It is as if Harry Potter had a cloak that also makes him unhearable. âMaybe a place of peace and quiet in the Christmas season,â say the KIT researchers, who succeeded in transferring the concepts underlying the optical invisibility cloak to acoustic waves in a plate.
âThe key to controlling waves is to specifically influence their local speed as a function of the ârunning directionâ of the wave,â 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. âContrary to other known noise protection measures, the sound waves are neither absorbed nor reflected,â says Professor Martin Wegener from the Institute of Applied Physics and coordinator of the DFG Center for Functional Nanostructures (CFN) at KIT. âIt is as if nothing was there.â Both physicists and Professor Martin Wilhelm from the KIT Institute for Chemical Technology and Polymer Chemistry have now published their results in the journal âPhysical Review Letters.â
âCirclingâ around the silent center: Design (top) and intermediate step of production (bottom) of the elastic invisibility cloak. (Graphics: AP, KIT)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) is a public corporation according to the legislation of the state of Baden-Württemberg. It fulfills the mission of a university and the mission of a national research center of the Helmholtz Association. KIT focuses on a knowledge triangle that links the tasks of research, teaching, and innovation.
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