An Inverse Method to Determine Acoustic Parameters of Polyurethane Foams

Polyurethane (PU) foams are widely used as noise and vibration damping materials in automobile applications. Their porous random microstructure is composed of a visco-elastic frame structure with an interstitial fluid filling the voids. Acoustic and mechanical energy absorption properties and their direct link with microstructure morphology are therefore of paramount importance in the design, prediction and optimisation of the material behaviour. Although some parameters can be directly measured, the complete acoustic characterisation of porous materials remains a real challenge. For that reason, an inverse calculation of acoustic parameters based on impedance tube measurement is proposed. Equivalent fluid models are used to study the wave propagation process within PU rigid foams, in which acoustic properties depend on the effective density and bulk modulus of the fluid. Conversely, Biot poroelastic model is used to study the multi-physical coupling between mechanical vibrations of the solid structure and acoustic waves in the fluid. Biot coupling is often present in the wave propagation through PU flexible foams, having a soft-like polymeric structure and semi-open microstructure. Finally, the macroscopic properties obtained by inverse numerical modelling are then linked to polymer characterisation obtained by Dynamical Mechanical Analysis and the material microstructure obtained with μCT.