Design and Deep-Subwavelength Low-Frequency Sound Absorption of a Coplanar Spiral-Varying-Channel Acoustic Metamaterial

This study proposes a novel coplanar spiral-varying-channel space-coiled acoustic metamaterial (CSV-SCAM) for efficient low-frequency noise control in the range of approximately 200–400 Hz. By integrating continuously graded spiral channels with secondary spiral branches, the proposed structure enables multi-stage acoustic impedance matching and enhanced thermo-viscous dissipation, effectively overcoming the bulkiness and limited low-frequency efficiency of conventional porous absorbers. Finite element simulations and impedance tube experiments demonstrate that the CSV-SCAM achieves near-unity deep-subwavelength sound absorption, with a peak sound absorption coefficient exceeding 0.99 around 750–850 Hz using a thickness of only 10 mm. Furthermore, hybrid configurations composed of units with different branch numbers significantly broaden the effective absorption bandwidth by more than 20% while maintaining high absorption levels. Compared with traditional Helmholtz resonators, the proposed metamaterial exhibits superior compactness, structural robustness, and design flexibility, providing a promising solution for practical low-frequency noise mitigation in space-constrained engineering applications.