Introduction to the special issue on non-reciprocal and topological wave phenomena in acoustics

For nearly two decades the field of acoustic metamaterials has seen an explosion of research activity focused on exploring the possibilities for controlling acoustic and elastic wave propagation through the design of materials across multiple length scales. Acoustic metamaterial research has previously focused on determining sub-wavelength structures that produce effective material behavior that can be characterized as negative dynamic density and bulk modulus or a high degree of material property anisotropy. Those studies were conducted primarily with the objective of creating novel devices like acoustic cloaks, lenses, or perfectly absorbing surfaces (Cummer et al., 2016; Haberman and Norris, 2016). At its core, metamaterials research focuses on challenging the assumptions of the limits of material response to propagating acoustic and elastic disturbances and, in so doing, can provide new insight to longstanding problems or suggest entirely new areas of research activity in acoustics. The content of this...