An active meta-layer for optimal flexural wave absorption and cloaking

Flexural wave propagation is common in thin structures and plays a key role in vibrations of structures. Designs of flexural wave absorbers for vibration control have been constantly pursued for decades aiming to find structures that can best balance tradeoffs between small sizes or lightweights and broadband operations. However, their absorption performance has long been evaluated on a case-by-case basis, and the theoretical limit characterizing the relationship between the absorption spectrum and the sample thickness and mass density is missing. In this study, by adopting an inequality as its electromagnetism and acoustics counterparts, the theoretical limit of flexural wave absorption is established. An active meta-layer composed of piezoelectric sensors and actuators with feedback control loops is proposed for optimal wave absorption. We experimentally demonstrate the active meta-layer for optimal broadband wave and vibration control in the beam and acting as skin cloaks of large voids in the plate. The active meta-layer is electrically programmable and scalable without losing stability. The approach proposed sheds light on designs of reconfigurable dynamic control devices and enables alternative solutions for ultrasonic sensing of complex engineering structures.