Thermo-viscous acoustics of micro-perforated panel absorbers with coiled cavities

Strategic utilization of cavity thermo-viscous effects in micro-perforated panel absorbers (MPAs) with coiled cavities for enhancing broadband noise reduction in ultra-thin compact configurations has received limited attention. This study addresses this critical research gap by systematically investigating the influence of thermo-viscous effects on sound absorption in compact MPAs with parallel coiled-cavities of different depths. An analytical prediction model that effectively incorporates both micro-perforation and cavity thermo-viscous effects is developed to predict absorption coefficients. The model is validated through finite element simulations coupling pressure and thermo-viscous acoustic fields under normal and oblique incidence, as well as impedance tube experiments. Based on the analytical model, detailed parametric studies are conducted on the thermo-viscous effects on MPAs with six parallel coiled sub-cavities, with widths ranging from 16 mm to 1 mm. Normal, oblique, and random incidence conditions are considered for a comprehensive analysis. Results show that cavity thermo-viscous effects within the absorber structure significantly enhance absorption performance by: (1) smoothing valleys in the absorption coefficient spectra within the 500–4000 Hz range, (2) causing slight shifts in absorption peaks, and (3) modifying the pressure distributions within the cavity. The impact of sub-cavity width is particularly pronounced when the width approaches the thickness of thermal and viscous boundary layers attached to cavity walls, revealing the critical role of thermo-viscous boundary layer matching in optimizing absorption performance. Case studies demonstrate that optimizing the sub-cavity width to 1 mm leads to remarkable improvements in average absorption coefficients by 5.4%, 9.8%, and 12.4% over the 260–4000 Hz range under normal, oblique, and random incidence conditions, respectively, achieving approximately 0.9, thereby enabling ultra-thin wideband high-performance absorption structures. This study not only advances the fundamental understanding of thermo-viscous energy dissipation mechanisms but also introduces innovative design strategies that significantly outperform conventional MPAs for next-generation ultra-thin acoustic absorbers in space-constrained applications.