A Lightweight and Transparent Composite-Perforated Janus Acoustic Metamaterial: Multi-Band Sound Absorption Realized via Asymmetric Integrated Design.

To address the bottlenecks of conventional resonant sound-absorbing structures-namely, fixed frequency bands, single functionality, and inapplicability to light-limited spaces-this paper designs and fabricates a composite-perforated Janus acoustic metamaterial using transparent SLA resin. By integrating acoustic impedance theory, parametric finite-element simulations, and impedance-tube experiments, we systematically investigate the sound absorption mechanisms under forward and reverse incidences, as well as the regulatory effects of aperture diameters and spacing on both sides. Results demonstrate that under forward conventional perforation incidence, the structure achieves near-unity low-frequency narrowband absorption at 322 Hz, with coefficients exceeding 0.8 across 308-337 Hz. Under reverse micro-perforated incidence, the absorption peak shifts to 658 Hz with a substantially broader effective band, maintaining coefficients above 0.8 from 611 to 710 Hz. The close agreement among simulations, theory, and experiments validates the reliability of our numerical models. Parametric studies reveal that the front-side perforations primarily tune the Helmholtz resonance frequency with marginal impact on peak magnitude, whereas the rear-side micro-perforations exhibit high sensitivity to aperture size, enabling independent and flexible tuning of absorption bands via bilateral geometric decoupling. By integrating two distinct absorption mechanisms within a single unit cell, this Janus architecture overcomes the limitations of narrowband low-frequency performance and functional singularity. Coupled with its optical transparency, it offers a novel noise-control solution for light-transmitting equipment and industrial pipelines, while providing a valuable reference for the design of multifunctional asymmetric acoustic metamaterials.