Fundamental Aspects of the Behaviour of Automotive Noise Control Composites

Abstract

Polyurethane foam cored automotive carpet composites are of interest because they provide good acoustic insulation performance coupled with manufacturing advantages. Systems for floorpan and engine bulkhead have, for some while, been the subject of research and development within the industry and by suppliers. However little information has been published on the fundamental aspects of their acoustic behaviour. Early studies of the transmission of noise through light-weight multi-element composites focussed on aircraft panels with rigid fibrous core filling. The results of this work cannot be applied directly to modern carpet composites because the systems differ in detail. For example with carpet systems the core material is elastic porous whereas in the aircraft work the filling was rigid porous. Hence theoretical models derived for the earlier systems have to be adapted to suite the needs of this application. At low frequencies the influence of the composite on overall transmission loss behaviour is quite small so the measurement and modelling strategy adopted is to remove the effect of the steel substrate and work in terms of the insertion loss (IL) of the composite. A laboratory based rig used to carry out systematic studies of design variables is described. This provides a much simpler geometery than a vehicle floorpan and facilitates the modelling of system behaviour. Experimental data obtained with this equipment for incident airborne noise is presented showing the IL behaviour of foam cores alone (no heavy layer) and the influence of the properties of the heavy layer and core foam on the IL response. The theoretical and experimental work identifies the resonance frequency of the panel + composite as a key parameter. At this frequency the IL value passes through a minimum. Measured data and model predictions show how design factors such as the surface density of the heavy layer and the thickness, modulus and damping of the core foam influence resonance and IL response. Information derived from these studies are used as a guide to material selection and system design.