Broadband near-perfect absorption of low-frequency sound by subwavelength metasurface

The emerging absorptive metasurface relies on arrays of structured meta-atoms with various geometries for customized sound localization, which can significantly enhance the energy dissipation. However, most of the existing absorbers are for given frequencies at an optimal incident angle. This limitation on the working frequency and incident angle remains a challenging obstacle for their practical applications, in addition to the perfect absorptance demand. Guided by the causality principle, a physical model is established in which the absorptive properties of such systems can be fully controlled by two simple parameters (i.e., leakage factor and loss factor) which are dictated by the geometrical properties of the underlying structures. We demonstrate a subwavelength metasurface absorber which shows near-perfect absorptance (at 95%) in a broad frequency regime from 228 Hz to 319 Hz (wavelength λ from 12.6 to 9.0 times thickness) and even allows 93% reduction with a large incident angle of 60°. We prove that this broadband near-perfect absorption behavior stems from the tunable damping conditions, which can be achieved by coupling an ordinary ultrathin surface sponge coating with an artificial underdamped multiband absorptive system. From the view of the causality principle, the subwavelength near-perfect absorptions originate from the finite working bandwidth. As the research premise, we also demonstrate a λ/21.7-thick, 16.7%-filling ratio ultrasparse absorber with unity absorptance by modulating the displacements between uniformly sized coiled space resonators. The paradigm may pave the way for versatile devices in noise remediation engineering.