Why are sound waves invisible

Physicists send sound waves unchanged through the labyrinth

Vienna physicists have succeeded in sending sound waves unchanged through a "labyrinth" riddled with obstacles. The experiment, about which the researchers from Austria, Greece and Switzerland report in the journal "Nature Physics", served to test an approach that could also be used in future cloaks.

Materials are opaque when light waves are reflected on the surface or are chaotically scattered and absorbed countless times inside. However, if the waves can traverse a structure unscathed, it becomes invisible. With this in mind, scientists around the world are working on various stealth methods.

In the past few years, a team led by Stefan Rotter from the Technical University (TU) Vienna has developed a theoretical concept in which a wave is continuously "repaired" on its way through a special material by means of precise local reinforcement or weakening. The wave can be controlled by the object as if it were not there. "From a mathematical point of view, it does not matter whether it is light waves, sound waves or quantum physical matter waves - but in acoustics the experiments can be carried out particularly clearly," said Rotter.

Tube with obstacles

To test the approach to wave manipulation, researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL) built irregular obstacles into an air-filled tube. Under normal circumstances, practically no sound arrives at the end of the meter-long pipe after this obstacle course.

However, if the loudspeakers in the tube were controlled according to the mathematical rules developed by the Viennese physicists, the sound wave came out unchanged at the other end. "The point is to manipulate the sound wave point by point and to guide it through the pipe, so to speak, so that it always has exactly the same strength at certain points in the pipe," explained Andre Brandstötter, co-author of the study.

The experiment shows that the concept is practical. "If the same can be done in three-dimensional space with light waves, you could in principle make objects invisible," said Rotter. There is still a long way to go before then, he admitted. However, the approach could soon be used in the field of messaging, the researcher said. (red, APA, July 2nd, 2018)