Railway noise and vibration mitigation: a review of technologies and applications

Noise and Vibration Mitigation for Rail Transportation Systems

A variety of isolation measures exists to reduce the vibration in the neighbourhood of railway lines. They can be roughly classified as elastic or stiffening systems. The following elastic elements are used (Fig. 1): railpads or resilient fixation systems between rail and sleeper, sleeper shoes under the sleepers, and ballast mats under the ballast. Stiffening systems (plates) are used as slab tracks, floating slab tracks, or mass-spring systems and in a different way, as an under ballast plate. The main interest of this contribution is ballast mats. Ballast mats are an efficient measure to reduce the vibrations near railway lines. The vehicle-track system gets a low eigenfrequency due to the insertion of an elastic ballast mat under the ballast. For frequencies higher than this low vehicle-track eigenfrequency, the forces, that are generating the vibration of the soil, are considerably reduced.

Based on the dynamic receptance method, a vehicle–track–bridge interaction model was developed to calculate the wheel–rail interaction forces and the forces transmitted to the bridge in an elevated urban rail transit system. A prediction model integrating the finite element method–boundary element method (FEM-BEM) and the statistical energy analysis (SEA) method was established to obtain the noise from the main girder, track slab, and wheel–rail system for elevated urban rail transit. The calculated results agree well with the measured data. Thereafter, the noise radiation characteristics of a single source and the total noise of elevated urban rail transit systems with resilient fasteners, trapezoidal sleepers, and steel spring floating slabs were investigated. The results demonstrate that the noise prediction model for elevated urban rail transit that was developed in this study is effective. The diversity of track forms altered the noise radiation field of elevated urban rail transit systems significantly. Compared to monolithic track beds, where the fastener stiffness is assumed to be 60 × 106 N/m (MTB_60), steel spring floating slab tracks (FSTs), trapezoidal sleeper tracks (TSTs), and resilient fasteners with a stiffness of 40 × 106 N/m (MTB_40) and 20 × 106 N/m (MTB_20) can reduce bridge-borne noise by 24.6 dB, 8.8 dB, 2.1 dB, and 4.2 dB, respectively. These vibration-mitigating tracks can decrease the radiated noise from the track slab by −0.7 dB, −0.6 dB, 2.5 dB, and 2.6 dB, but increase wheel–rail noise by 0.4 dB, 0.8 dB, 1.3 dB, and 2.4 dB, respectively. The noise emanating from the main girder and the track slab was dominant in the linear weighting of the total noise of the elevated section with MTBs. For the TST and FST, the radiated noise from the track slab contributed most to the total noise.

Because of the recent emphasis on rail transit infrastructure expansion and rebuilding in the United States, there is concern about the potential adverse vibration effects on existing activities that are sensitive to vibration and ground-borne-noise located in proximity to new or reactivated rail system rights-of-way. A Southern California at-grade/elevated LRT system that is currently under construction required the design and specification of extensive vibration control features. The final design needed to include the entire range of vibration mitigation tools presented in the Federal Transit Administration guidance manual. Designs and Contract Specifications included simple single-layer ballast mats; multi-layer ballast mats; high-resilience direct fixation fasteners; potential wheel-squeal solutions; low-vibration special trackwork; continuously-supported, street-running, floating slab track; and discrete, steel-helical-spring-supported floating slab track. It is noteworthy that the discrete steel-spring-supported FST is the first use of this vibration control approach in an at-grade rail system in the United States. This paper will discuss the overall approach, data analysis, solutions development, and the final designs prepared for this project and how this information may be beneficial to other projects with similar issues.

To predict the vibration transmission of over-track buildings and investigate the effectiveness of countermeasures, a fully coupled three-dimensional vibration prediction model for over-track buildings with consideration of the pile–soil interaction (PSI) was proposed. The accuracy of this prediction model was validated with field data from in-situ vibration monitoring in a metro depot. The consideration of PSI improved prediction accuracy about the vibration of over-track buildings. Then the effectiveness of different mitigation measures (rail damper and ballast mat) was investigated by this prediction model. It was found that the vibration of over-track buildings was dominated by vertical vibration components. Rail dampers significantly reduced the vibration in the high-frequency range above 80 Hz, while the vibration in the low-frequency range below 10 Hz was amplificated. The insertion loss of ballast mats reached 10 dB in the frequency range above 40 Hz except at the resonance frequency of the track, and mitigation effectiveness gradually decreased with increasing story height. This proposed method can predict the vibration level within over-track buildings and is an effective tool to guide the vibration mitigation design of over-track buildings.

Railway GBN impact has raised increasing concerns due to underground transportation expansion. Reducing train speed or replacing standard baseplates with high resilient ones are adopted for GBN control. Both mitigations are not satisfactory considering the installation difficulty and limited performance, e.g., only ~3dB noise reduction for 30% train speed reduction. On the other hand, FST (Floating Slab Track) are commonly used. In many cases, to accommodate the FST, tunnel diameter is enlarged for the TBM (Tunnel Boring Machine) tunnel section. Also, rail dampers are used for air-borne noise control, but never used for GBN control due to its relatively small mass. P2 resonance is a main cause of railway GBN. It is a simple harmonic motion of a lump mass (wheel and rail combined) oscillating on top of resilient baseplates. Laboratory test was conducted with a 6m fastened rail and a ~450kg mass to simulate train wheel and track system. A retrofit rail damper with TMD (Tuned Mass Damping) oscillators was tested. The mass of TMD oscillators along the rail with ~2m effective length of P2 resonance is more than 10% of the wheel. Around 7dB vibration reduction was recorded at the rail and floor when allowing the TMD oscillation.

The frequently moving subway trains often cause large vibration of the rack buildings especially tall buildings and noise problem, which lead to the discomfort of the occupants. Therefore, it is highly necessary to adopt measures to decrease the vibrations. The finite element model of vehicle-rail-foundation was established by using ABAQUS program, and vibration mitigation ability by using ballast mats was investigated. The results showed that the ballast mat has ability to mitigate train-induced environmental vibrations. The acceleration level can be highly reduced within 30 m from the track centerline on the ground. Increasing the thickness of the ballast mat and increasing the static foundation modulus can enhance the vibration mitigation effect. The research results can provide references for the train-induced environmental vibration mitigation design.

Roughness in rail surface accumulates with train operations and grows faster on curved tracks than tangent tracks. Higher roughness leads to higher noise radiation. In Hong Kong, railway noise prediction includes a 3dB correction for rail surface deterioration. Depending on rate of deterioration, different rail grinding cycles (typically between 3 and 24 months) are scheduled to ensure train operations without excessive noise radiation. Before rail dampers installation, noise levels were monitored for 1.5 years and around the noise limit with variation +/- 3dB(A) due to rail roughness variation. After installation of rail damper, railway noise was generally maintained at 7dB(A) below the noise limit with variation +/-1.5dB(A) for more than 3 years. Noise reduction at dominant frequencies 630 Hz and 800 Hz were more than 10dB in 1/3 Octave analysis. The rail dampers were installed at alternative spaces such that additional rail dampers can be installed to double the amount in case further noise reduction is required. This paper examines the five years' monitoring data in detail, showing the noise levels with smaller variations over grinding cycles after rail damper installation, and explores rail damper potential ability to slow down the growth rate of rail roughness levels.

Vibration of the elevated urban rail transit (URT) severely affects the health of nearby residents, threatens the integrity of adjacent historic buildings, and aggravates the performance of vibration-sensitive instruments in buildings, and the accompanied annoying structure-borne noise always arouses public complaint. Vibration and noise mitigation measures through track structures and/or noise barriers are increasingly favored to deal with these challenging issues. This paper presents systematic field measurements on noise and vibrations of elevated URT. The vibration experiment covers vibration of track structures, bridge girders and piers, and ground soil under three different track structures, i.e., embedded sleeper track, ladder sleeper track, and floating slab track (FST) with rubber mats. Noise measurements were also conducted considered the effect of track structures and with or without fully enclosed noise barriers. It is shown that ladder sleeper track and FST were more effective in control bridge vibration than ground vibration. The overall vibration level of the bridge is 8~10 dB greater than the ground vibrations. The noise reduction effect through track structure was limited for far-field ground. Furthermore, it is found that the noise barrier was more effective to reduce near-field wheel/rail rolling noise rather than far-field noise. Good correlation between structure-borne noise and vibration was observed for both the embedded sleeper track and FST at the bottom slab of the box girder bridge.

Floating slab tracks (FSTs) are used to reduce the impact of vibration on precision instruments and historical relics along metro lines; however, ground vibration is universally amplified at the natural frequency of the tracks. In this study, a full-frequency control method that considers frequency matching for environmental vibrations, in combination with metro vibration sources and propagation paths, was developed based on the bandgap theory of the periodic structure. The effectiveness of this method was analysed by establishing a three-dimensional metro train–FST coupled model and a finite element analysis model of track bed–tunnel–soil–row piles. The results show that ground vibration can be reduced by approximately 3–5 dB at the natural frequency of the FST by adjusting the bandgap range of the periodic piles to 7–9 Hz, eliminating the adverse effect of vibration amplification at the natural frequency of the FSTs. The proposed control method shows good vibration control effects and can effectively minimise ground vibration in the full-frequency range.

TDR measures the rate of vibration decay along rail in dB/m. Higher TDR leads to lower noise radiation. However, new railways are often having low TDR due to the use of resilient fasteners (to provide vibration isolation between rail and supporting structure for ground borne noise concerns), which leads to high noise radiation. Rail damper is used to increase TDR (thus reduces railway noise), where TMD is an efficient damping mechanism dissipating the vibration energy of the rail. TMD provides reactive damping force, maximised after a few cycles of oscillations. TMD force is stronger with continuous excitation than impulse excitation. For convenient purposes in the industry, TDR measurements are primarily conducted by impulse method, which do not allow sufficient time to include reactive TMD force. Therefore, impulse excitation TDR is smaller than continuous excitation TDR. Continuous excitation TDR is considered to reflect more of the real case of wheel/rail interaction excitation during train running. Besides, TDR in terms of time decay in dB/s is an alternative approach for evaluating noise performance of the rail. This paper presents TDR measurement results under different conditions, e.g., impulse excitation against continuous excitation; freely supported short rail (~6m), short rail (~6m) with resilient fastener support as well as site measurements in continuous welded rails.

A mass-spring-series model was assembled and tested using displacement-controlled vibrations in order to validate the effectiveness of vibration mitigation in a floating slab track. A mass mounted with an accelerometer was isolated with a spring from the vibration source. The other end of the mass was also connected with a spring to the fixed end. The spring coefficients and natural frequencies of the mass-spring combinations were carefully measured and verified. The time histories of the vibration amplitude indicated that properly tuned combinations of mass and springs effectively retain vibrations in the designated mass and reduce propagation and reflection. It was determined that using a stiff spring to confront the excitation source and a soft spring as a foundation isolator may alleviate vibration propagation and reduce vibrations reflected to the excitation. The drawbacks of the experimental design were also discussed.

As the main vibration source of metro depot, the throat area has seriously affected the work and life of the occupants in over-track building. However, there are few researches on vibration mitigation of turnout. Aiming at the No. 7 single turnout with 50 kg/m rails in ballast bed, the design and effect predictions of the combined vibration mitigation with under sleeper pads (USP) and under ballast mats (UBM) is carried out through the stiffness homogenization, dynamics calculation and three-degree-of-freedom vibration calculation. The study results show that: the USP not only helps to improve the turnout stiffness irregularities longitudinally, and can significantly improve the vibration mitigation effect. When using the UBM alone, the vibration mitigation effect is optimal up to 9 dB. However, after laying the USP, the vibration mitigation effect can reach about 15 dB, which increased by 67%, and meet the vibration mitigation need in the metro depot.

The noise and vibration effects of rails can have a significant impact on the environment surrounding the railways. Rail dampers are elements that are attached to the sides of the rail and can improve the track decay rate of rail and then enhance the rails’ ability to attenuate noises and vibrations. However, in practical applications, the most efficient rail damper design still cannot adjust its own parameters to adapt to different requirements because their stiffness and damping are fixed after designed. In this work, a tunable magnetorheological elastomer rail damper that works on the principle of a dynamic vibration absorber has been designed, analysed, characterised, and experimentally tested for the suppression of railway noise and vibration. The new rail damper incorporates variable stiffness magnetorheological elastomer layers, whose stiffness can be controlled by an externally applied magnetic field, to realise adaptive characteristics. Experimental characterisations of the magnetorheological elastomer rail damper were performed with an electromagnetic shaker. Subsequently, theoretical predictions of the track decay rate of a UIC-60 rail with different rail dampers and without rail damper were conducted; simulation results verified that magnetorheological elastomer rail dampers can improve the track decay rate of rail over a wider frequency range compared to conventional rail dampers and thus the performance of the magnetorheological elastomer rail damper outperforms other conventional rail dampers on rail noise reduction.

MODELLING OF RAIL VEHICLES AND TRACK FOR CALCULATION OF GROUND-VIBRATION TRANSMISSION INTO BUILDINGS