Realizing non-Hermitian tunneling phenomena using non-reciprocal active acoustic metamaterials

Non-reciprocal systems have been shown to exhibit various interesting wave phenomena, such as the non-Hermitian skin effect, which causes accumulation of modes at boundaries. Recent research on discrete systems showed that this effect can pose a barrier for waves hitting an interface between reciprocal and non-reciprocal systems. Under certain conditions, however, waves can tunnel through this barrier, similar to the tunneling of particles in quantum mechanics. This work proposes and investigates an active acoustic metamaterial design to realize this tunneling phenomenon in the acoustical wave domain. The metamaterial consists of an acoustic waveguide with microphones and loudspeakers embedded in its wall. Starting from a purely discrete non-Hermitian lattice model of the system, a hybrid continuous-discrete acoustic model is derived, resulting in distributed feedback control laws to realize the desired behavior for acoustic waves. The proposed control laws are validated using frequency and time domain finite element method simulations, which include lumped electro-acoustic loudspeaker models. Additionally, an experimental demonstration is performed using a waveguide with embedded active unit cells and a digital implementation of the control laws. In both the simulations and experiments the tunneling phenomenon is successfully observed.