Scientists Develop One-Way Sound Wave Propagation, Paving the Way for Advances in Electromagnetic Wave Technology

Recently, researcher at the Swiss-based ETH Zurich university was able to create one-way sound waves. Hence, there could be future uses that could be applied in technology relating to electromagnetic wave.

Usually, waves like water waves, wave of light, sound waves move either in forward direction or in backward direction. This characteristic let us always understand each other from the distance, as we can hear the interlocutor as clearly as he or she hears us, which is crucial for dialogue. But in certain specific technical situations it would be desirable if the waves propagate only in one direction for instance to block back reflecting light or microwaves.

Ten years ago, researchers successfully control sound waves in such manner that the waves cannot propagate backward yet at the same time, forward waves are relatively weaker.

The research team at ETH Zurich that includes Nicolas Noiray with contribution from Romain Fleury of EPFL has now engineered a way of stopping sound waves from going backward in their direction without inhibiting their forward progress.

Described in detail recently in Nature Communications, this method could be used for electromagnetic waves in the future.

The premise for this directional control of sound waves is based on self-oscillations in which a dynamic system exhibits the same behavior in a cyclic manner. “Actually, I have spent most of my working life attempting to stop such occurrences,” adds Noiray.

Noiray’s works and interests involve the analysis and understanding of how reactive re-acoustic oscillations can exist through the interference of sound waves and flames in the combustion chamber of an aircraft engine, a phenomenon that can result in serious vibrations that, in the worst of scenarios, would reduce the engine to a heap of metallic debris.

Harmless and useful self-oscillations

Noiray had it in mind to have the self-excited benign aero-acoustic oscillations in order to facilitate the propagation of the sound waves in one direction only by the use of a device called a circulator, without loss. In this approach, the signal attenuation of the sound waves is compensated through the resonance oscillations existing inside the circulator, which leads to synchronization with the incoming waves and thus, they gain energy from the oscillations.

It should include a flat disk like structure for the formation of a cavity in the form of a hollow through which a forced air stream is blown from a single side from an opening at the middle of the circulator’s structure. In the cavity, certain air speed and swirl intensity leads to a whistling sound in the cavity, in fact, the correct balance of the two will achieve this.

This new whistle does not produce sound in the same manner as a standing wave that is prevalent in most conventional whistles, whereas this whistle makes use of spinning wave, according to Tiemo Pedergnana, a former doctoral student in Noiray’s group and the author of the study.

There was a certain time period to advance from concept to the attempt. First, Noiray and his team analyzed the fluid mechanics of the spinning wave whistle and connected three acoustic waveguides in a triangular configuration at the circulator rim.

In particular, if sound waves are input through the first waveguide the can be output through the second. However, a wave entering through the second waveguide cannot go backward through the first but it can travel through the third waveguide.

Using sound waves as a model

The different components of the circulator were designed and theoretically analysed by the ETH researchers over several years. Finally, but rather inconveniently, they were able to provide the proof to stated that the loss-compensation method worked experimentally. They produced an ultra sound wave of about 800 Hertz (this is the high G of a soprano) through the first waveguide and recorded how far it goes to the second and the third waveguides.

As expected there is no transfer of a sound wave to the third wave guide. But from the second waveguide towards the forward direction, there was manufactured a sound wave which was stronger than the first one.

“This idea of loss compensated nonreciprocal wave mode of transport is, in my opinion, a major advancement that can be extended to other application systems as well,” informs Noiray. He sees the sound wave circulator as one good instance of the wave manipulating concept using synchronized self oscillations, which could also be used for metamaterial manipulating electromagnetic waves.

The application of this method could enhance the control of the motion of microwaves into the radar systems as well as bring about formation of the topological circuits for future communication systems.

Reference: Tiemo Pedergnana et al, Loss-compensated non-reciprocal scattering based on synchronization, Nature Communications (2024). DOI: 10.1038/s41467-024-51373-y

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