Broadband acoustic meta-liner with metal foam approaching causality-governed minimal thickness

Acoustic absorbers based on resonant cavities or porous materials have been extensively investigated for developing acoustic liners but still suffer from narrow working frequency bands or bulky sizes. Here, we present a meta-liner capable of high-efficiency and broadband sound attenuation via the causality-governed minimal thickness, which utilizes the coherent couplings among the resonant structures and porous materials to enhance the absorption efficiency. The meta-liner consists of a series of parallel-coupled neck-embedded Helmholtz resonators (NEHRs) and a micro-perforated panel (MPP) backed with a metal-foam-filled gap. In constructing the meta-liner, the metal foam plays an essential role in achieving the over-damped condition and the suppression of impedance oscillation, which therefore facilitates the realization of the minimum thickness and the consistently-high absorption avoiding absorption dips. Distinct from the design methods employed in previous studies that take deficient consideration of the effect of high-order acoustic waves, this work introduces the acoustic grating diffraction theory to comprehensively modulate the coupling effects of meta-liners’ components from fundamental and high-order waves, which enables more precise modulation of meta-liners and leads to improve absorption performance in practice. As a proof-of-concept demonstration, we theoretically designed and experimentally validated a meta-liner supporting unanimously high-efficiency sound absorption from 800 Hz to 3200 Hz with a thickness of only 40 mm. Our work enriches the design concepts of acoustic liners and provides an efficient pathway to construct broadband meta-liners against absorption dips via the causality-governed thinnest structures, which may benefit the applications in noise-control engineering and impedance engineering. • The presented meta-liner with metal foam achieves unanimously high-efficiency sound absorption from 800 Hz to 3200 Hz via a thin structure of 40 mm. • Metal foam is utilized to modulate the intrinsic losses of the meta-liner in light of the causality-governed optimal condition. • The theoretical model enables efficient modulation of high-order couplings of complex acoustic liners and contributes to improved absorption performance. • The coupling effects among the meta-liners’ components of dominants the overall absorption performance.