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Schematics of the experimental set-up (left) and wave propagation (right): Waveguide 1 can be heard perfectly by waveguide 3, but not by waveguide 2, and waveguide 3 can be heard perfectly by waveguide 2, but not by waveguide 1. As expected, the waves can only travel in one direction. Credit: Nicolas Noiray / ETH Zurich