4D Printing of Reconfigurable Acoustic Metamaterials with Multiband Low-Frequency Absorption

Controlling low-frequency noise presents a significant challenge for traditional sound absorption materials, such as foams and fibrous substances. Recently developed acoustic absorption metamaterials, which rely on local resonance can effectively balance the volume occupation and low-frequency absorption performance. However, these materials often exhibit a very narrow and fixed absorption band. Inspired by Helmholtz resonators and bistable structures, we propose bistable reconfigurable acoustic metamaterials (BRAMs) that offer multiband low-frequency absorption. These BRAMs are fabricated using shape-memory polylactic acid (SM-PLA) via four-dimension (4D) printing technology. Consequently, the geometry and absorption performance of the BRAMs can be adjusted by applying thermal stimuli (at 55 °C) to switch between two stable states. The BRAMs demonstrate excellent low-frequency absorption with multiband characteristics, achieving an absorption coefficient of 0.981 at 136 Hz and 0.998 at 230 Hz for stable state I, and coefficients of 0.984 at 156 Hz and 0.961 at 542 Hz for stable state II. It was found that the BRAMs with different inclined plate angles had linear recovery stages, and the recovery speeds range from 0.75 mm/s to 1.1 mm/s. By combining a rational structural design and 4D printing, the reported reconfigurable acoustic metamaterials will inspire further studies on the design of dynamic and broadband absorption devices.