Physicists Create Sound Waves That Move Exclusively in One Direction

Imagine three people arranged in a circle, where each can only hear one other directly. Scientists have created a device that directs sound waves in just this way, allowing them to travel in a single direction only.

Developed by researchers at ETH Zurich and the Swiss Federal Institute of Technology Lausanne, the device consists of a disk-shaped cavity with three evenly spaced ports that can both send and receive sound. In its inactive state, sound from one port reaches the other two at equal volume, creating echoes. However, when activated, sound from port 1 only reaches port 2, thanks to a stream of air swirling in the cavity at a specific speed. This air flow aligns sound waves in a one-way pattern, keeping them from dissipating—like a “roundabout” for sound.

The researchers believe this approach could inspire future technologies in communications and materials, potentially affecting electromagnetic wave control as well. “This concept of controlled, one-way wave travel is an important finding that may apply to other systems,” says senior researcher Nicolas Noiray. Normally, sound waves in any typical medium are reciprocal, meaning they can move equally in both directions. This new method breaks that symmetry, opening up possibilities for unidirectional wave propagation.

Sometimes, making sound travel in just one direction—like for noise reduction—can be very useful. In 2014, researchers at the University of Texas at Austin developed an acoustic circulator using fans to blow air through a resonant ring. This setup directed sound to a single port, making it non-reciprocal. However, the sound waves lost energy as they traveled.

ETH Zurich’s Acoustic Circulator Design Boosts Sound Waves for One-Way Transmission

To tackle this, ETH Zurich researchers designed an acoustic circulator that prevents this energy loss. Here, air swirls into the ring from the centre, creating oscillations in the cavity that align with the incoming sound waves. This tuning allows the waves to gain energy, arriving stronger at their destination without weakening.

Testing their design, the team sent sound waves at 800 Hz through one waveguide and measured the output at two others. The waves arrived stronger at the second waveguide, while no waves reached the third, achieving both directional control and energy preservation.

Interestingly, this energy-boosting concept stemmed from research on reducing oscillations, which can be harmful in systems like aircraft engines. Now, this technology could advance studies in sound wave manipulation and may even be applied to electromagnetic waves to improve radar and communication systems.

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