Unit cell interactions of non-local active acoustic media

Abstract

Active acoustic metamaterials have greatly expanded upon the unique and unusual ways that passive metamaterials manipulate sound. Previously, we have shown how spatially non-local forces in an acoustic system can create band gaps and break acoustic reciprocity, and how such effects can be realized in practice using a non-local active acoustic metamaterial (NAM). We demonstrated experimentally the remarkable ability of a single NAM unit cell to generate large, subwavelength, broadband nonreciprocity using an open loop feedforward control mechanism where a measured local pressure is transmitted by an electronic controller downstream to actuate an acoustic source. In NAM systems with multiple unit cells, this control scheme strongly couples each cell with every other cell, and thus requires special design considerations. Here, we discuss the advantages and challenges of developing NAM systems with multiple unit cells by first considering a system with two cells, detailing the impacts of their complex interaction on both performance and stability. We validate model predictions with experiments and highlight the exciting prospects offered by larger multi-cell NAM arrays.