Experimental Modeling Techniques for Understanding Road Noise

Abstract

Low frequency road noise phenomena in passenger vehicles have generic characteristics associated with tyre, suspension and body dynamics for a wide range of vehicle styles and classes. Experience has shown that transmission path concepts are inappropriate due to low modal densities and that understanding road noise requires a modal approach. The current techniques for experimental modal acquisition are limited by non-linearities of the suspension and tyre dynamics. Likewise the analysis processes are limited containing ‘ noise’ as a result of statistical variance, high levels of modal damping, and a complex force generation mechanism. In the fields of modal analysis and operational measurements MIRA has developed a number of experimental techniques to overcome some of the current limitations and provide a better understanding of the interaction between suspension and tyre dynamics within the typical operating environment. With a good understanding of both the natural and forced modes of the suspension system it will be possible to optimise the design of future suspension systems for NVH. MIRA has been involved in the measurement and development of suspension systems for road noise for many years. Road and tyre noise is increasingly a subjectively dominant part of the in-vehicle noise for a wide range of vehicles, particularly at low and mid- speeds. Analysis of the in-vehicle noise field for road induced noise has shown that the 63 and 125 Hz octave bands are both significant contributors to the total acoustic energy and correlate strongly with the subjective perception of low frequency tyre noise. Experience has shown that, not surprisingly, most of the energy in this frequency range is structure-borne. To better understand the structural paths MIRA has employed many of the techniques traditionally available to NVH development engineers such as Transfer Path Analysis (TPA), Principal Component Analysis (PCA), and virtual running mode analysis. These techniques have been specifically developed to address the analysis of multiple incoherent sources. Whilst these have proven invaluable in identifying major paths and have helped to