An investigation of the structurally induced acoustic field in a car passenger compartment

The causes and characteristics of structurally induced interior noise in cars in relation to the boom problem is discussed and the relevant theory developed. The work is concerned with structural excitation of the air within the passenger compartment associated with the second order component of the engine crankshaft out of balance forces. This limits the frequency range of interest to below 200 Hz. Firstly, the acoustic modes of a Rover Metro passenger compartment are predicted using the finite element method. A new method for the experimental acoustic modal analysis of cavities is introduced and verified for a rectangular rigid walled room. The method is then applied to the untrimmed passenger compartment of a Rover Metro to determine its acoustic modes and to study the interaction of the structural modes of the vehicle body and acoustic response of the cavity. The acoustics of a Rover Metro passenger compartment are modelled using the finite element method with experimental structural FRF data from the car body as a forcing function. This model is used to predict the noise spectra associated with the second order component of engine excitation experienced by occupants for the bare body. The acoustic effects of various items of trim are added to the model as acoustic absorption coefficients to show the Significance of trim in reducing low frequency boom in car passenger compcompartments. The necessary absorption coefficients were measured with an impedance tube