Researchers succeed in making sound waves propagate in only one direction, with implications for electromagnetic technology

Researchers from ETH Zurich have succeeded in making sound waves travel in only one direction – a method that could potentially be used in future for technological applications based on electromagnetic waves.

Water, light and sound waves normally travel equally well in both directions, forward and backward. So when we are talking to someone standing some distance away, they can hear us just as well as we can hear them. This is useful when having a conversation, but in some technical applications it may be desirable for waves to travel in only one direction, for example to avoid unwanted reflections of light or microwaves.

A decade ago, researchers managed to suppress the backward propagation of sound waves, but this also attenuated the forward-propagating sound waves.

A research team from ETH Zurich led by Nicolas Noiret, professor of combustion, acoustics and flow physics, in collaboration with Romain Fleury at EPFL, has developed a way to prevent sound waves from propagating backwards without worsening their forward propagation.

In the future, this method will be Nature CommunicationsIt can also be applied to electromagnetic waves.

This one-way principle of sound waves is self-oscillation, in which a dynamical system repeats its motion periodically. “I’ve actually spent a good part of my career trying to prevent this from happening,” Noirey says.

He studies how the interaction of sound waves and flames in the combustion chambers of aircraft engines can generate self-sustaining thermoacoustic oscillations that can lead to dangerous vibrations. In the worst case scenario, these vibrations can destroy the engine.

Harmless and useful self-oscillation

Noiret came up with the idea of ​​using harmless self-sustaining aeroacoustic vibrations to allow sound waves to pass without loss in only one direction through a so-called circulator. In his plan, the inevitable attenuation of sound waves would be compensated for by their self-oscillations in sync with the incoming waves in the circulator, allowing the sound waves to gain energy from the vibrations.

The circulator itself consists of a disk-shaped cavity with a central opening into which swirling air is blown in from one side. A particular combination of blowing speed and swirling strength produces a whistling sound within the cavity.

“In a normal whistle, the sound is produced by standing waves in a cavity, but in this new whistle, the sound is produced by rotating waves,” explains Tiemo Pedergnana, a former doctoral student in Noirey’s research group and lead author of the study.

It took a while to go from idea to experiment: First, Noirey and his colleagues investigated the fluid dynamics of a rotating wave whistle, then they added three acoustic waveguides arranged in a triangle along the edge of the circulator.

Sound waves sent through the first waveguide can exit the circulator through the second waveguide, but waves entering through the second waveguide cannot exit “backwards” through the first waveguide, but can exit through the third waveguide.

Sound waves as a toy model

The ETH researchers have been developing and theoretically modelling the various components of the circulator for several years, and now they have finally been able to experimentally demonstrate that their loss-compensation approach works: They sent sound waves with a frequency of about 800 Hertz (roughly the high note g on a soprano) into the first waveguide and measured how well it was transmitted to the second and third waveguides.

As expected, the sound waves did not reach the third waveguide, but the second waveguide (in the “forward” direction) emitted a sound wave that was even stronger than the first one sent.

“This concept of loss-compensated nonreciprocal wave propagation is, in our view, an important achievement that can be applied to other systems,” says Noirey, who sees the acoustic circulator primarily as a powerful toy model for a general approach to wave manipulation using synchronous self-oscillations that could be applied, for example, to metamaterials for electromagnetic waves.

In this way, it will be possible to better guide microwaves in radar systems and realize so-called topological circuits to route signals in future communication systems.