Acoustic performance optimization of natural-fiber micro-perforated panels backed by an optimized polyurethane–fibrogranule composite

Noise pollution poses a major threat to public health and urban sustainability, necessitating effective and environmentally compatible acoustic solutions. Here we report a high-performance hybrid absorber that integrates a natural-fiber micro-perforated panel (MPP) fabricated from alkali-treated flax and rice husks with an optimized polyurethane-fibrogranule (PU-FG) composite backing reinforced by the same renewable fillers. Using response surface methodology based on central composite design, we systematically optimized composite formulation, panel porosity, and air-gap geometry to achieve superior broadband absorption. The resulting system, a 1.61% porosity MPP, 28.5 mm front air gap, 40 mm PU-FG backing, and a 30 mm rear air gap, achieves a sound absorption average (SAA) of 0.82 and a noise reduction coefficient (NRC) of 0.85 across the frequency range 100-2500 Hz. This configuration provides effective broadband absorption through the combined action of Helmholtz resonance from the MPP and visco-thermal losses in the porous backing. Morphological analysis via field-emission scanning electron microscopy confirms hierarchical pore structures enhancing tortuosity and interfacial adhesion. By substantially increasing the renewable content of both the MPP and the porous backing, this lightweight, high-efficiency hybrid offers a practical and scalable pathway toward more sustainable noise-control materials for architectural, transportation, and urban applications.