Sound spreads both forwards and backward in a space, meaning someone standing nearby can hear you as well as you can hear them (even if you’d rather they couldn’t). A team of researchers has found a way to make sound waves propagate in one direction only, which could help avoid unwanted reflections and possibly pave the way for the same to be done with electromagnetic waves.
“Wave propagation is said to be reciprocal if it is symmetric with respect to exchanging the locations of any sender-receiver pair,” the team from ETH Zürich explains in their paper. “Due to its close links to time-reversal symmetry, reciprocity is ubiquitous in physics. Breaking reciprocity is necessary in applications that aim to guide waves in one direction while isolating another, for example, to protect a source from unwanted reflections, or to induce strong topological protection against obstacles or imperfections.”
In the new study, the scientists managed to achieve that. They built on work from a decade ago which managed to prevent sound from propagating backward, but weakened it in the forward direction. In the new work, they were able to prevent the backward propagation without this deterioration in the forward direction – in fact, the sound was stronger.
To do so, the team used self-sustaining aero-acoustic oscillations inside a circulator which swirling air was blown into, creating a whistling sound. “In contrast to ordinary whistles, in which sound is created by a standing wave in the cavity, in this new whistle it results from a spinning wave,” Tiemo Pedergnana, lead author of the study, explained in a statement.
The waveguides ensure that sound can only travel in one direction.
Image credit: Nicolas Noiray / ETH Zurich
The experiment placed three waveguides on the circulator. Sound traveling through the feeder waveguide traveled in the forward direction and left through a second waveguide, but not the third. In normal circumstances, the waves would have propagated in both directions.
“This concept of loss-compensated non-reciprocal wave propagation is, in our view, an important result that can also be transferred to other systems,” Nicolas Noiray, professor for Combustion, Acoustics and Flow Physics, who led the team, added. One of these could be better guiding electromagnetic waves used in radar systems.
The study is published in the journal Nature Communications.
