Experimental and Semi-Phenomenological Investigation on Sound Absorption Performance of Natural Granular Sound Absorber: A Case Study on Rice Bran Composites

The sound absorption performance of rice bran composites was quantitatively investigated through an improved semi-phenomenological approach. Rice bran (RB) was employed as the primary and structural component in the creation of granular-type sound absorbers with urea-formaldehyde (UF) adhesive. The sound absorption coefficient (SAC) was measured by the two-microphone impedance tube method. Samples with a rice bran per volume ratio lower than 253 kg/m3 show peak-valley characteristics in the saturation region of their SAC spectrum. Five non-acoustic parameters for each sample were obtained by direct measurement and fitting the experimental SACs to the semi-phenomenological Johnson-Champoux-Allard (JCA) equivalent fluid model using the least-squares fitting method. Samples with higher proportions of RB demonstrate lower porosity (ϕ), viscous characteristic length (Λ), and thermal characteristic length (Λ'). Flow resistivity (σ) was the only parameter that noticeably increases when RB increased while tortuosity (α_∞) did not show a strong correlation. For the uncertainty analysis of the experimental SAC, multivariate method was used in this study. A new model (NM) was predicated on the power-law relation introduced by Delany and Bazley, in which the SAC was a function of flow resistivity alone. The new model predicted the SAC of RB composites more precisely than the standard Delany-Bazley model (¯Δ_"abs(DBM)" ≈4.0¯Δ_"abs(NM)" ). The proposed model had the potential to be extended into a more unified empirical model of SAC for granular-typed sound absorbers in future investigations with a broader spectrum of granular materials.