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Talk about the ultimate physical security program, the invisibility cloak is becoming a reality thanks to progress made in metamaterials in nanotechnologies.

Technology allows for light waves to go around an object in such a way the object appears to be non-existent. This concept applied to electromagnetic light waves may also transfer to other types of waves, such as sound waves.

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Researchers from Karlsruhe Institute of Technology (KIT) have now succeeded in demonstrating for the first time an invisibility cloak for elastic waves. Such waves also occur in strings of a guitar or drum membranes.

“The key to controlling waves is to specifically influence their local speed as a function of the ‘running direction’ of the wave,” said Dr. Nicolas Stenger from the Institute of Applied Physics (AP).

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In his experiment, he used a smartly microstructured material composed of two polymers: A soft and a hard plastic in a thin plate. The vibrations of this plate are in the range of acoustic frequencies at 100 Hz, which you can see directly from above.

The scientists found they can guide the sound around a circular area in the millimeter-thin plate in such a way that vibrations can neither enter nor leave this area.

“Contrary to other known noise protection measures, the sound waves are neither absorbed nor reflected,” said Professor Martin Wegener from the Institute of Applied Physics and coordinator of the DFG Center for Functional Nanostructures (CFN) at KIT. “It is as if nothing was there.”

The scientists explain their idea by the following story: A city, in the shape of a circle, suffers from noisy car traffic through its center. Finally, the mayor has the idea to introduce a speed limit for cars that drive directly toward the city: The closer the cars come to the city area, the slower they have to drive. At the same time, the mayor orders to build circular roads around the city, on which the cars can drive at higher speeds. The cars can approach the city, drive around it, and leave it in the same direction in the end. The time required corresponds to the time needed without the city. From outside, it appears as if the city was not there.

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