Normally, sound and other waves travel equally in both forward and backward directions. Great for conversations as you can hear each other’s voices. However, in some technical applications, like light or microwaves, it may be better for the waves to travel in only one direction to avoid unnecessary reflections.
About 10 years ago, researchers discovered a way to prevent sound waves from traveling backwards. But this also weakened the advancing wave.
A research team at ETH Zurich, led by Professor Nicolas Noiray, in collaboration with EPFL’s Romain Fleury, has developed a way to keep sound waves moving forward powerfully, while stopping them from traveling backwards.
This one-way sound system is based on self-oscillation, where a dynamic system periodically repeats its behavior. The researchers say the method could also be applied to electromagnetic waves.
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Professor Nicolas Noiray has devised a clever way to move sound waves in only one direction using harmless, self-sustaining aeroacoustic vibrations via a circulator. In this setup, the natural attenuation of the sound waves is balanced by the circulator’s self-oscillation, helping it capture energy in sync with the incoming waves.
The circulator is a disc-shaped cavity into which air is blown from one side through an opening in the center. Adjusting the blow speed and swirl strength will create a whistling sound inside the cavity.
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Unlike regular whistles, which generate sound from standing waves in a cavity, this new whistle generates sound from rotating waves.
It took time to experiment with ideas. First, Noiret and his team studied the hydrodynamics of a rotating wave whistle. Next, we added three acoustic waveguides arranged in a triangular shape around the edge of the circulator.
Sound waves enter through the first waveguide and exit through the second waveguide. However, if a wave enters through a second waveguide, it cannot travel “in the opposite direction” through the first waveguide. Instead, it exits through a third waveguide.
After several years of development and theoretical modeling, ETH researchers were finally able to test the circulator in experiments. The researchers sent a sound wave at a frequency of about 800 hertz (a sound similar to the high G note a soprano sings) into the first waveguide and then looked at how well it traveled into the second and third waveguides. was measured.
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As expected, the sound waves did not reach the third waveguide. But a sound wave, even stronger than the original wave they sent, came out of the second waveguide (in the “forward” direction).
“We believe this concept of loss-compensated non-reciprocal wave propagation is an important achievement that can be applied to other systems,” said Noiray.
Reference magazines:
Pedergnana T, Faure-Beaulieu A, Fleury R, Noiray N: Loss-compensated nonreciprocal scattering based on synchronization. Nature Communications 15, 7436 (2024). DOI: 10.1038/s41467-024-51373-y
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