We are constantly dealing with waves that are distracted in a complicated way: A beam of light passes through a glass of milk and is scattered in all directions. Electromagnetic waves from the cell phone pole are scattered or absorbed so that we are annoyed by poor reception indoors.
At the TU Vienna, methods are being developed to manipulate waves in a targeted manner so that they can move practically undisturbed. In cooperation with a research group of the École polytechnique fédérale de Lausanne (EPFL) and the University of Crete, this method has now been implemented in experiments. With precisely controlled loudspeakers it was possible to send a sound wave through a tube with various obstacles. In the long term, such technologies could lead to manipulating light waves and making objects invisible.
In order to test the concept for lossless wave transport, it was decided to use sound waves. "Our technology can be applied to any kind of wave," says Prof. Stefan Rotter from the Institute of Theoretical Physics at Vienna University of Technology. "Mathematically speaking, it does not matter whether the waves are light waves, sound waves, or quantum-physical matter waves - but in acoustics the experiments can be carried out in an especially illustrative manner.
In order to manipulate the wave in exactly the right way, energy must be supplied or drawn off at certain locations. This is achieved with special loudspeakers mounted along a meter-long sound tube. "However, the speakers are not designed to simply reproduce the original sound wave on the other side of the tube - that would be too easy," explains Andre Brandstötter, a co-author of the study and doctoral student in Stefan Rotter's group. "It's about manipulating the sound wave point by point and guiding it through the tube so that it always has exactly the same strength at certain points in the tube.
The speakers are controlled so that the wave is locally amplified or attenuated. "This enables us to counteract the complicated scattering that would otherwise be unavoidable when the wave encounters an obstacle," says Rotter.
The experiment was carried out with an air-filled tube in which irregular obstacles were installed. If you send a sound wave through this pipe, practically no sound arrives at the end. However, if the loudspeakers inserted into the tube are controlled according to the mathematical rules developed by the team at TU Vienna, the sound wave leaves the tube as if it had not encountered a single obstacle on the way.
The experiment in Lausanne shows that the wave manipulation technologies of the TU Vienna are actually suitable for practical use. The goal is now to further expand the possibilities of this technology. "If the same can be achieved in three-dimensional space with light waves, objects could in principle be made invisible," says Stefan Rotter. While some further development steps are of course necessary for a possible "stealth cap", the new technology could already be of great interest for various communications applications today.