Preventing sound waves from traveling backward without deteriorating their propagation in the forward direction

https://phys.org/news/2024-09-implications-electromagnetic-technology.html

"harmless, self-sustaining, sound wave propagation through synchronization with incoming waves allowing them to gain energy from those oscillations... circulator: disk-shaped cavity through which swirling air blown from 1 side through opening in center, whistle tailored to specific blowing speed/ intensity... 3 acoustic waveguides arranged triangularly along edge... sound leaving 1st waveguide enters 2nd, 3rd, but not back... radar, topological circuits... future: apply to metamaterials for electromagnetic waves"