Interior noise level scatter in motor vehicles

A major problem of noise control in motor vehicles and other forms of transportation is the variability in the noise characteristics of nominally identical vehicles. Scatter ranges as high as 10 db have been reported for four cylinder motor vehicles. Differences in noise levels of this magnitude are very substantial, and can lead to a situation where a percentage of cars are not acceptable to customers. Scatter of structural acoustic origin is far more difficult to control than the scatter which originates in mechanical subsystems. Finite element models of the vehicle structure and the cavity, and which include coupling of the panels with the airspace, have recently been developed, and they may be used as a reliability model by which the sources of scatter can be investigated. A review of the potential sources of variability indicates that only two appear to be significant: damping and phase angle. These parameters define the polar vector diagrams for determining the panel participation in the generation of noise at a point. The sources which affect coupling with the cavity mode via shifts in panel natural frequency are deemed to be of much lesser significance.