Doppler effect reversed by metamaterial

This latest research takes the principles of negative electromagnetic refraction and applies them to acoustic vibrations. Here the parameters to be made negative are material density and modulus, the latter relating to a material’s elasticity. Until now engineers have only created metamaterials with either of these properties, but Kim and colleagues have successfully combined them to create the world’s first “double-negative” acoustic metamaterial (arXiv:0901.2772v2 ). Their acoustic tube is constructed from thin membranes under tension fed by a carefully controlled air flow, and this manages to create a negative phase velocity for sound travelling through.