Phys. Rev. Lett. 103, 104301 (2009)

Diodes act as one-way filters for electric current, protecting delicate devices from sudden reversals in flow. Sound waves can also travel easily in both directions along a given path, like electricity does, so acoustic devices could block wrong-way reflections. Alas, acoustic diodes do not yet exist.

Jian-chun Cheng of Nanjing University in China and his colleagues have now described a possible way to build one consisting of a sandwich of acoustic layers. Key to the structure would be a layer of nonlinear material that, by changing the frequency spectrum of incoming sound waves, could act as a filter. The researchers suggest that acoustic diodes could be useful in improving ultrasound devices such as those used to break up kidney stones.