Proceedings of the Institute of Acoustics

 

 

Assessment of the noise reduction impact from application restrictions on rail dampers

 

Christoph Gramowski¹, Schrey & Veit GmbH, Sprendlingen, Germany

Roxana Donner², Acouplan GmbH, Berlin, Germany

 

ABSTRACT

 

In the last decades, rail dampers had become a common noise abatement measure at several mainline and metro networks. Rolling noise reductions outdoor/indoor up to 4/8 dB had been found. While the beginning of damper application was started with multiple test sections, infrastructure managers had raised installation restrictions from other related technology departments like signaling and communication. This is leading to defined sections where one rail has to remain partially free from dampers or the track has to remain completely free from dampers. These sections become relevant when the damper application is caused by noise tackling action plans. Therefore, the acoustic impact was assessed by a) laboratory test track analysis (partially free) and b) environmental noise prognosis software (completely free).

 

It is shown, that also for a partially free rail, a cancellation of the noise reduction can be expected for single 1/3 octave frequencies. This can be relevant for the overall noise emission. The effect from the completely free rail is much larger.

 

Railway noise mitigation has to consider these results or, if possible, the rail damper technology has to be improved to avoid these restricted sections.

 

1. INTRODUCTION

 

In the last decades, multiple approaches on ‘innovative’ rail noise abatement had been realized. However, only a small number of technology approaches had been stepped into a ‘serial’ usage. One of these technologies are rail dampers. This label represents all vibration reducing devices, which are directly applied to the rail, independent from their material, design and/or manufacturer. Common characteristic is that these devices are acting passive, thus without external energy supply. Core advantage is that a rolling noise reduction is introduced with low effort, compared to other trackside measures like noise walls. Also, most of the damper types are maintenance free, do not need operating supplies (e.g. lubricants) and are made for a long lifetime. Most of all track maintenance works remain possible.

 

 

Figure 1: Rail damper at mainline (left) and metro track (right)

 

Rail dampers are mostly used at mainline tracks and metro networks, see Figure 1. For the latter, the low rail fastening stiffnesses’ at slab tracks are leading to remarkable indoor noise reductions of up to 8 dB [1] [2]. At mainline tracks, usually different train types like freight, intercity and regional trains are operating. This is leading to a design where the damper benefit is balanced between the relevant train types, decreasing the noise reduction to common values of 2-3 dB [3] [4]. Also, for passengers inside trains, this rolling noise reduction is perceivable, if this is not masked by additional noise sources.

 

In the beginning, dampers had been applied following the approach “the more – the better”. From different track technology related reason this approach has been reduced by restrictions. However, noise prognosis calculations, which are often executed previously, are usually not considering these restrictions.

 

From the acoustic point of view, it is interesting in which amount the rail damper’s noise reduction is affected by these restrictions. In-field measurements are in general possible but suffer from high organizational, personal and material efforts.

 

2. EXISTING RESEARCH

 

Related investigations in the acoustic effect of reduced damper amounts had not been in the focus since general damper application questions are from a higher interest. Two publications are known from Ho et al. [5] and Croft et al. [6].

 

Ho et al. research is motivated by continuously less dampers per rail due to limited track access time (for installation) or a limited budget for purchase and installation work. For a ballasted track, it is shown that the Track Decay Rate (TDR) [7] starts to increase when only ¼ of the dampers are installed. Furthermore, “half installed” dampers are further increasing the TDR, leading to only small differences in most frequencies compared to a full damper installation. In contrast, for a booted sleeper track, the TDR improvement from ¼ dampers is less pronounced and it remains a remarkable TDR difference between “half installation” and “full installation”.

 

Rail vibration measurement results are confirming these findings at most of the considered locations at the rail.

 

Croft et al. are focusing on slab track applications with different technical constellations (rail type, fastening type etc.). The damper applications vary also in term of ‘density per rail meter’, similar to Ho et al. Results are presented both for in-field measurement and also a “short rail test” according the Stardamp approach [8]. The in-field results are determined by the rail roughness which makes an objective comparison difficult. In contrast, the “short rail” test offers neutral boundary conditions where only single parameter can be adjusted – but finally no sound reduction can be assessed. It is shown, that different damper densities are leading to damping changes which also differ in the frequency range but it is trending to decrease with less dampers.

 

Both papers are addressing the damper density but not track sections, which are completely free of dampers.

 

3. RESTRICTIONS FROM INFRASTRUCTURE MANAGERS

 

The beginning of rail damper application was started with multiple test sections [9] [10]. The installation was aiming on gathering noise reduction experience – more or less without installation restrictions.

 

However, a deeper communication inside the Infrastructure Managers (IM) had raised installation restrictions [11] from other related technology departments like signaling and communication (SC). For some railway networks, this is leading to appointed sections which have to remain completely free from dampers or where the rail has to remain partially free from dampers.

 

Examples for completely free sections are switches and crossings, both limited to their inner length but also with adjacent sections. Also, some IM specify tracks at platforms and road crossing, both with adjacent sections, to be completely damper free. Partially free sections are currently defined in some networks for sections where a SC cable is placed on the rail foot, where an electric isolation rail joint and/or where guide rails are located, for example.

 

These sections become relevant when the damper application is caused by noise tackling action plans – which are often based on public regulations like Germany [12], the Netherlands [13] or Austria [14]. Due to the view on the whole (national) networks, a detailed analysis of these ‘short’ sections cannot be executed. This bears the risk that a locally decreased noise reduction is arranged. The resulting resident protection difference should initially be avoided in any way. Furthermore, it is possible that residents are taking legal actions if they get aware that the promised and expected noise reduction is not realized on their location.

 

Therefore, an investigation in the acoustic effect of restricted damper track sections, always in relation to a completely damper applied track, can give a view in the impact on receiver’s location.

 

4. METHODOLOGY

 

The analysis is executed in two independent ways.

 

4.1. Analysis of Partially Damper Free Sections

 

In track noise assessment tools like Stardamp and related overall noise assessment tools like the German railway noise calculation scheme Schall03 [15] or the Dutch railway noise calculation scheme RMG2012 [16], a partially (reduced at one rail) damper installation cannot be introduced. Thus, an approach on laboratory conditions has to be used. This is usually executed by the common Stardamp approach where dampers are assessed at a ‘free’ 6 m rail. However, a modification at only one rail is changing only this noise emission while in reality the other rail remains unchanged. This ongoing noise emission has to be considered. A test track with two rails would avoid this deficit – however, sleepers and rail fastenings are determining the result and a comparison to Stardamp Decay Rates became impossible.

 

It has to be noted, that this test track can only be excited at one rail head. The other rail is not ‘directly’ but ‘indirectly’ excited by the vibration from the excited rail and its transformation through the sleepers. This rail’s decreased noise emission is therefore accentuating modifications on the excited rail. This leads to an overestimation of the resulting effect – although, it cannot exactly be quantified.

 

The Schrey & Veit laboratory is providing this type of test track, see Figure 2.

 

 

Figure 2: Test track at Schrey & Veit laboratory. Left rail with dampers solely at one side the rail, dampers from the back are only indicated by the spring clamp ends under the rail foot.

 

Technical parameters of the test track are: Rail length 6 m, profile UIC54, medium hard rail pads, sleeper B70 in 0.6 m spacing. The excitation is applied by an electrodynamic shaker (outside the image in Figure 2) in an angle of 45 ° at the railhead with a white noise force spectrum. At the three rail ends, where the shaker is not acting, several passive damping devices are fixed to decrease the reflecting effects and leading to more ‘infinite’ rail behaviour.

 

Rail dampers of Schrey & Veit type VICON AMSA 54 VS are installed, similar to the real application, in each sleeper bay. Here, one half of the dampers at the excited rail is left out, see left rail in Figure 2.

 

Assessment parameters are the airborne sound levels at four positions in 0.3 m horizontal and vertical distance to the excited rail, see also Figure 2. These small distances allow an assessment of the rail’s emission with a reduced influence from room reflection noise. Furthermore, this includes different emission characteristics in vertical and lateral direction. An additional microphone is placed in 1.6 m distance on top of the test track (not visible in Figure 2) to receive noise from both rails and both vibration directions.

 

The resulting rail acceleration is analyzed at 22 positions of the rail web.

 

4.2. Analysis of Completely Damper Free Sections

 

Effects of tracks without dampers, also on ‘short’ sections, can straightforwardly be assessed with common environmental noise prognosis software tools. Multiple immission points can be used to objective the noise reduction differences in different locations – e.g., distance to the track and acoustic surrounding.

 

Here, the immission differences should be assessed in an environment which is representative for rail damper application. Due to the dense location from railway line and (sub)urban areas, dampers in Europe are often installed at sections of river valley railway lines, see e. g. damper applications at German sections of the Rhine (>50 km) and Elbe valley (>30 km). This leads to mixed train types from freight, intercity and regional trains with medium speed ranges due to the curved aligning, see Table 1.

 

Table 1: Simulation parameter

 

 

Two immission points are used due to the possibility to assess noise level results also at color plots. Further parameter for sound propagation etc. are chosen according Schall03.

 

At these valley lines, usually some adjacent hills/mountains are impacting the acoustic scene by e. g. reflecting mechanism. This is depending on the local geographical situation and to deviate an ‘representative’ site is difficult. It was chosen to avoid this subjective influence by using a straight line at a plane surrounding.

 

Furthermore, for this question, only trains in the “Day” schedule are applied. Although the resulting absolute noise levels can differ from “Night” schedule calculations, the effect from restricted damper installations will not significantly be impacted.

 

The simulation is executed by the German railway noise calculation scheme Schall03 in octave band resolution. Resulting value is the “Beurteilungspegel” (rating level) which can be used for subsequent assessments on noise annoyance etc. Note: Here, only a “Day” train schedule is applied which precludes such assessments.

 

Four cases are assessed:

Case A: No dampers

Case B: Dampers

Case C: No dampers at 200 m track length

Case D: No dampers at 500 m track length

 

An application restriction specifies for stations a damper free section of platform length ±15 m while for switches this is specified to the switch length ±60 m. If resulting damper sections are less than 100 m length, no dampers have to be applied.

 

Case C is representing a track section at a station or at switches. A damper free section of 200 m is resulting for platform length of 170 m (typical for train stops at railway lines). A similar length is resulting for switch length of 80 m, a typical value for mainlines.

 

Case D is deviated from constellations, where a stations and /or switches are located in a distance, that the intermediate section is about 100 m and has also to remain damper free.

 

Damper free sections at road crossings are not assessed due to rarely occurring road crossing at mainlines which are common for damper applications.

 

The simulation was executed with CadnA Rev. 2022.

 

5. RESULTS

 

5.1. Partially Damper Free Sections

 

The analysis of the location in 1.6 m above the test track is showing that for partially damper free sections (dampers at only one side of one rail), a cancellation of the noise reduction can be expected for single 1/3 octave frequencies, see Figure 3. However, other frequency ranges are not affected by the damper absence. Here, it is irrelevant for this effect at which side of the rail the damper is left out.

 

 

Figure 3: Sound pressure level in 1.6 m above the test track

 

Similar results are obtained from the other SPL measurement locations although the spectral shapes differ. Especially at the locations vertical 0.3 m above the excited rail, up to 10 dB decreased noise reductions are visible in the range 1 to 2.5 kHz, see Figure 4.

 

 

Figure 4: Sound pressure level in 0.3 m above the excited rail

It is important to keep in mind that these laboratory track results are showing only track noise effects, train (especially wheel) noise aspects are left out.

 

5.2. Completely Damper Free Sections

 

The analysis results are shown in color plots for an adjacent area of sizes 500 m (rectangular to the railway line) and 1000 m in train running direction.

 

It is visible, that the noise levels are reduced by the dampers in the whole assessed area, see Figure 5 and 6. Reductions of about 2.5 dB are resulting in both immission points. Note: With increasing distance to the railway line, the noise level iso lines lost the parallelism due to the boundary effects from the finite length of the simulated track.

 

 

Figure 5: Rating level L_r with no dampers

 

 

Figure 6: Rating level L_r with full damper application

 

When damper free sections are introduced, the noise levels at the adjacent areas are restored to the initial values from “No dampers”, see Figures 7 and 8. The areas of noise increase are located along these sections but also farther away. By increasing the distance to the track, the isoline shapes are changing from line source to point source contours (‘getting curved’). This is more pronounced at the longer damper free section.

 

 

Figure 7: Rating level L_r at 200 m damper free section

 

 

Figure 8: Rating level L_r at 500 m damper free section

 

The spatial distributions of noise level differences are shown in Figures 9 and 10, both referencing to the constellation “full damper”.

 

 

Figure 9: Rating level L_r differences at 200 m damper free section

 

 

Figure 10: Rating level L_r differences at 500 m damper free section

 

The highest level differences are located close to the damper free section with values of up to 2.5 dB. This implies a neutralization of the damper’s noise reduction effect. It is interesting to see that the noise is also increased at locations, which are rectangular to damper sections.

 

Results of the spectral analysis for both immission points are shown in Figure 11 and 12.

 

 

Figure 11: Rating level L_r spectra at 25 m line distance

 

 

Figure 12: Rating level L_r spectra at 100 m line distance

 

The octave band at 1 kHz is dominating the rating level. With dampers, this octave band level is decreased by about 3 dB at both immission points. For the immission point in 25 m distance, both damper free sections are fading out the damper noise reduction and the initial levels are present again. For the immission point in 100 m distance, this is also the case for a damper free section of 500 m length while at 200 m length a small noise reduction from about 0.2 dB is remaining.

 

6. CONCLUSIONS

 

Both analysis approaches are indicating the impact on real damper applications. If one side of one rail cannot be applied with rail dampers, e.g., due to a SC cable on the rail foot, laboratory tests are showing a decrease in the damper’s noise reduction. Although this takes not place in the whole relevant frequency range, a similar noise reduction (compared to full dampers) potential cannot be secured. To meet the specified noise reduction potential (see e. g. rail damper product specifications from the Netherlands [17] or Germany [18]), a full damper application is indicated from this laboratory analysis.

 

If track sections are completely free from rail dampers, the simulation shows a decreased noise reduction in the adjacent areas. For some immission points, the decrease in noise reduction is from a high amount, resulting in a completely damper effect neutralization at some areas. This effect is ‘starting’ close to the track chainage point where dampers are left out. With increasing free damper section length, also the rectangular distance increases, where the damper’s noise reduction is decreased. In this example, a 500 m length free damper section is fading away the noise reduction at rectangular distances up to 200 m.

 

Both results are pointing on a reduced noise reduction from partially and full damper free sections, both compared to a completely damper section. This should be avoided to realize the noise reduction in the initially desired amount. For example, the damper technology has to be improved to become compatible with rail located cables.

 

7. REFERENCES

 

1. Zoontjes, L.; Welsh, L.; Croft, B.: Predicting and managing rolling noise emissions from trains on the Perth metro passenger rail network, Acoustics 2017, Perth

2. Croft, B.: Rail dampers transit noise reduction outcomes, Wheel Rail Interaction 2021 - Rail Transit Seminar

3. Thompson, D. J.: Railway Noise and Vibration - Mechanisms, Modelling and Means of Control, Elsevier, Oxford 2009

4. Margiocchi, F.; Poisson, F.; Gramowski, C.; Tartary, J. P.: A 8-years complete assessment of rail damper performances on an operated track in France, World Congress on Rail Research 2016

5. Ho, W.; Cheung, C.; Cheng, M.; Lin, L.: Half Installation of Rail Damper. In: Degrande, G. et al.: Noise and Vibration Mitigation for Rail Transportation Systems, Proceedings of the 13^th International Workshop on Railway Noise, Ghent 2019

6. Croft, B.; Miller, A.; Gramowski, C.: Track decay rate analysis and rail damper noise reduction for slab tracks. In: Degrande, G. et al.: Noise and Vibration Mitigation for Rail Transportation Systems, Proceedings of the 13^th International Workshop on Railway Noise, Ghent 2019

7. Jones, C. J. C.; Thompson, D. J.; Diehl, R.: The use of decay rates to analyse the performance of railway track in rolling noise generation. Journal of Sound and Vibration 293 (2006), pp 485-495

8. Asmussen, B. et al.: STARDAMP Standardisation of Damping Technologies for the Reduction of Rolling Noise. Final report 27^th May 2013

9. van Haaren, E.; van Keulen, G. A.: New Rail Dampers at the Railway Link Roosendaal Vlissingen Tested within the Dutch Innovation Program. In Schulte-Werner, B. et al.: Noise and Vibration Mitigation for Rail Transportation Systems, Proceedings of the 9^th International Workshop on Railway Noise, Munich 2007

10. DB Netz AG: Innovative Maßnahmen zum Lärm- und Erschütterungsschutz am Fahrweg, Schlussbericht, Berlin 2012

11. DB Netz AG 2021, internal paper

12. Vierundzwanzigste Verordnung zur Durchführung des Bundes-Immissionsschutzgesetzes - Verkehrswege-Schallschutzmaßnahmenverordnung (24. BImschV)

13. Wet Geluidhinder 1979 (Noise Abatement Act 1979)

14. Verordnung des Bundesministers für öffentliche Wirtschaft und Verkehr über Lärmschutzmaßnahmen bei Haupt-, Neben- und Straßenbahnen (Schienenverkehrslärm Immissionsschutzverordnung - SchIV) 1993

15. Anlage 2 der Sechzehnten Verordnung zur Durchführung des Bundes Immissionsschutzgesetzes (Verkehrslärmschutzverordnung) Berechnung des Beurteilungspegels für Schienenwege (Schall 03) 2015

16. Staatssecretaris van Infrastructuur en Milieu: Reken- en meetvoorschrift geluid 2012

17. ProRail: Productspecificatie Raildempers, SPC00323, 2021

18. DB Netz: DBS 918 290 – Technische Lieferbedingungen „Schienenstegdämpfer“, 2017

 

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¹ christoph.gramowski@sundv.de

² roxana.donner@acouplan.de