A numerical investigation of noise mitigation for railway track

Abstract

Railways are considered to be environmentally friendly, but the noise and vibration from railways is an obstacle to the promotion of railways. The main source of railway noise is rolling noise, which is caused by the interaction between the rail and wheel irregularities. For the mitigation of rolling noise, various control methods at source have been developed and used, as they are more cost–effective than conventional noise barriers. A promising technique is to use sound absorbing materials to improve the performance further. To be able to predict their performance accurately, it is necessary to understand how sound absorbing materials behave as a part of noise mitigation measures. To address this issue a 2.5D finite element model for poro–elastic media has been developed and implemented in an existing in-house finite element/boundary element model. The developed model has been validated, highlighting differences between the poro–rigid model and the poro–elastic model. The input parameters, which are introduced by the Biot-Allard model, are also presented and ways to determine each parameter have been shown. A melamine foam has been characterised and used in the validation of the numerical model. The flow resistivity has been measured directly, and the other fluid parameters have been inferred from the measured absorption coefficient and impedance. The frame properties have also been identified using a dynamic stiffness measurement and associated simulation. The acoustic absorption of railway ballast has been investigated, comparing a locally– reacting formulation with the current extended reaction model. Three cases from previous studies are revisited: the sound absorption of the ballast layer, the effects of the ballast on the sleeper noise radiation and the effects on the rail noise radiation. It was found that the extended reaction model shows a better agreement with the measurements, although some discrepancies still exist. In addition, the noise radiation from the ballast vibration has been calculated using a 3D finite element model in COMSOL. The iv results showed that the ballast can radiate as much noise as the sleepers below 200 Hz, although the sound power of the ballast will be overestimated due to the limitation of the model. The ground stiffness affects the noise level at low frequencies below 200 Hz, but the relative contribution of the ballast to the sleeper noise is unaffected. The dynamic stiffness of the ballast for use in the track dynamic models has also been calculated, showing good agreement with a previous measurement.
Measurements were conducted in an anechoic chamber to study the effect of absorptive panels laid on the track. A 1:5 scale ballasted track was used, and the insertion loss of the panels was measured reciprocally. A low–height noise barrier was also used for comparison. It was found that the noise barrier is more effective than the absorptive panels. In most cases the combination of the two showed the best performance. A comparison with results from the numerical model showed moderate agreement. The effects of a noise barrier with a porous lining and absorptive blocks placed on a slab track have been studied separately. The materials for the lining have been characterised from impedance tube measurements. It was found that the barrier with the porous lining becomes more effective in the presence of a car body, due to increased reflections. Noise from the slab has also been calculated, and found to have an impact only on the low frequency noise. The effects on rolling noise of absorptive blocks made of porous rubber have been calculated. From a study of the blocks of the same geometry but with different absorbing properties, it was found that the shielding effect is dominant. This was improved when the blocks were absorptive, compared with the case of the rigid blocks, by up to 2 dB(A) for the selected configuration and material. In total, an overall reduction of 4 dB(A) was found in the rolling noise from the numerical calculation by application of the absorptive blocks.

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

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