Determination of viscous and thermal characteristic lengths in rigid porous materials via ultrasonic and Brewster angle measurements

This paper introduces a practical and cost-efficient methodology for independently determining the viscous (Λ) and thermal (Λ') characteristic lengths in rigid-frame porous materials. The proposed approach synergistically combines analytical expressions for transmission and reflection coefficients at high frequencies, derived from the Johnson-Champoux-Allard model, with experimental measurements of the magnitudes of transmission coefficients at oblique incidence and Brewster angles. Notably, this method dispenses with a fixed ratio assumption between Λ' and Λ, and does not rely on computationally intensive and noise-sensitive inversion techniques nor on the use of dual-fluid saturation, which may compromise the integrity of the material's microstructure. By incorporating multiple angles of incidence, the method enables the extraction of additional information from the medium, potentially revealing anisotropic or heterogeneous features and thereby improving model accuracy. The experimental setup employs a standard ultrasonic measurement system with a signal generator, two 100 kHz transducers, and a digital oscilloscope. The results obtained for melamine and polyurethane foams, which are widely used in building acoustics and the automotive industry, exhibit overall consistency with values reported in the literature, further supporting the robustness and reliability of the proposed method.