Characterizing and modelling the sound absorption of wood wool cement boards (WWCB)

The present article aims to characterize and, by using impedance models, predict the sound absorption of wood wool cement boards (WWCB). The main challenge lies in the inhomogeneity of the WWCB; the samples taken from different commercial boards do not only greatly differ in density, but also in wood-to-binder ratio. Different models, able to predict the acoustic impedance of rigid-frame porous materials, have been analysed and their suitability for the WWCB has been evaluated. It is concluded that the Johnson-Champoux-Allard (JCA) model was found to be most appropriate to fit the normal incidence sound absorption values as measured in the impedance tube. From the five input parameters for this impedance model, the flow resistivity has been measured. The open porosity, tortuosity and viscous and thermal characteristic lengths, have been determined by making use of an inverse calculation method, a curve fitting approach, based on the measured acoustic absorption coefficients in the impedance tube. The tested WWCBs are made of three different strand widths (1.0, 1.5 and 2.0 mm) and of different thicknesses, densities and wood cement ratios. By making use of the found relations between the bulk density and the input parameters, it is concluded that it is possible to predict the normal incidence sound absorption of the WWCB by only making use of the bulk density and thickness as input parameters.