Structure and design optimisation of composites for noise suppression in vehicles.

Multi-layered noise control systems are used worldwide in the automotive industry to control vehicles interior noise quality. These composites, which include moulded, and slabstock, polyurethane foam cored carpet systems, are intended to attenuate the ingress of airborne noise and suppress the radiation of structure-borne noise. However, little information has been published on their performance characteristics and the optimisation of their design. This thesis reviews previous theoretical work on the dynamics of panel vibration and the airborne acoustic insulation and the structure-borne isolation provided by composite systems. Mathematical models are developed for the acoustic behaviour of unbonded multilayer foam cored carpet composites as experienced on the experimental test rig. The models identify the important material and system parameters governing behaviour. These models, together with experimental evidence are used to optimise the design of the polyurethane foam core and rank the foam materials in order of performance. The experimental and theoretical studies are not intended for vehicle interior noise prediction purposes. The experimental facility uses a horizontal steel test panel, 1mm thick, about lmxlm, and provides acoustic and vibration excitation. Data are obtained for the effective damping (loss factor) of the 4C panel and the insertion loss (IL) of foam cored composites for both vibration and airborne excitation. The intensity method was used to measure the transmitted noise. The precision of the insertion loss measurements was shown to be better than 1.7dB for frequencies below 4kHz. For airborne excitation reproducibility was better than 1.5dB (f<2kHz). For vibration excitation the reproducibility was less. This was attributed to the coupling method used. Experimental and theoretical studies are divided into three sections: (i) loss factors, (ii) vibration insertion loss and (iii) airborne insertion loss. Particular attention is given to airborne insertion loss since the precision of measurement allows a detailed analysis to be made. It is shown that IL passes through a minimum governed primarily by the modulus and thickness of the foam core and the surface density of the septum and steel substrate. The level of the IL depends in a complex way on material and design parameters, including the damping of the foam core. The damping of the septum mass is shown to have little effect upon behaviour. The design optimisation procedure described in the thesis takes account of the loudness of vehicle interior noise at relatively high frequencies (f>500Hz) and the annoyance of discrete tonal noise at relatively low frequencies. It is shown that for a particular incident noise spectrum the carpet composite can be designed to provide the most acceptable noise quality in a vehicle. For a typical large volume production vehicle the optimum resonance frequency is predicted to be about 300Hz. Ways of achieving this frequency with different combinations of design parameters are described.