'Ghost tunnels' guide sound waves in one direction while staying invisible to others

Acoustic metamaterials are a fast-evolving family of materials which manipulate sound waves in ever more advanced ways. Now, a team led by Changqing Xu at Nanjing Normal University in China has engineered an acoustic metamaterial, a "ghost tunnel": a structure which acts as a near-perfect waveguide for sound entering through its ends, while being essentially invisible to waves incident on its sides. The results, published in Physical Review Letters, could open new avenues for manipulating sound waves in complex signal environments.