Researchers at ETH Zurich have recently made a groundbreaking discovery in the field of wave propagation, allowing sound waves to travel in only one direction. This innovative breakthrough opens up new possibilities for technical applications involving electromagnetic waves, where unidirectional wave travel is essential.
Conventional water, light, and sound waves propagate in both forward and backward directions, leading to bidirectional wave travel. While this is advantageous for everyday communication, it can pose challenges in technical applications where unwanted reflections or interference need to be avoided. Ten years ago, researchers managed to suppress backward sound wave propagation; however, this also resulted in attenuation of waves traveling forward.
Led by Professor Nicolas Noiray, a team of researchers at ETH Zurich, in collaboration with Romain Fleury at EPFL, developed a method to prevent sound waves from traveling backward without compromising their forward propagation. This groundbreaking technique, recently published in Nature Communications, could potentially be extended to electromagnetic waves in the future.
The key to enabling one-way sound wave propagation lies in harnessing self-oscillations within a dynamic system. Noiray, known for studying thermo-acoustic oscillations in aircraft engine combustion chambers, proposed using self-sustaining aero-acoustic oscillations to create a unidirectional sound wave path through a circulator. By utilizing the circulator’s self-oscillations to compensate for wave attenuation, sound waves could pass through in one direction without losses.
The circulator, consisting of a disk-shaped cavity with swirling air blown through it, creates a unique spinning wave whistle. By adding three acoustic waveguides arranged in a triangular shape along the circulator’s edge, sound waves can enter through one waveguide and exit through another, preventing backward transmission. The carefully designed system allows for loss-compensated unidirectional wave propagation.
After extensive theoretical modeling and development of the circulator components, the researchers conducted experiments to verify the effectiveness of their approach. Sending a sound wave through the circulator at a frequency of around 800 Hertz, the team observed successful transmission in the forward direction, with the wave emerging even stronger than the original input. This experimental validation confirmed the feasibility of loss-compensated non-reciprocal wave propagation.
Noiray views the sound wave circulator as a valuable model for manipulating waves using self-oscillations, applicable beyond sound waves to electromagnetic systems like metamaterials. This concept could enhance the guidance of microwaves in radar systems and enable the development of topological circuits for advanced communications systems, revolutionizing wave propagation across various domains.
The achievement of unidirectional sound wave propagation represents a significant advancement in wave manipulation technology, with far-reaching implications for a wide range of applications. The innovative circulator design and loss-compensation approach pioneered by the researchers at ETH Zurich have paved the way for future advancements in wave engineering and communication systems.