In our everyday world, waves are stubbornly democratic. Whether it’s the sound of a conversation, the glow of a lightbulb, or the undulations of the ocean, waves tend to flow equally in both directions. You speak, and your voice travels to your friend standing across from you — just as theirs reaches back to you.

We like it this way. But what if we needed waves to move in only one direction, free from interference, like cars on a one-way street?

That’s the kind of control a team of researchers at ETH Zurich has just achieved. After years of effort, they’ve figured out how to direct sound waves so that they travel forward — but never backward. It’s a feat that could have vast implications for future technologies, from communications systems to radar, and they’ve done it without weakening the sound’s strength.

The Problem of Reflected Waves

The idea of controlling wave propagation — directing it to flow in only one direction, for instance — has appealed to scientists for years. That’s because they would like to solve real-world problems. In applications like radar or communications, reflected waves can cause interference, garbling signals or reducing efficiency.

Ten years ago, researchers managed to stop sound waves from bouncing back. But there was a catch. The waves moving forward weakened in the process.

So, a group of researchers tackled the problem head-on. The researchers were based at ETH Zurich led by Nicolas Noiray, a professor of combustion, acoustics, and flow physics. With the help of Romain Fleury from EPFL, the team has finally found a solution

Their breakthrough hinges on self-oscillations — cyclical movements in a system that repeat without external influence. While these oscillations are usually a nuisance (and even dangerous when they cause vibrations in airplane engines), Noiray realized we could harness them for good: to make a one-way street for sound waves.

Whistles and Waves

Noiray’s solution is as ingenious as it is simple. Picture a disk-shaped cavity, through which air is blown with just the right intensity to create a whistling sound. But this is no ordinary whistle. Instead of creating a standing wave, where sound bounces back and forth inside a space, this one generates a spinning wave.

The ETH Zurich team spent years working out the fluid mechanics behind this spinning wave. Then, they added three pathways, or waveguides, arranged in a triangular pattern. When a sound wave enters the first waveguide, it travels smoothly through the circulator to the second waveguide. But if the sound tries to enter through the second waveguide, it can’t go back. It’s forced into a third, separate path, ensuring that the sound only moves one way.

The team tested their design with sound waves at a frequency of about 800 Hertz — roughly the pitch of a high soprano note. And it worked. Not only did the sound wave travel forward without reflecting backward, but it even emerged stronger than before, thanks to the energy boost from the circulator’s self-oscillations.

“This concept of loss-compensated non-reciprocal wave propagation is, in our view, an important result that can also be transferred to other systems,” says Noiray.

The possibilities go far beyond sound. This kind of one-way control over wave movement could be a game-changer for technologies that rely on electromagnetic waves — like radar systems, where precision and directionality are crucial. Future communication systems, too, could benefit from this newfound ability to route signals without interference, guiding them efficiently along topological circuits.

In a field that often faces hard limits, Noiray’s team has discovered a new way forward — literally.

The findings appeared in the journal Nature Communications.