Parity-time acoustic metamaterials and unidirectional invisible sensors

We theoretically and experimentally investigate the unique scattering properties of parity-time acoustic systems. We observe acoustic invisibility of airborne sound at audible frequencies in a parity-time symmetric meta-molecule made of perfectly balanced gain and loss subunits. This is achieved in a waveguide configuration using electro-acoustic resonators loaded with non-Foster elements, which are tailored to induce invisibility at the design frequency. This unidirectional reflectionless system is fully stable and self-sensing, allowing for robust implementation of parity-time acoustic metamaterials for loss compensation, non-invasive sensing and advanced acoustic signal manipulation.