5 Analysis of the Engine-Shield System

The computational prediction of the effect of a noise shields on engine noise has received the least attention of the problems considered in this paper. In order that the noise shields do not add significantly to the weight of the vehicle, they will generally be made of light, flimsy materials and hence they may also vibrate significantly. The vibration of the noise shields may arise through mechanical coupling with the vehicle vibration or because of acoustic coupling between the engine and shields.

The frequencies that largely determine the noise produced by the engine-shield system are the frequencies at which the engine vibration is strong, the structural resonant frequencies of the shield and the acoustic resonant frequencies that may occur in the gap between the engine and the shield. Moreover, should any of the resonant frequencies of each of these components coincide in the frequency spectrum then the shield may well increase rather than decrease the noise. Clearly, therefore, much useful information about the system can be derived by considering the properties of the engine, the shield and the gap in isolation. However, in order to model the full system an advanced computational method termed the boundary and shell element method is required. It is this method that we consider applying in this paper.