Boundary element analysis of vehicle interior noise

The work in this thesis examines the structural-acoustic interaction problem within enclosures using both analytical and numerical solutions. The analytical solutions for the noise generated within a uniform linear duct and a three-dimensional rectangular cavity are given. It is shown how the acoustic field inside the linear duct could be controlled by altering the boundary conditions. This gives a basic understanding of the interaction problem and of the measures available in the much more complex case of vehicle noise control to reduce or avoid boom problems. An analytical solution for the acoustic pressure within a closed rectangular cavity with one flexible simply supported panel is given. The results of this analysis are used to find the effect, in terms of acoustic stiffness, of such a backing cavity on panel vibration. Both the acoustic field and the structure are analysed. It is shown that using mobility measurement as input data to the acoustic analysis gives better results than using structural analysis. Numerical modelling of interior noise problems, with application to the interior of vehicles, is presented. An efficient and accurate computer program has been developed and tested on a variety of enclosures. The code is written in FORTRAN and uses the Boundary Element Method as the analysis tool to calculate the interior sound level in a volume surrounded by boundaries of arbitrary shape. The input data required for the numerical model are a surface mesh of boundary elements and the vibrational data on the surface. The vibrational data in this thesis are obtained from mobility measurements. The acoustic field is then modelled by the Boundary Element Method. The numerical technique is applied initially to simple geometries, such as a uniform linear duct and a three-dimensional rectangular cavity, and later the technique is applied to a scale-model of a vehicle. Predicted results are compared with the measured pressure response in the interior of these cavities.