Active nonreciprocal metamaterial using a spatiotemporal modulation control strategy

A class of active nonreciprocal metamaterial (ANMM) is presented which consists of an acoustic duct with periodically placed active diaphragms that are controlled by a spatiotemporal modulation strategy. The acoustic nonreciprocities can be realized by modulating a system's properties spatiotemporally. Such an approach has been extensively employed by many investigators to break the reciprocity in acoustic and elastic metamaterials. However, our proposed ANMM distinguishes itself from the above-mentioned methods by introducing actively tunable space-time modulated feedback gain of the controllers. The controller is implemented in an analog manner to enable fast response at high modulation frequencies. By discretizing a 1D acoustic duct into multiple acoustic unit cavities, we introduced a time-varying gain with a phase difference between adjacent acoustic cavities. Directional band gaps of the modulated system are numerically analyzed as the asymmetric acoustic wave propagation can be realized by converting the acoustical energy from the fundamental mode to higher order modes. In addition, nonreciprocal behavior of the proposed ANMM was experimentally demonstrated using a waveguide with periodically placed condenser microphones (sensors) and speakers (actuators).