ETH Zurich Researchers Unlock the Key to One-Way Sound Waves

While sound waves usually travel in two directions, researchers at ETH Zurich have discovered how to guide—and amplify—sound waves in one direction without losing energy.

Researchers from ETH Zurich recently developed a resonator that enables sound waves to travel unidirectionally along waveguides.

Wave propagation, the movement of waves, travels in both a forward and backward direction. This bilateral movement enables two-way communication, be it with sound or electromagnetic waves. The ETH Zurich team, however, suggests that some applications may benefit from mitigating unwanted reflections of light or microwaves. In the combustion chamber of an aircraft engine, for example, the thermo-acoustic oscillations born from the interplay between flames and soundwaves can generate dangerous vibrations that can, in some cases, destroy the engine. 

ETH Zurich researchers found that self-oscillations (red and blue) caused sound waves (purple, green, and orange) to travel through the circulator in one direction.

While researchers have previously suppressed wave propagation moving backward, their efforts also attenuated waves in the forward direction. The ETH Zurich team, in collaboration with researchers at EPFL, has now developed a way to prevent sound waves from traveling backward without deteriorating propagation waves traveling forward.

Aero-Acoustic Oscillations: The Key to One-Way Sound Waves

Resonant cavities, or resonators, are hollow devices that support wave oscillations and amplify a wave. These resonant cavities can generate unidirectional sound waves with a biased airflow. They can also scatter sound waves and break their symmetry using an approach similar to the electronic Zeeman effect. The Zeeman effect splits spectral lines into multiple components in the presence of a magnetic field. 

Previous research used linear resonators to achieve unidirectional sound waves. These devices, however, also suffered large energy losses and wave transmission limitations. The ETH Zurich team overcame these challenges by creating a cyclic resonant cavity. This device architecture allowed incident sound waves to synchronize with self-sustained oscillations inside the cavity and use that energy to amplify the signals in one direction along waveguides.

How the Cyclic Cavity Moved Waves in One Direction

A cyclic cavity resonator, known as a circulator, allows incident waves to gain energy. The resonator's Zeeman-like bias causes the sound waves to transmit in a non-symmetric pattern once they enter the resonator. The resonance inside the cavity is converted into a limit cycle that emits self-sustaining radiation to prevent loss in the sound wave. When multiple waveguides (known as ports) are connected to the resonator, the self-sustained waves can then move down specific waveguides.

The experimental setup for waveguide propagation. 

The researchers achieved one-directional wave movement when they applied the bias to the resonator, creating a special whistle from a spinning wave inside the cavity. The team used a blower to inject air into a disk-shaped cavity in the circulator. As they blew air into the wind channel, the team controlled the swirl intensity of the air jet before it entered through a hole in the center of the cavity. While the Zeeman effect uses a magnetic field to control waves, ETH Zurich leveraged air to control waves. Once the researchers applied "air bias," they broke the symmetry of the sound wave, allowing it to only travel in one direction around the cavity.

Testing the Cyclic Cavity with 800 Hz Waves

The researchers attached waveguides to the cavity, so incident sound waves entering the cavity could exit through the waveguide ahead. The one-way circular motion of the spinning acoustic field prevented the wave from moving backward to the previous waveguide, forcing it to exit through the next waveguide in the forward direction instead.

Three waveguides attached to the circular cavity, forcing sound to travel in one direction only.

The researchers tested the cyclic cavity by feeding a sound wave with a frequency of about 800 Hz into Waveguide 1. Because the waves only traveled in one direction around the resonator cavity, Waveguide 1 could only be heard by Waveguide 3, but not by Waveguide 2. Likewise, Waveguide 3 could be heard by Waveguide 2, but not by Waveguide 1. In another realization, the researchers found that the sound wave that emerged from Waveguide 2 was stronger than the sound wave they originally sent in, showing that the resonator could amplify a signal without experiencing energy loss.

From One Wave to Another: Potential Applications Abound

While the researchers see their work as a toy model, they believe their approach to wave manipulation using synchronized self-oscillations can be further developed. The resonator prototype can direct and amplify waves in one direction for a number of applications, such as guiding microwaves in radar systems and routing signals in topological circuits for future communication systems. The team also suggested that their work could be applied to metamaterials for electromagnetic waves.