Microstructure influence on acoustical properties of multi-scale porous materials

This thesis investigates the microstructure influence on the acoustical properties of multi-scale porous materials. The theory of sound propagation in multi-scale materials is extended to account for physical processes specific to very small pores, e.g. rarefaction and sorption effects. First, models of sound propagation through single porosity fibrous and granular materials are investigated. These are then extended to account for fibre and particle porosity respectively. A first approach to modelling the acoustical properties of triple and quadruple porosity materials is then introduced. An investigation of the acoustical properties of resonant absorbers backed with multiscale porous materials follows. Finally, a description of two practical applications is presented. The models introduced in this thesis are validated by performing sound absorption coefficient and sound transmission loss measurements. Accurate predictions are obtained provided that the material microstructure is adequately described and the multi-scale physics involved is accounted for. It is concluded that the material microstructure has a significant influence on the acoustical properties of multi-scale materials. Furthermore, these materials exhibit greater sound absorption and sound transmission loss at reduced weight compared with conventional single porosity materials with similar mesoscopic characteristics and the same layer thickness. The findings of this thesis could provide a basis for designing or finding new porous materials with tailored acoustical properties