The influence of baseplate fastening systems on railway rolling noise

Abstract

The focus of this study is placed on railway rolling noise that is in most situations the dominant source of railway airborne noise for conventional train speeds. This work aims to investigate and quantify the influence of the rail fastening system on the rolling noise. For this purpose, a commercial two-stage baseplate system mounted on a slab track is studied experimentally and numerically. The research aims to identify the key design parameters that affect the rolling noise, pointing the way towards controlling noise. These aims are addressed by using laboratory and field measurements of vibration and noise as well as prediction models. Rail fastening systems perform an important role in distributing the train load to the sleepers or the slab, as well as in protecting them from excessive dynamic loading. Their types range from simple systems such as rubber rail pads, an elastic layer inserted between the rail and sleeper, held down by fastening clips, to more sophisticated ones that include fastening systems with baseplates and more than one rubber pad. The dynamic properties of the rail fastening system, such as its stiffness and damping, play a key role in the dynamic performance of the track and its noise emission. The dynamic stiffness affects the degree of coupling between the rail and the foundation as well as the vibration decay rate along the rail. The dynamic stiffness of rail pads and rail fastening systems fitted on railway tracks has been measured in the laboratory. By definition, dynamic stiffness is the complex frequency-dependent ratio between the applied dynamic force and the deflection. An experimental procedure based on a standardised indirect method is applied for measuring the dynamic stiffness of rail pads and more complex rail fastening systems. The method allows the dynamic stiffness of resilient track elements to be obtained as a function of the excitation frequency for known loading conditions. The results are obtained in the frequency range 50 Hz - 1000 Hz. The track decay rate (TDR) is the rate of vibration attenuation with distance along the track. It is usually determined for both the vertical and lateral waves propagating in the rail. It is a measurable quantity and it is closely related to the rolling noise performance of the track. For the purpose of this work, the TDR has been measured for four different configurations (rail pads) on a non-operational slab track section located at the National College for High-Speed Rail at Doncaster, UK.
The properties of rail pads and rail fastening systems in the context of noise and vibration are discussed using simple mass-spring models to explain features of the measured results. Initially, a model of a beam on an elastic foundation is used to calculate the TDR. This simple model does not provide a sufficient match with the measured TDR and thus a detailed 2.5D finite element (FE) model of the rail coupled with an FE model of the flexible fastening system (baseplate) is deployed. The 3D FE model for the baseplate and the 2.5D model for the rail are combined into a model of a discretely supported railway track using a receptance coupling technique. This model is used to predict the track mobilities and TDRs. The predicted results agree well with the measured ones for all the cases considered. By using the baseplate response predicted from this model the sound radiation of the baseplate is estimated. Two models are used to calculate the baseplate sound radiation: an analytical model based on the Rayleigh integral and a more detailed boundary element (BE) model that can include the scattering effect of the rail. Although the BE model can account for more details, it is shown that adequate results are obtained using the analytical model. The predicted baseplate sound radiation is then compared with measured data obtained on a full-scale slab track section in the reverberation chamber. A good agreement is found for most configurations.
Finally, in order to predict the rolling noise from a slab track, the TWINS model is used with the results from the flexible baseplate model and the effect on the overall noise is determined. The results show that the contribution of the baseplate noise is 5 – 10 dB lower than that from the rail. If the thickness of the baseplate is increased, the rolling noise is reduced between 200 Hz and 1 kHz due to changes in the TDR, whereas the noise from the baseplate itself is increased. The change in the total noise is quite small. By comparing results for different stiffnesses of railpad it is found that the optimum stiffness of the railpad is around 500 MN/m.

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Identifiers

ORCID for David Thompson: orcid.org/0000-0002-7964-5906

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