Computational and experimental investigation of low frequency noise in passenger vehicles

Acoustic comfort of passenger vehicles has become a significant competition factor in the market, as much as the others, i.e. styling, power, fuel consumption and budget. Vibro-acoustic response studies play an important role in developing countermeasures to noise problems, before or after vehicle launch. In this study, low frequency noise characteristics of passenger vehicles are addressed. Vehicle noise variability and dominant paths that cause low frequency booms are investigated. To diagnose the cause of variability, a systematic approach is proposed, where all steps are explained briefly. It is deduced that predominant paths, which are found to be the main contributors of diagnosed booms, are also the root causes of variability observed. Current practice of experimental transfer path analysis is discussed in the context of trade-offs between accuracy and time cost. An overview of methods, which propose solutions for structure borne noise, is given, where assumptions, drawbacks and advantages of methods are stated theoretically. Applicability of methods is also investigated, where the engine induced structure borne noise of the sedan studied is taken as a reference problem. Sources of measurement errors, processing operations that affect results and physical obstacles faced in the application are analyzed. Effects of damping, reasons and methods to analyze them are discussed in detail. In this regard, a new procedure, which increases the accuracy of results, is also proposed. Coupled vibro-acoustic response of the sedan is analyzed, and the effect of folding rear seat aperture is studied. An analytical solution is proposed to calculate acoustic eigenfrequencies. Then, uncoupled acoustic eigenfrequencies of the actual cavity, where trunk and cabin cavities are connected through the aperture are computed. It is shown that planar acoustic eigenfrequencies of the sedan can approximately be calculated using the analytical solution proposed. To further clarify the impact of folding rear seat aperture, coupled vibroacoustic response of the sedan is analyzed through different case studies. It is observed that booms are highly correlated with the calculated uncoupled planar acoustic eigenfrequencies. It is concluded that proposed analytical solution can be effectively used in calculation of acoustic eigenfrequencies and identification of booms, rather performing a detailed computational work. Experimental modal analysis studies are carried out to update the computational model. The updated model is then used in modification prediction studies. Finally, it is shown that the procedures proposed works well with real problems.