Spatiotemporally modulated reflective acoustic metasurfaces

Abstract

Nonreciprocal acoustic wave propagation has been the topic of intense study for nearly a decade with the objective of understanding physical requirements to increase control over propagating acoustic waves [Nassar et al., Nat. Rev. Mater. 5(9), 667–685 (2020)]. One approach to achieve non-reciprocity is deterministic spatiotemporal modulation of the material properties of the medium in which waves propagate. Achieving this modulation requires external sources of energy throughout the domain of interest, which presents a significant scientific and technical challenge. Furthermore, the non-reciprocal nature of the acoustic response is not guaranteed for finite domains and finding parameter sets of modulation amplitude, frequency, and wavenumber that lead to significant non-reciprocal behavior is non-trivial [Goldsberry et al., Phys. Rev. B 102(1), 014312 (2020)]. Input impedance modulation of finite acoustic metasurfaces (AMS) presents a more tractable technical challenge. This work considers semi-analytical and numerical modeling of modulated reflective AMS. We present parametric studies of the static input impedance profiles and modulation functions as a linear, time-varying boundary condition of an acoustic half-space. We consider static admittance profiles that are uniform, discontinuous, and continuously varying and how this may influence the effectiveness of spatiotemporal modulation in achieving performance objectives such as acoustic diffusivity.