Speaker
Description
Foam-formed plant fibre materials are promising sustainable sound absorbers, but their manufacture creates dense drainage layers, producing internal densitygradients that violate the homogeneity assumption of standard equivalent-fluid models such as the Johnson–Champoux–Allard–Lafarge (JCAL) model. We investigatefoam-formed reed and lupine fibre materials using impedance tube measurements from both the open surface (Side A) and the drainage-mesh surface (Side B). The measured absorption depends clearly on orientation: with the dense layer near the rigid backing (Side A), JCAL inversion and analytical fibrous models reproduce the absorption well, whereas with it facing the source (Side B), the enhanced low-frequency performance cannot be captured by homogeneous modelling. Model parameters obtained by inverse characterisation are therefore configuration-dependent rather than intrinsic. Standard airflow resistivity measurements are further shown to be dominated by the thin drainage layer. These results establish Side A characterisation as the appropriate basis for deriving intrinsic fibre parameters in inhomogeneous foam-formed materials.