One-way ticket to the tune

Reciprocity is a standard characteristic of acoustic wave transmission, but breaking the symmetry can lead to greater control and potential applications. Here, the authors report non-reciprocal acoustics by achieving large breaking of reciprocity in sounds composed of harmonics spanning several octaves, via non-Hermitian physics.

This non-reciprocal device is applied to physically transpose by one octave real musical instruments, and allow one-way propagation on the harmonized sound.