Broadband Low-Frequency Near-Perfect Sound Absorber via Coupled Metasurfaces

We propose a simple yet effective method for low-frequency broadband acoustic absorption. The absorber consists of two concentric space-coiling resonators with distinct resonance frequencies, with the inner resonator characterized by a low-quality factor (Q) and the outer resonator by a high Q factor. The coupling between the two resonators enables efficient broadband absorption within a deep-subwavelength range exceeding 15 times the structural thickness. Numerical simulations, theoretical analysis, and experimental measurements demonstrate that highly efficient (greater than 80 percent) low-frequency broadband absorption is achieved in the range of 198-315 Hz, as well as a 58 percent fractional bandwidth spanning 183-334 Hz. Furthermore, with the outer dimension fixed, adjusting the parameters of the internal resonators enables flexible tuning of the absorption band across a broad frequency range. This work presents a powerful design methodology that eliminates the need for traditional complex spatially arranged multi-resonator assemblies. By employing a single class of resonant units, thin and efficient broadband absorbers can be achieved, offering various application prospects in the field of low-frequency sound absorption.