Variation of Vehicle NVH Properties due to Component Eigenfrequency Shifting - Basic Limits of Predictability

Many papers have been published on variation in noise and vibration as well as transfer function characteristics between individual vehicles with nominally identical design [1], [2] and [3]. However, prediction of Noise Vibration and Harshness (NVH) properties is mostly based on detailed, deterministic modelling with FE- and BE-methods. Time and computer resources for creation and experimental updating of these models need to be optimised with respect to achievable prediction accuracy, and in this context statistical, energy flow based methods (SEA, EFA etc.) should be considered as an efficient alternative for medium and high frequency NVH prediction.

A basic study of variability for transfer function of multimodal systems, using ideal acoustic and structural components with parameters corresponding to vehicle body plates and cavities is performed. Well known theory on variability, originally developed for room acoustics, is demonstrated to apply also for simple plates. This theory applies for any multimodal system with a sufficient modal density and damping to obtain well overlapping modes in a frequency range. It is shown that a fundamental standard deviation of 5-6 dB can be expected also for rather small eigenfrequency shifts or damping variations for the modes in this frequency range.