Researchers at ETH Zurich have developed a groundbreaking method to make sound waves travel in a single direction, with potential applications in electromagnetic wave technology. This discovery could lead to advancements in various fields, from radar systems to future communication networks.
The Reversal of Reversibility
In general, the waves of sound (compression), water (waves) and light work in this way when traveling in both directions. In typical use-cases like phone calls, this bi-directional property means that both sides can hear one another. On the other hand, in some technological experiments you would rather have waves only be able to go in one direction without being bounced back or interfering with each other.
Efforts before to block sound wave propagation in the reverse direction worked, but they also dampened waves trying to move forward. A research team at ETH Zurich, with Nicolas Noiray, professor for AEROX (Aerothermal and Optical Shopping), has now found a way to prevent the waves from also travelling back — while simultaneously allowing them to still do this in the opposite direction. The results could open the door to similar headways in the arena of electromagnetic wave technology, which has wide-ranging implications.
Harnessing Self-Oscillations
The secret to creating a one-way sound wave that passes without influence in the other direction lies in managing self-oscillation. The periodicity of these phenomena … that was becoming more and more obvious, is indeed precisely what Noiray had tried to prevent in the study of combustion chambers or aircraft engines for example. Normally, such an “aero-acoustic device,” which enables sound waves to travel across a circulator in one direction, loss-free, is formed by making these self-sustaining acoustic oscillations disruptive but this time around the researchers were able to transform them into something only harmlessly resonant.
The circulator is itself a disk-shaped chamber… with the swirling air entering from one side through a central opening. The 17-meter blade can therefore set up a natural frequency that causes the air to spin in its wake at just the right rate for this dynamic combination of blowing speed and swirl intensity to generate a squeal or whistling sound — not unlike how an ordinary whistle makes noise by setting up a standing wave. The researchers controlled acoustic waveguides to move in a circular direction, using three acoustic waveguides arranged in a triangular shape on the edge of their circulator.
Conclusion
Given the many similarities between the waveforms of sound and light, this represents a significant advance in electromagnetic wave technology for the research team. This idea of loss-compensated non-reciprocal wave propagation can be adapted into different systems, like the metamaterial structures for microwaves used in radar systems or future communication networks. Efficient and error-free wave control is possible by manipulating waves based on self-oscillations presented in a synchronized manner, which further opens up new breakthroughs for various applications such as telecommunications, radar technology, and more.
