Efficient Modelling of Absorbing Boundaries in Room Acoustic FE Simulations

Thanks to the rapidly increasing computational power of contemporary computer systems, acoustic finite element (FE) applications are nowadays widely used to predict the modal behaviour of airborne and structure-borne sound waves in enclosed cavities. In room acoustics it is by now mostly possible to simulate the whole frequency range below the Schroeder frequency using FEM in reasonable simulation times, thus making it a promising contender for extending the validity of typically ray-based room acoustic simulations to lower frequencies. However, while contemporary CAD and meshing tools facilitate the design of high quality geometric models with regular and sufficiently fine meshes, the Achilles' heel of room acoustic FEM simulations appears to be the realistic representation of the room boundaries. The present study investigates the influence of different boundary representations of porous absorbers on the simulated sound field in small rooms. Therefore measurements and FE simulations of the sound field in a scale model room with a well defined geometry and variable boundary conditions are conducted. Two different approaches are followed to model the porous absorbers on the room boundaries. Firstly the absorber layers are modelled with their exact 3D dimensions using an 'equivalent homogeneous fluid' (EHF) model. Secondly the acoustic characteristics of a porous absorber are modelled by an acoustic surface impedance for either normal or field/diffuse incidence, which is determined by measurement or model calculation. The paper clearly points out the limitations of the applied boundary models and also brings out the differences between these models. As a conclusion the paper shows that by using field incidence impedances instead of normal incidence impedances the quality of room acoustic FE simulation results can in some cases be increased considerably and comparable results to those obtained with a 3D absorber model can be achieved even for non-locally reacting materials.