Broadband flexural wave focusing based on partitioned acoustic black hole

Abstract The flexural wave gradient-index (GRIN) lens is a flat, structured plate designed to manipulate flexural waves in thin plate structure. It achieves wave focusing by spatially varying its effective refractive index across the lens aperture. However, estimating the refractive index for the lens is generally nontrivial and computationally intensive, often requiring substantial iterative calculations and numerical simulations. This complexity significantly increases the design difficulty and limits the applicability of the lens. To address this challenge, we propose a novel partitioned acoustic black hole (PABH) design where neighboring layers are separated by partitions. This isolation prevents wave interaction between layers, enabling precise control of the phase gradient across the lens aperture. In addition, the effective refractive index of the PABH can be simply obtained theoretically, bypassing iterative methods and numerical simulations. This work proposed the PABH for the realization of flexural wave focusing in thin plate structures. A detailed description of the theoretical methodology for PABH is presented, and numerical simulations and experimental validations of flexural wave focusing confirm the feasibility of the PABH for wave-manipulating applications.