In context: Sound waves typically propagate in forward and backward directions. This natural movement is problematic in some situations where unwanted reflections cause interference or reduced efficiency. So, researchers developed a method to make sound waves travel in only one direction. The innovation has far-reaching applications that go beyond acoustics, such as radar.
After years of research, scientists at ETH Zurich have developed a method to make sound waves travel in a single direction. The study was led by Professor Nicolas Noiray, who has spent much of his career studying and preventing potentially dangerous self-sustaining thermo-acoustic oscillations in aircraft engines, believed there was a way to harness similar phenomena for beneficial applications.
The research team, led by Professor Nicolas Noiray from ETH Zurich’s Department of Mechanical and Process Engineering, in collaboration with Romain Fleury from EPFL, figured out how to prevent sound waves from traveling backward without weakening their forward propagation, building upon similar work from a decade ago.
At the heart of this breakthrough is a circulator device, which utilizes self-sustaining aero-acoustic oscillations. The circulator consists of a disk-shaped cavity through which swirling air is blown from one side through a central opening. When the air is blown at a specific speed and swirl intensity, it creates a whistling sound in the cavity.
Unlike conventional whistles that produce sound through standing waves, this new design generates a spinning wave. The circulator has three acoustic waveguides arranged in a triangular pattern along its edge. Sound waves entering the first waveguide can theoretically exit through the second or third but cannot travel backward through the first.
The critical component is how the system compensates for the inevitable attenuation of sound waves. The self-oscillations in the circulator synchronize with the incoming waves, allowing them to gain energy and maintain their strength as they travel forward. This loss-compensation approach ensures that the sound waves not only move in one direction but also emerge stronger than when they entered the system.
To test their design, the researchers conducted experiments using sound waves with a frequency of approximately 800 Hertz, comparable to a high G note sung by a soprano. They measured how well the sound was transmitted between the waveguides and found that, as expected, the waves did not reach the third waveguide but emerged from the second waveguide even stronger than when they entered.
“In contrast to ordinary whistles, in which sound is created by a standing wave in the cavity, in this new whistle it results from a spinning wave,” said Tiemo Pedergnana, a former doctoral student in Noiray’s group and lead author of the study.
While the current prototype serves as a proof of concept for sound waves, the team believes their loss-compensated non-reciprocal wave propagation method could have applications beyond acoustics, such as metamaterials for electromagnetic waves. This research could lead to advancements in areas such as radar technology, where better control over microwave propagation is essential.
Additionally, the technique could pave the way for developing topological circuits, enhancing signal routing in future communication systems by providing a method to guide waves unidirectionally without energy loss. The research team published its study in Nature Communications.