CHAPTER 10: Molding Sound Propagation and Scattering with Acoustic Metamaterials and Metasurfaces

Acoustic metamaterials are artificially structured materials that are capable of interacting with acoustic waves in anomalous ways, leading to exotic physical properties, not found in natural materials. In this chapter, we review and highlight how the extreme sound interaction properties associated with acoustic metamaterials can be exploited to reach a new degree of acoustic wave control, enabling fascinating wave phenomena with large application potentials, including cloaking and sound manipulation via anomalous matching and tunneling effects. We discuss the analogue of plasmonic cloaking for acoustic waves, as well as exceptional matching phenomena that occur at the interface between a natural acoustic medium and a passive acoustic metamaterial with near zero effective density. We also discuss active acoustic metasurfaces obeying parity-time symmetry conditions, which can induce anomalous tunneling effects with large reflection asymmetry, and demonstrate that such active planar surfaces have the potential to relax some of the limitations associated with bulk passive 3D metamaterials, including sensitivity to losses, bandwidth of operation and imperfect isotropy. Finally, the asymmetry in the transmission properties of acoustic meta-atoms is discussed within the context of linear nonreciprocal acoustics.