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Portrait Monika Landgraf
Monika Landgraf
Head of Corporate Communications, Chief Press Officer

Phone: +49 721 608-47414
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Press Release 186/2011

Not Only Invisible, but Also Inaudible

KIT Researchers Transfer the Concept of an Optical Invisibility Cloak to Sound Waves
Im „Kreisverkehr“ um das ruhige Zentrum: Design (oben) und Zwischenschritt der Herstellung (unten) der elastischen Tarnkappe (Grafik: AP, KIT)
“Circling“ around the silent center: Design (top) and intermediate step of production (bottom) of the elastic invisibility cloak. (Graphics: AP, KIT)

Progress of metamaterials in nanotechnologies has made the invisibility cloak, a subject of mythology and science fiction, become reality: Light waves can be guided around an object to be hidden, in such a way that this object appears to be non-existent. This concept applied to electromagnetic light waves may also be transferred to other types of waves, such as sound waves. 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.

It is as if Harry Potter had a cloak that also makes him unhearable. “Maybe a place of peace and quiet in the Christmas season,” say the KIT researchers, who succeeded in transferring the concepts underlying the optical invisibility cloak to acoustic waves in a plate.

“The key to controlling waves is to specifically influence their local speed as a function of the ‘running direction’ of the wave,” says Dr. Nicolas Stenger from the Institute of Applied Physics (AP). 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, that is some 100 Hz, and can be observed directly from above. The scientists found that the sound waves are guided 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,” says 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.” Both physicists and Professor Martin Wilhelm from the KIT Institute for Chemical Technology and Polymer Chemistry have now published their results in the journal “Physical Review Letters.”

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 towards 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 are allowed to 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.


Karlsruhe Institute of Technology (KIT) pools its three core tasks of research, higher education, and innovation in a mission. With about 9,300 employees and 25,000 students, KIT is one of the big institutions of research and higher education in natural sciences and engineering in Europe.

KIT – The Research University in the Helmholtz Association

le, 20.12.2011

For further information, please contact:

Margarete Lehné
Phone: +49 721 608-48121
Fax: +49 721 608-45681
margarete lehnePfi3∂kit edu
The photo of printing quality may be requested by presseHbh3∂kit edu or phone: +49 721 608-47414. The press release is available as a PDF file.