for elastic waves – the kind that also occur in strings of a guitar or drum membranes. By transferring the concepts underlying the optical invisibility cloak to acoustic waves in a plate, the researchers may have paved the way for an invisibility cloak that is also inaudible. The team believes the key to controlling waves is to specifically influence their local speed as a function of the 'running direction' of the wave. In an experiment, they used a smartly microstructured material composed of two polymers – a soft and a hard plastic in a thin plate. The vibrations of the plate were around 100Hz, within the range of acoustic frequencies, and could be observed directly from above. Observations revealed that the sound waves were guided around a circular area in the millimetre thin plate in such a way that vibrations could neither enter nor leave this area. Professor Marin Wegener from the Institute of Applied Physics and coordinator of the DFG Center for Functional Nanostructures (CFN) at KIT, said: "Contrary to other known noise protection measures, the sound waves are neither absorbed nor reflected. It is as if nothing was there." The findings have been published in Physical Review Letters.