Acoustic nonreciprocity

Breaking acoustic reciprocity is essential to create robust one-way propagation where sound waves or elastic vibrations are permitted to travel in only one direction. This unidirectional response forms the basis for devices such as acoustic isolators and circulators, and it also unlocks new functionalities for complex systems such as acoustic topological insulators. After reviewing the principles of acoustic reciprocity, we look at techniques to achieve large reciprocity breaking, including nonlinearities, moving media, spatiotemporal modulation, and nonlinear bianisotropy. We then discuss the recent surge of progress in nonreciprocal surface acoustic wave devices and topological acoustic systems, areas which we predict will continue to flourish in the coming years. We anticipate that these and other applications of nonreciprocity will continue to enhance acoustic technology and form the basis for new acoustic devices. Reciprocity is a fundamental principle in wave manipulation, and techniques for breaking its symmetry will continue to be discovered, refined, optimized, and applied to several acoustic domains as the understanding of the underlying principles and new technologies mature.