Broadband sound absorption of wood-derived porous acoustic composites via multi-step pore restructuring

To overcome the limited sound absorption of natural sapwood, this study developed high-strength, broadband sound-absorbing wood-derived porous acoustic composites from Balsa ( Ochroma pyramidale ) and Paulownia ( Paulownia tomentosa ) via a sequential delignification–carbonization–waterborne polyurethane (WPU) impregnation strategy. The initial delignification and carbonization (350 °C) transformed occluded cell cavities into hierarchical interconnected pores. Balsa exhibited a higher specific surface area (15 m² g⁻¹) than Paulownia (10 m² g⁻¹), intensifying visco-thermal dissipation to yield a mid-frequency absorption peak of ∼0.8. Subsequent WPU impregnation formed a conformal interfacial coating, regulating airflow resistance and possibly contributing to interfacial damping, thereby sustaining high-frequency (>3000 Hz) absorption above 0.6. Notably, Balsa exhibited a broader effective absorption band due to its inherent vascular advantages. COMSOL simulations combined with Johnson–Champoux–Allard (JCA) model inversion revealed the anatomy-dependent dissipation mechanisms: the lower tortuosity of Balsa enables broadband impedance matching and continuous layered dissipation, whereas the higher tortuosity of Paulownia induces tortuosity-induced localized dissipation. Mechanically, WPU impregnation formed an interpenetrating polymer network that mitigates the brittleness of the mildly carbonized scaffold and increases the compressive strength of Balsa and Paulownia composites to 8.5 MPa and 20 MPa, respectively. This work established an integrated structure–mechanism–performance relationship for wood-derived, load-bearing porous acoustic composites.