Multi-order hierarchical broadband tunable acoustic metamaterials

Abstract To address the issues of narrow low-frequency absorption bandwidth and fixed performance in conventional Helmholtz resonator unit cells, this study proposes a multi-order hierarchical design strategy at the scale of the unit cell. We develop a multi-order hierarchical nested labyrinth Helmholtz acoustic metamaterial (MHAM). It comprises reconfigurable geometric modules and octagonal slit tubes with subwavelength thicknesses. Through the organic integration of multi-order gradient layering, core rotation adjustment mechanisms, and modular design, MHAM significantly expands its inherent bandwidth and possesses broadband tunable acoustic absorption capabilities. The performance of MHAM was evaluated through theoretical analysis, numerical simulation, and experimental testing. The research findings demonstrate that MHAM endows the acoustic metamaterial unit cell with exceptional broadband tunability and exhibits near-perfect absorption properties in low-frequency sound control applications. The half-peak absorption bandwidth of this unit cell exhibits exponential growth, with a tunable absorption range spanning from 323 Hz to 1147 Hz. This advancement provides a novel strategy for complex frequency-varying acoustic environments caused by rotating machinery and variable-frequency domestic appliances.