The influence of the non-linear stiffness behaviour of rail pads on the track component of rolling noise

The stiffness of the rail pad is a critical parameter in determining the noise from the track. Both theoretical and experimental studies show that a stiff pad, by coupling the rail to the sleeper, leads to a high attenuation of rail vibration with distance along the track and an increase in the sleeper vibration. In this way the rail-radiated component of noise is reduced but the sleeper component is increased. Conversely a soft pad leads to a low sleeper noise component but greater rail noise, as the rail can vibrate over a greater length. In modern track, soft pads are used, as they lead to reduced track forces and less damage to track components such as sleepers. In this paper, measured vibration characteristics of track are presented and compared with predictions. A recent study of the effect of the load dependence of the rail pad and ballast stiffnesses on the dynamic behaviour of the track is extended to give a practical assessment of the effects on the noise radiation. It is shown that, for a typical soft pad with a high load dependence, neglect of the preloading effect would lead to a significant overprediction of the track noise component. It is thus shown that soft pads need not be so disadvantageous for noise as has been suggested.