Characterisation of anisotropic acoustic properties of porous materials - inverse estimation of static flow resistivity

Abstract: The production processes of porous materials introduce an inherent geometric anisotropy in the material at micro scale, which influences the material properties at macro scale. In this thesis, the focus is turned to one of these macroscopic properties, the flow resistivity, which is a measure of the resistance felt by the sound pressure waves as they propagate through a porous space. In the current work, two different porous materials have been studied, a fibrous glass wool and a Melamine foam. The two materials are expected to show different degrees of anisotropy with respect to flow resistivity. Glass wool is assumed to be transversely isotropic, as a result of the stacking of layers of fibres. The level of anisotropy in Melamine foam is not as obvious, and might be related to production specific aspects, such as the rise-direction, and the position of the injection nozzles. The thesis begins by giving an introduction to porous materials in general, and to glass wool and Melamine foam in particular, followed by an introduction to flow resistivity, together with two methods to measure the flow resistivity. The full anisotropic flow resistivity of glass wool and Melamine foam samples is determined by means of measurements and inverse estimation. An eigenvalue and eigenvector decomposition of the flow resistivity tensor provides an insight into the connection between the directionality of the flow resistivity in each material, and its production process. A study of the homogeneity in density and flow resistivity for the two materials shows that these properties vary within the block of material. However, for each material, there seems to be no connection between the variation in the two properties, investigated at the macroscopic scale.