Multistable mechanical metamaterials for sound absorption.

Designing acoustic metamaterials with high sound absorption coefficients under low-frequency and broad-bandwidth conditions remains a highly challenging task. This paper proposes a tunable acoustic metamaterial based on a multistable structure to achieve low-frequency broadband sound absorption. The metamaterial integrates multistable thin-walled tube (MTWT) units with embedded neck structures to form Helmholtz-type resonators. The introduction of multistability enables a synergistic combination of multiple dissipation mechanisms: beyond classical Helmholtz resonance, the structure incorporates acoustic soft boundaries induced by thin-wall vibrations of the multistable units, as well as enhanced thermoviscous dissipation within the confined narrow regions. This multi-mechanism coupling not only enriches acoustic energy dissipation pathways but also provides a structural basis for tunable sound absorption. Furthermore, the multistable characteristic offers discrete and self-sustained geometric configurations, allowing the absorber to switch between well-defined acoustic states without continuous external actuation, thereby ensuring robust and energy-efficient tunability. Experimental and simulation results demonstrate that, considering low frequency, bandwidth, and structural compactness, the metamaterial achieves near-perfect sound absorption within the frequency range of 436-1141 Hz, demonstrating significant potential for broad applications in low-frequency broadband noise control. Notably, within this range, the absorption coefficient can be continuously tuned from 0 to 1, representing a highly flexible and nontrivial capability enabled by the multistable design. This work provides a new strategy for the design of acoustic metamaterials with multi-dimensional tunability.