In the ETH Zurich experiment, self-oscillations (blue-red) cause sound waves (green, orange, purple) to propagate in the circulator in only one direction. Credit: Xin Zou
Researchers at ETH Zurich have succeeded in ensuring that sound waves propagate in only one direction. This method could also be used in technical applications involving electromagnetic waves in the future.
Water, light, and sound waves generally propagate forward and backward in the same way. Therefore, when we talk to someone who is some distance away from us, they can hear us just as well as we can. This is useful in a conversation, but in some technical applications it would be preferable for waves to be able to propagate in only one direction, for example to avoid unwanted reflections of light or microwaves.
Ten years ago, researchers succeeded in suppressing the backward propagation of sound waves; however, this also attenuated the forward propagating waves.
A team of researchers from ETH Zurich led by Nicolas Noiray, professor of combustion, acoustics and flow physics, in collaboration with Romain Fleury from EPFL, has now developed a method to prevent sound waves from traveling backwards without impairing their forward propagation.
In the future, this method, which was recently published in Nature Communicationscould also be applied to electromagnetic waves.
The basis of this one-way phenomenon for sound waves is self-oscillation, in which a dynamic system periodically repeats its behavior. “I have spent a good part of my career preventing such phenomena,” Noiray explains.
In particular, he studies how self-sustained thermoacoustic oscillations can occur in the combustion chamber of an aircraft engine from the interaction between sound waves and flames, which can lead to dangerous vibrations. In the worst case, these vibrations can destroy the engine.
Schematic of the experimental setup (left) and wave propagation (right): waveguide 1 is perfectly audible to waveguide 3, but not to waveguide 2, and waveguide 3 is perfectly audible to waveguide 2, but not to waveguide 1. As expected, the waves can only propagate in one direction. Credit: Nicolas Noiray / ETH Zurich
Harmless and useful self-oscillations
Noiray had the idea of using self-sustaining and harmless aeroacoustic oscillations to allow sound waves to pass in a single direction and without any loss through a circulator. In his system, the inevitable attenuation of the sound waves is compensated by the self-oscillations of the circulator which synchronize with the incoming waves, allowing them to draw energy from these oscillations.
The circulator itself was to consist of a disc-shaped cavity through which swirling air is blown from one side through an opening in its center. For a specific combination of blowing speed and swirl intensity, a whistling sound is thus created in the cavity.
“Unlike ordinary whistles, in which the sound is created by a standing wave in the cavity, in this new whistle it results from a rotating wave,” explains Tiemo Pedergnana, a former doctoral student in Noiray’s group and lead author of the study.
From idea to experiment, it took time. Noiray and his colleagues first studied the fluid mechanics of the rotating wave whistle, then added three acoustic waveguides, arranged in a triangle shape along the edge of the circulator.
Sound waves entering through the first waveguide can exit the circulator through the second waveguide. However, a wave entering through the second waveguide cannot exit “backwards” through the first waveguide, but can do so through the third waveguide.
Sound waves as a toy model
The ETH Zurich researchers developed and theoretically modeled the individual parts of the circulator over several years. They were finally able to demonstrate experimentally that their loss compensation approach works. They sent a sound wave with a frequency of about 800 hertz (about the high G of a soprano) through the first waveguide and measured the quality of its transmission to the second and third waveguides.
As expected, the sound wave did not reach the third waveguide. However, from the second waveguide (in the “forward” direction), an even more powerful sound wave emerged than the one originally sent.
“This concept of non-reciprocal wave propagation with compensation of losses is, in our opinion, an important result that can also be transferred to other systems,” says Noiray. He sees his sound wave circulator primarily as a powerful toy model for the general approach of wave manipulation using synchronized self-oscillations that can, for example, be applied to metamaterials for electromagnetic waves.
In this way, microwaves from radar systems could be better guided and so-called topological circuits could be realized, with which the signals could be routed in future communication systems.
More information:
Tiemo Pedergnana et al, Loss-compensated nonreciprocal diffusion based on synchronization, Nature Communications (2024). DOI: 10.1038/s41467-024-51373-y
Quote: Researchers make sound waves travel in one direction, with implications for electromagnetic wave technology (2024, September 6) retrieved September 6, 2024 from
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