Non-reciprocal propagation in an acoustic waveguide with a thermally graded metal lattice core

Acoustic reciprocity describes the behavior of a large class of acoustical systems where the signal, generated by a sound source and recorded by a receiver, remains the same if the positions of source and receivers are interchanged. In an acoustic waveguide, the reciprocity can be seen in terms of equal transmission of the sound power when acoustic waves propagate in upstream and downstream directions. When reciprocity is broken (non-reciprocity is enabled), unusual unidirectional functionalities are allowed, and one-way propagation can be established. Achieving acoustic non-reciprocity is generally not an easy task, but it can be useful to design new acoustic devices such as "acoustic diodes", including isolators, acoustic cloaks and topological insulators, to name a few. In this study, two additively manufactured metal lattice cores undergoing longitudinal temperature gradient are used to obtain thermo-acoustically induced non-reciprocity in an acoustic waveguide provided with anechoic terminations. The unit cell size of the lattice affects the thermal diffusion occurring at different frequencies, while the effect of the temperature is that of facilitating the wave propagation in the direction of the thermal gradient and hindering the propagation in the opposite direction.