A comparison of two acoustical methods for estimating parameters of glass fibre and melamine foam materials

Using the Bies–Allard and Kino–Allard models, parameters including porosity, flow resistivity and characteristic lengths of glass wool, polyester fibre, melamine foam and compressed melamine foam materials have been deduced from knowledge of the mean fibre diameter, bulk density and density of the raw solid material from which the fibres are made. Also these parameters have been deduced from ultrasonic measurements and inverted from impedance tube measurements using commercial software Foam-X. It has been found that the ultrasonically-measured viscous and thermal characteristic lengths are in good agreement with the predictions from fibre diameters and densities. However the values of tortuosity, flow resistivity and characteristic lengths inverted from impedance tube data using the commercial software differ significantly from the values obtained by the ultrasonic slope method. Differences between bulk densities of materials predicted by the Bies–Allard and Kino–Allard models using viscous characteristic length values derived from the Foam-X and values deduced by the slope method are explored in detail. In particular it is shown that the outputs from the commercial software do not distinguish between materials with similar flow resistivity but rather different microstructures.