Low‐Frequency Acoustic Metamaterials: Mechanisms, Designs, and Broadband Absorption Strategies

This review provides a mechanism‐oriented survey of low‐frequency sound‐absorbing acoustic metamaterials for overcoming the thickness and tunability limitations of conventional porous and fibrous absorbers. Acoustic metamaterials composed of engineered subwavelength structures are classified into four interrelated categories: (i) resonant‐type deep‐subwavelength absorbers, including cavity/Helmholtz‐resonator‐based and membrane‐ or plate‐type designs; (ii) slow‐sound and coiled‐channel metamaterials that employ folded or labyrinthine paths to realize path‐lengthened, dispersion‐controlled absorption; (iii) porous–metamaterial hybrid and hierarchical broadband absorbers that couple resonant units with porous media for impedance matching and critical coupling; and (iv) adaptive, tunable, and bio‐inspired architectures that leverage smart materials, mechanical reconfiguration, or biological morphologies to achieve reconfigurable and multifunctional responses. For each category, the working mechanisms, structural strategies, and low‐frequency absorption characteristics are comparatively analysed using unified metrics including frequency range, peak and average absorption, normalized thickness, and auxiliary functions such as ventilation and energy dissipation. Recent advances from 2023 to 2025, experimental and numerical limitations, and challenges in scalable manufacturing, durability, and application‐driven integration are highlighted, and future opportunities in AI‐assisted inverse design, advanced additive manufacturing, and strongly coupled acoustic–mechanical multifunctional systems are outlined.