Adjustable acoustic labyrinthine metamaterials for multiband noise absorption

Acoustic metamaterials offer promising solutions for mitigating noise pollution, but addressing noise sources with frequency-varying remains challenging. This study introduces an Adjustable Acoustic Labyrinthine Metamaterial (AALM), with a fixed thickness of 60 mm, designed for tunable, multiband sound absorption targeting the critical 100-2000 Hz frequency range. The AALM's acoustic properties are tuned via two mechanisms. The primary control involves changing the number of coiled-up spaces; as the number of channels is increased, the effective sound path lengthens, causing a significant decrease in the peak absorption frequency while also increasing the absorption coefficient. Secondly, fine-tuning and impedance matching are achieved by adjusting the wave entrance cross-sectional area; decreasing the opening size leads to a minor decrease in absorption frequency but a notable increase in the peak absorption coefficient. Numerical simulations (FEM) and experimental impedance tube measurements validate the design. The AALM, configurable into 24 distinct modes, achieves near-perfect sound absorption (α≈1) at targeted frequencies across a wide spectrum from 220 Hz to 1750 Hz. Altering the coiled spaces enables major frequency adjustments (≈120-530 Hz shifts), while entrance area changes allow for finer tuning (≈10-50 Hz shifts). This adaptable design surpasses the limitations of conventional passive absorbers and shows significant potential for integration into smart systems capable of real-time noise adaptation.