VTI rapport 559A www.vti.se/publications Published 2006
Noise emission from railway traffic
Mikael Ögren
Utgivare: Publikation: VTI rapport 559A
Utgivningsar:˚ Projektnummer: Dnr: 2006 12547 2005/0574-24
581 95 Link¨oping Projektnamn: Bulleremission fr˚an sp˚arburen trafik
Forfattare:¨ Uppdragsgivare: Mikael Ogren¨ VTI
Titel: Bulleremission fr˚an sp˚arburen trafik
Referat (bakgrund, syfte, metod, resultat) max 200 ord: I Europa ¨onskar man ¨oka andelen gods och personer som transporteras p˚aj¨arnv¨ag i f¨orh˚allande till v¨ag och om det lyckas s˚akommer problemet med buller fr˚an sp˚arburen trafik att ¨oka. Det ¨ar sedan tidigare k¨ant att ˚atg¨arder direkt p˚ak¨allan ¨ar effektivare ¨an indirekta ˚atg¨arder som byggnation av bullersk¨armar eller byte av f¨onsterglas i bost¨ader, ¨aven om dessa ˚atg¨arder ocks˚ahar en viktig funk- tion. Denna rapport ¨ar en litteratur¨oversikt ¨over hur bullret genereras och hur olika ˚atg¨arder vid k¨allan kan minska bulleremissionen. Rapporten beskriver ocks˚akortfattat vilka gr¨ansv¨arden och regler f¨or bulleremissionen som g¨aller nu och kommer att g¨alla den n¨armaste framtiden inom EU. Dessutom diskuteras de ber¨akningsmodeller som anv¨ands f¨or att ber¨akna ljudniv˚an i olika mottagarpositioner (immissionen) utifr˚an de olika sp˚arfordonens emission.
Nyckelord: T˚agbuller, bulleremission, buller˚atg¨arder ISSN: Sprak:˚ Antal sidor: 0347–6030 Engelska 37 Publisher: Publication: VTI rapport 559A
Published: Project code: Dnr: 2006 12547 2005/0574-24 SE-581 95 Link¨oping Project: Noise emission from railway traffic
Author: Sponsor: Mikael Ogren¨ VTI
Title: Noise emission from railway traffic
Abstract (background, aims, methods, results) max 200 words: European authorities hope to be able to increase the volume of freight and passengers that are trans- ported on railway systems compared to those transported on roads. If that policy is successful the problem of noise from railway traffic will increase. It is known from previous research that measures against the noise are more efficient if applied directly at the source itself rather than using indirect measures such as noise barriers and increased window insulation. This report is a literature survey on how the railway noise is generated, and to what extent different measures at the source can reduce the noise emission. The report also briefly describes what limits and recommendations on noise exposure are enforced now and in the near future. Furthermore the methods used for calculating the noise level at different receiver positions (noise exposure) from the noise emission are discussed.
Keywords: Railway noise, noise emission, noise measures ISSN: Language: No. of pages: 0347–6030 English 37 Preface This project started in September 2005, and has been funded by VTI.
I appreciate the valuable contributions and comments by Lennart Folkeson (VTI), Ulf Sandberg (VTI), Bertil Hyl´en (VTI), Krister Larsson (SP The Swedish National Testing and Research Institute) and Karin Blidberg (Banverket).
G¨oteborg, September, 2006
Mikael Ogren¨
Cover: Gray X40 train, Leif-Erik Nyg˚ards Red noise barrier, VTI/ Hejdl¨osa bilder Quality review Review seminar was carried out on 12 December 2005 where Krister Larsson (SP Swedish National Testing and Research Institute) reviewed and commented on the report. Mikael Ogren¨ has made alterations to the final manuscript of the report. The research director of the project manager Lennart Folkeson examined and approved the report for publication on 26 October 2006.
Kvalitetsgranskning Granskningsseminarium genomf¨ort den 12 december 2005 d¨ar Krister Larsson (SP Sveriges Provnings- och Forskningsinstitut) var lekt¨or. Mikael Ogren¨ har genomf¨ort justeringar av slutligt rapportmanus. Projektledarens n¨armaste chef Lennart Folkeson har d¨arefter granskat och godk¨ant publikationen f¨or publicering den 26 oktober 2006. Table of contents
Listoffigures ...... 5 Listoftables...... 6 Summary ...... 7 Sammanfattning...... 9 1 Introduction...... 11 1.1 Railwaynoise ...... 11 1.2 Annoyance and health effects of railway noise ...... 11 1.3 Scopeofthisreport ...... 13 2 Noisesources...... 14 2.1 Rollingnoise...... 14 2.2 Wheel/railroughness...... 17 2.3 Curvesqueal...... 19 2.4 Aerodynamicandsecondarysources ...... 19 3 Measures ...... 20 3.1 Addressingthesource...... 20 3.2 Wheel/railmeasures ...... 20 3.3 Summaryofreductionpotential ...... 22 4 Determiningrailvehiclenoiseemission ...... 24 4.1 Soundpowerandsoundpressure ...... 24 4.2 Maximumandequivalentlevel ...... 24 4.3 ISO3095...... 25 4.4 TWINS...... 26 5 European limits, targets and calculation methods...... 28 5.1 Noiseemission...... 28 5.2 Noiseexposure...... 28 5.3 Standardisedcalculationmethods ...... 29 6 Discussion...... 31 6.1 Increasing traffic volumes and noise exposure ...... 31 6.2 Railaccesscharges...... 31 6.3 Whoisresponsiblefortherollingnoise? ...... 32 Glossary...... 33 References...... 35 List of figures
1.1 Polynomial approximation of percentage of subjects highly annoyed by transportationnoise...... 12
2.1 Sketch of wheel-rail interaction and the track including sleepers and pads. . 14 2.2 Power flow from the contact patch that connects wheel and rail...... 14 2.3 Illustration of the effect of wheel rotation on the mechanical waves in the wheel propagating away from the contact patch...... 15 2.4 Typicaltrackstructure...... 16 2.5 Sketch of the mechanical components of the track in the vertical direction. 17 2.6 Examplesofcalculatedmodesonarail...... 17 2.7 Sketch of wheel-rail interaction and the track including sleepers and pads. . 18 2.8 Rail roughnessamplitudeandwavelength...... 18
3.1 Railgrinding...... 21 3.2 Illustration of screens both on the vehicle and close to the track in com- bination...... 22
4.1 Noise emission vs. exposure and sound power level Lw vs. sound pres- sure level Lp...... 24 4.2 Illustration of sources on a train that contribute to the maximum sound pressure level at different distances...... 25 4.3 Sound pressure level as a function of time during train passage...... 26 4.4 FlowchartoftheTWINSmodel...... 27
5.1 Illustrationof emissionand exposure...... 28 5.2 Harmonoiseprojectstructure...... 30 List of tables
3.1 Noisereductionpotentialofmeasures...... 22
VTI notat 53-2004 Noise emission from railway traffic by Mikael Ogren¨ VTI (Swedish National Road and Transport Research Institute) SE-581 95 Link¨oping
Summary
European authorities hope to be able to increase the volume of freight and passengers that are transported on railway systems compared to those transported on roads. If that policy is successful the problem of noise from railway traffic will increase. It is know from previous research that measures against the noise are more efficient if applied di- rectly at the source itself rather than using indirect measures such as noise barriers and increased window insulation. This report is a literature survey on how the railway noise is generated, and to what extend different measures at the source can reduce the noise emission. The report also briefly describes what limits and recommendations on noise exposure are enforced now and in the near future. Furthermore the noise propagation methods used for calculating the noise level at different receiver positions (noise expo- sure) from the noise emission are discussed.
VTI rapport 559A 7 8 VTI rapport 559A Bulleremission fran˚ sparburen˚ trafik av Mikael Ogren¨ VTI SE-581 95
Sammanfattning
B˚ade gods- och persontransporter v¨antas ¨oka inom j¨arnv¨agssektorn. F¨orhoppningen fr˚an det Europeiska j¨arnv¨agsforskningsr˚adet (ERRAC) ¨ar att passagerar- och godsvolymerna skall tredubblas fram till 2020. Om detta besannas kommer problemen med buller fr˚an j¨arnv¨agstrafik att stadigt ¨oka i framtiden. Det ¨ar v¨alk¨ant att det ¨ar effektivast att ˚atg¨arda bullret vid k¨allan, dvs att angripa ljudet som str˚alar ut fr˚an hjul och r¨al snarare ¨an att bygga bullersk¨armar eller ¨oka fasadiso- leringen hos n¨arliggande bost¨ader. Denna rapport ¨ar en kunskaps¨oversikt ¨over olika s¨att att angripa bullret direkt vid k¨allan, t.ex. via vibrationsd¨ampare och slipning av hjul och r¨al. De grundl¨aggande processerna hur ljudet genereras och utbreds g˚as ocks˚aigenom, samt hur dessa modelleras exempelvis i mjukvaran TWINS, som har utvecklats inom ett flertal europeiska forskningsprojekt p˚aomr˚adet. En viktig slutsats ¨ar att b˚ade r¨alen och fordonen bidrar till det utstr˚alade ljudet, vilket inneb¨ar att ansvaret f¨or bullret inte vilar enbart p˚aBanverket eller t˚agoperat¨oren, utan p˚ab˚ada. Rapporten beskriver ocks˚af¨orh˚allandet mellan riktv¨arden f¨or bullerniv˚an hos boende i n¨arheten (immissionen) och f¨or den k¨allstryka som varje fordon representerar (emis- sionen). I Sverige finns inga riktv¨arden f¨or emissionen annat ¨an de som g¨aller f¨or hela EU via de s˚akallade TSI-dokumenten, som anger de specifikationer som j¨arnv¨agsfordon m˚aste uppfylla f¨or att f˚atrafikera det gemensamma europeiska j¨arnv¨agsn¨atet. F¨or im- missionen d¨aremot finns klara riktv¨arden, vilket skapar en obalans i hur ˚atg¨arder f¨or att minska bullret s¨atts in. Eftersom f˚aeller inga krav st¨alls p˚afordonen ˚aterst˚ar en- dast att bygga bullersk¨armar, ¨oka fasadisoleringen p˚abefintliga bost¨ader eller att planera banstr¨ackningen s˚aatt f˚aboende blir ber¨orda.
VTI rapport 559A 9 10 VTI rapport 559A 1 Introduction
1.1 Railway noise Noise is loosely defined as unwanted sound. Railway traffic generates sound mainly due to the vibrations induced by the small roughness on the wheel and rail surfaces. These vibrations radiate sound from the wheels and the rail, and also from other vibrating sur- faces such as parts of the boogie and suspension, rail car sides and so on1. This sound propagates through the atmosphere and arrives at the listeners, who are the ones who determine if the sound is unwanted (i.e. noise) or not. Whether the sound is unwanted or not, it can damage the hearing of persons subjected to it if the sound level is high enough. At lower levels it will interfere with speech and can mask other possibly important sounds. But there are positive effects also, such as alerting people to the fact that a train is about to pass, or making it possible to detect damaged components or other faults by changes in the radiated sound. Noise radiation is by no means the only environmental effect of the running train. If it is a diesel train there will be exhaust emissions, all trains emit particles due to wheel, rail and brake pad wear. Fast and heavy trains cause vibration in the ground and buildings. The presence of railway traffic also has an impact on wildlife [1]. The traffic volumes of both passengers and freight over the European rail network are expected to increase in the future, both as a result of a deliberate policy within the Euro- pean Union to shift traffic from other modes and due to increased transportation needs. The European Rail Research Advisory Council (ERRAC) mentions a tripling of passen- ger transport and more than a tripling of freight transport between 2000 and 2020 [2]. The International Union of Railways (UIC) and partners have almost as high expecta- tions in their research strategy [3]. If the traffic volumes increase that much, the environmental impact will be substantial, both from expanding the network with new sections and from the increased traffic on existing tracks. A very rough estimate for the noise emission would be an increase in the equivalent overall noise level of about 5 dB(A) and a tripling of the number of loud events during the night.
1.2 Annoyance and health effects of railway noise The effects of noise from railway transports upon the public can be divided into three main areas, general annoyance, sleep disturbance and cardiovascular effects. Here gen- eral annoyance refers to the disturbance experienced by the public as reported in ques- tionnaires. When exposed for a long time this annoyance together with sleep distur- bances is believed to lead to cardiovascular effects apart from the impact on the quality of life in general. The relationship between the general annoyance reported in questionnaires and the sound level is described in [4, 5], where data from several large studies have been analysed to- gether in a meta analysis. The reported annoyance is lower for railway traffic than for road and air traffic. An example of the curves for the percentage of subjects that an- swered they were “highly annoyed” is given in Fig. 1.1.
1The different generation mechanisms are discussed in chapter 2
VTI rapport 559A 11 This lower annoyance for railway traffic is often reflected in limits where a bonus is given to rail traffic based on these results. However, recent studies indicate that in some cases the reported annoyance from rail traffic is equal to or higher than that of road traf- fic [6–8]. Cardiovascular effects have also been studied by meta analysis of data from question- naires and laboratory experiments [9] for transportation noise in general. This analysis shows that for subjects that have been exposed to high noise levels (Level Day Evening 2 Night (Lden) > 65 dB) for extended periods of time there is an increased risk for my- ocardial infarction (lethal infarction). There is also evidence of an increased risk of high blood-pressure among subjects exposed to high noise levels, at least for male sub- jects [10].
70 Air Road Rail 60
50
40
30 Highly annoyed [%] 20
10
0 45 50 55 60 65 70 75 LDEN [dB] Figure 1.1 Polynomial approximation of percentage of subjects highly annoyed by transportation noise. From [5].
2 The noise level metric Lden is explained in chapter 5
12 VTI rapport 559A 1.3 Scope of this report This report is a literature survey and knowledge synthesis on noise emission from rail- way traffic. The effects of vibration, both on buildings close to the railway and inside the rail vehicle itself, are not included. Neither is noise inside the rail vehicle, only noise emitted to the exterior of the rail vehicle is dealt with in this report. The method used is a literature survey, where references have been collected with the aid of the transport library at VTI3. Literature in Nordic languages and English published after 1990 was used. The focus is on Sweden as an EU member and therefore part of the future European rail network. For this network interoperability and abolishing trade obstacles are important concepts [3].
Chapter 2 focuses on the source mechanisms such as rolling noise, aerodynamic noise and curve squeal. Chapter 3 is a review of different noise abating measures such as screening, silent brake pads and rail grinding. The reduction potential for different measures are summarised in a table based on several references. Chapter 4 describes how to determine the noise emission strength of a rail vehicle us- ing measurements with the recently updated standard ISO 3095. The maximum and equivalent levels are explained and the quantities sound power and sound pressure are discussed.
Chapter 5 discusses the limits and targets for noise from railway traffic used in coun- tries within the European Union. The different metrics are also explained and dis- cussed together with the calculation methods used to predict the noise level today, and the future calculation method Harmonoise is presented.
Chapter 6 summarises the report and discusses the noise emission challenges faced by the rail industry and society as a whole due to the expected growth in railway traffic volumes in the future.
3The VTI library is accessible online at http://www.transguide.org
VTI rapport 559A 13 2 Noise sources
2.1 Rolling noise 2.1.1 Wheel/rail interaction The primary physical process responsible for the vibrations that radiate as noise is the contact between the train wheel and the rail. Fig. 2.1 shows a sketch of a wheel on a rail- way track. The wheel tread rests on the rail head, and the mechanical contact is within a contact patch approximately 10 to 15 mm long and about as wide [11, ch.1]. When the wheel is rolling on the rail the small unevenness of both wheel and rail cause forces on both of them. These forces excite vibrations throughout the whole system which in turn radiates sound. This noise generation mechanism is known as rolling noise.
Figure 2.1 Sketch of wheel-rail interaction and the track including sleepers and pads.
The mechanical power flow through the system can be described by the simple flowchart in Fig. 2.2 The power is generated by the forces on the contact patch, and via the vi- bration patterns of the wheel, rail and sleepers it can propagate away as ground waves, radiate as sound or be dissipated as heat.
Figure 2.2 Power flow from the contact patch that connects wheel and rail.
14 VTI rapport 559A 2.1.2 Wheel vibration and sound radiation The wheel is a resonant structure with a shape relatively close to that of a (flat) bell. Like the bell the freely suspended structure is very undamped. The force from the con- tact patch will excite vibrations in the wheel much like the force from the clapper in a bell. There are differences though, the wheel is pressed against the rail by a constant preload (the part of the train mass carried by the wheel) and it also rotates around the axle. For the wheel the contact with the rail can be seen as added damping since vibration en- ergy can be transmitted from the wheel into the rail where it propagates away and never comes back. This changes the typical loss factor from 10−4 of the freely suspended wheel to 10−3 for a wheel preloaded against the rail, see [12] and [11, ch.1&6]. This also makes it important to test measures that increase the damping of the wheel under preloaded conditions, since testing on a freely suspended wheel may give misleading results both for the vibration of the wheel and the sound radiation from it. The rotation of the wheel makes it a bit more complicated to understand than a struc- ture at rest. Instead of a single mode or vibration pattern corresponding to one resonance frequency, some modes are broken up into two parts. As opposed to a single frequency related to a certain vibration pattern, there will be two frequencies. This is due to the dif- ferent wave propagation speeds for waves travelling from the contact patch and forward or backward, respectively. A wave travelling forward gets a shorter wavelength than a wave travelling backward due to the motion of the wheel, see Fig. 2.3. This is similar to the Doppler effect for sound waves. The effect becomes more pronounced as the rota- tional speed of the wheel increases.
Figure 2.3 Illustration of the effect of wheel rotation on the mechanical waves in the wheel propagating away from the contact patch.
VTI rapport 559A 15 2.1.3 Rail vibration and sound radiation The rail is an infinite beam-like structure. The forces from the wheel-rail contact patch move along the rail as the wheels move along it, leading to similar effects as for the rota- tion of the wheel. The rail is fastened to the sleepers via resilient pads, and the sleepers are spaced periodically along the track (although random spacing has been suggested, see [11]). The sleepers are typically placed in a ballast, but in some cases the rail might be fas- tened with or without pads or sleepers directly into a stiff underground. Alternatively the rails can be mounted on a concrete slab which rests on some kind of resilient springs. The majority of the tracks are of the rail–pad–sleeper–ballast type, see Fig. 2.4.
Rail
Pad
Sleeper Ballast Inert ground Figure 2.4 Typical track structure.
At very low frequencies the track is very stiff in the vertical direction. The first reso- nance corresponds to the mass of the entire track and the stiffness of the ballast. The second resonance is typically slightly higher in frequency and involves the stiffness of the pads and the mass of the rail and the sleepers, where the rail and the sleepers are moving in anti-phase. The resonance frequencies can of course vary a lot depending on the rail mass, pad stiffness and other factors, but typical values for a modern rail on concrete sleepers are around 100 Hz for the first and 500 Hz for the second resonance. Between those resonances there is an anti resonance where the track acts like a tuned ab- sorber. At frequencies higher than the second resonance the rail is essentially decoupled from the sleepers by the resilient pads. The mechanical system can be seen as a number of masses and springs acting in the vertical direction as in Fig. 2.5. Note that the damp- ing is not included in this simple representation. The bending and longitudinal waves that travel along the length of the rail are filtered by the complex structure of the periodically spaced sleepers. Waves at some frequencies are heavily damped, while others travel along the track relatively unhindered. The important factor for the radiated sound is the damping as an average over a frequency band, often expressed as the damping ratio per metre (dB/m). Fig. 2.6 shows a few of the vibration patterns that can propagate along the rail.
16 VTI rapport 559A Rail mass
Pad stiffness
Sleeper mass Ballast stiffness Figure 2.5 Sketch of the mechanical components of the track in the vertical direction.
Figure 2.6 Examples of calculated modes on a rail. From [13].
At low frequencies the rail together with the sleepers are dominating the sound radia- tion. The wheel has a smaller radiating area and has few modes at low frequencies. In the mid-frequency range the sleepers become uncoupled from the rail and the rail is the dominating source. At high frequencies the wheel is the dominant source since the damping is lower than in the rail. As noted above the damping in the wheel is deter- mined by the damping introduced by the contact with the rail, i.e. the damping in the rail.
2.2 Wheel/rail roughness The small unevenness or roughness on the wheel and rail surface is the physical struc- ture behind the forces on the contact patch that cause the vibrations and hence the sound radiation, which is illustrated in Fig. 2.7. The study of forces and deformations due to mechanical contact between bodies in general is called contact mechanics, see [14]. Ap- plied on the case of a railway wheel and rail it can explain the process behind the force generated at the contact patch and give insights useful when modelling the contact inter- action. The roughness on either wheel or rail can be characterised by the length scales or wave- length present and on the roughness amplitudes at those length scales, see Fig. 2.8. Mea-
VTI rapport 559A 17
Figure 2.7 Sketch of wheel-rail interaction and the track including sleepers and pads. suring the height with a sensitive displacement sensor1 along the wheel or rail surface gives a roughness profile. Taking the Fourier transform of the profile will yield a rough- ness spectrum; a decomposition of the profile into amplitudes at different length scales.
Length scale Wavelength Amplitude
Figure 2.8 Rail roughness amplitude and wavelength.
Depending on the speed of the train a certain length scale will cause vibrations of differ- ent frequencies according to f = v/λ, (2.1) where f is the frequency in Hz, v the train speed in m/s and λ the length scale or wave- length in metres. In [11, ch.1] it is stated that the length scales important for noise radia- tion are between 5 and 200 mm, with amplitudes from 1 µm upwards. One important effect of the size of the contact patch is what is known as the contact filter. For roughness lengths that are shorter than the size of the contact patch (about 15 mm) the excitation is less than expected due to the averaging effect of the patch. This acts like a low-pass filter on the roughness when seen as an input signal into the system. In Fig. 2.7 the roughness is illustrated in a two dimensional manner, but in the real case the problem is three dimensional. The wheel and the rail are rough not only in the di- rection along the rail but also in the lateral direction. If the roughness is random in both directions there is less excitation than if it is correlated in the lateral direction. This is important for braking systems that apply brake pads directly on the wheel tread, which can lead to a correlated roughness profile which is worse than a random profile.
1Commonly using Laser or LVDT (Linear Variable Differential Transformer) technology.
18 VTI rapport 559A The generation of noise for a car tire rolling over a rough asphalt surface is a related re- search area. One of the differences between the rubber tire and the railway wheel is the adhesion. For car tires a certain roughness is desirable since a very smooth surface can lead to slightly more noise generation due to increased adhesion [15, ch.11]. For the railway wheel the optimum surface would be perfectly flat, since the adhesion compo- nent is less influential in this case.
2.3 Curve squeal Curve squeal is the intense tonal noise that can set in when a rail vehicle traverses a curve or switch. The process starts with either lateral creeping in the contact patch be- tween rail and wheel or rubbing of the flange of the wheel against the rail. When the stick-slip process at the patch or the flange becomes unstable, i.e. when there is a feed- back that leads to instability, the wheel will radiate the tonal noise. In [16] a theoretical approach and measurements in a special rig is used to investigate the phenomenon. The main results are a theoretical model that can predict when squeal will occur which together with the measurement rig makes it possible to investigate the effectiveness of measures such as lubrication, wheel damping and rail transverse profile adjustment. In practice lubrication is often used to reduce squeal noise. The lubricant can be water or petroleum/vegetable products. They reduce the friction which in turn removes the instability.
2.4 Aerodynamic and secondary sources 2.4.1 Aerodynamic sources Aeroacoustic sources are closely related to airflow around the train and the optimisa- tion for a low air resistance. Where the airflow is turbulent, sound will be emitted, and at high speeds (>300 km/h) the contribution can be substantial. The pantograph is a typi- cal problem area, as well as other structures that are protruding from the exterior of the train. Recesses are also important, and the turbulent boundary layer around the surfaces of the train. The sound power emitted by the aeroacoustic sources is strongly dependent on the train speed, between v4–v8 depending on the source characteristics. Another important prop- erty of aeroacoustic sources is that they are predominantly high frequency sources, so rolling noise will still be important at lower frequencies even if the train is very fast. In the special case with a barrier that shields the wheel-rail source but not the panto- graph, aeroacoustic sources can be important at lower speeds. Without the barrier the aeroacoustic sources are masked by the rolling noise, but the barrier reduces the rolling noise thereby making the aeroacoustic noise audible.
2.4.2 Secondary sources Secondary sources are noisy machinery on the train such as cooling fans and power transmissions. If poorly designed they may contribute to the total noise emitted by the vehicle. They may also be very important even if they are not audible on the running ve- hicle when it is not moving. Then there is no rolling noise or aeroacoustic sources that can mask the noise from fans or other auxiliary equipment.
VTI rapport 559A 19 3 Measures
3.1 Addressing the source A general truth when dealing with noise problems is that they are best addressed at the source. Measures concerning the propagation are generally less cost effective than ac- tions taken directly at the source. Source-directed measures usually mean modification of the wheel or rail whereas secondary measures typically mean constructing sound bar- riers or increasing the fac¸ade insulation of nearby buildings. In [17] the favourable cost/benefit ratio of wheel/rail measures compared to screening or fac¸ade insulation is reported, but how come that screens and fac¸ade insulation are used to the extent they are? One explanation is that these measures are often under the direct control of the infrastructure managers, but limits on the permissible sound levels from individual vehicles are an issue for the legislative bodies. In the European perspectives strict legislation in one country but not in its neighbours is effectively an obstacle for railway interoperability and trade. This means that the process of tightening the emis- sion limits must be handled in an international perspective. Barriers and fac¸ade insula- tion are measures that can be taken locally and have an effect much faster in comparison with the introduction of quieter vehicles.
3.2 Wheel/rail measures 3.2.1 Roughness measures As discussed in chapter 2 the roughness on the wheel and on the rail is the primary source mechanism. If the roughness amplitude can be decreased, or the roughness spec- trum altered to avoid “bad” wavelengths, the sound emission will decrease. Grinding the rail and the wheel tread to get a smooth surface on both is an effective way of reducing the roughness. The reduction potential is given as 10 dB in [11], but if the track and the wheels that traffic them are already in a good condition such high reduc- tions can not be expected. Not much is known on how frequent such grinding operations must be to ensure acceptable roughness levels, but a scheme is presented in [18] where a database is used to decide when to grind a particular section. Fig. 3.1 shows a rail grind- ing vehicle in operation in Italy. One important source of wheel and rail roughness is vehicles with tread braked wheels. The brake pads create a roughness on the wheel, which in turn makes the rail rough over time. Therefore replacing cast iron brake blocks with composite material blocks, or changing the braking system altogether, would be beneficial for all vehicles that travel on the same track. UIC has stated that a reduction of noise emission from tread braked freight waggons of 8 dB(A) is possible by retrofitting the brake blocks using existing technology, but in [19] it is claimed that no proven retrofitting solutions were available at the time of writing (2003). In [17] an example is given where it is estimated that all European freight waggons with tread braked wheels can be retrofitted within a 5 year period at a cost of 20 · 109 e. Al- though this is a crude example it still illustrates that it is not an impossible scenario to retrofit the current fleet. Restricting the measures to fitting new waggons with more silent braking systems would only have an influence in the long term since the lifespan of the existing rolling stock is at least 50 years [17].
20 VTI rapport 559A Figure 3.1 Rail grinding. c Mecnafer, Italy 2006.
Another approach to reduce the influence of the roughness is to make the contact patch larger, thereby changing the contact filter effect. This can be achieved by lowering the stiffness of the tread, but that can be difficult due to increased mechanical stress and wear.
3.2.2 Wheel and rail damping Adding damping to the wheel or the rail makes those structures less resonant, which lowers the vibrations and thus the sound radiation. As stated in chapter 2 it is very im- portant to test such systems under realistic conditions, since the damping of a freely sus- pended system might be considerably less than if it is preloaded. There is also a com- plex interaction between wheel and rail, and if the treatment on one of them affects the contact between them the other part might radiate more sound. The wheel measures that have been reported include constrained layer or viscous layer damping added to the wheel. A theoretical study of what can be achieved for a few ex- ample situations is reported in [20]. The effect of the constrained layer on total sound radiation is between 3.0 and 5.1 dB(A) depending on the wheel radius and braking sys- tem. Adding tuned absorbers to the wheel works in a similar way as constrained layers, ex- cept that they are effective in a more limited frequency region. In [21] a number of stud- ies are mentioned where the effect is 4–8 dB(A) for different forms of tuned absorbers.
3.2.3 Screens close to the wheel/rail As an alternative to measures directly at the wheel or rail a screen can be mounted on the boogie or the vehicle frame. In combination with a screen on the ground close to the track this can almost totally seal the wheel-rail source in a “moving room” as illustrated in Fig. 3.2. If sound absorbers are present in this virtual room the radiated sound will be reduced even more. The major drawback with such solutions is that they can interfere with the permissible gauge of vehicles on that particular track, i.e. some waggons might not clear the ob- stacle that the stationary screen represents. In [21] an example is given from the Silent Freight and Silent Track projects where a gap due to international gauging constraints led to an insertion loss of only 3 dB(A). The effectiveness of such screen combinations can be further reduced if the rail is the dominant sound radiator since the wheel is fully enclosed by the screens but the rail is only partly shielded.
VTI rapport 559A 21 Vehicle mounted screen Stationary Wheel screen Rail Figure 3.2 Illustration of screens both on the vehicle and close to the track in combi- nation.
One important benefit of combining a low barrier with a vehicle mounted screen com- pared to a standard high barrier is that the low barrier can be stepped on or over when evacuating a disabled vehicle. A high barrier forms a difficult obstacle when evacuating. Another benefit is the lower visual barrier effect, both for the passenger (scenery) and for the housing environment close to the track.
3.3 Summary of reduction potential In Tab. 3.1 a summary of reported reduction potential for different measures is given. The results are taken from [21–24], and the majority are based on field tests but some are based on theoretical data. Note that in all cases (except perhaps for wheel grinding) a poorly designed measure can have no effect at all, or possibly even worsen the situation. But a well designed and tested solution can attain the reduction levels given in the table. Table 3.1 Noise reduction potential of measures. Compiled from [21–24]. Measure Reduction potential dB(A) Wheelrailroughness Regularwheelgrinding 10 Regularrailgrinding 10 Compositebrakepads 8 Wheel rail damping Wheel damping 5 Raildamping 5 Pad stiffness optimised forlownoiseemission 5 Screens close to wheel/rail Overlapping screens 10 Screens with gap 3 Combinations of measures can have unexpected results and it is very likely that com- bining two measures will have a slightly less effect than the sum of the reductions, i.e. grinding the rail might give 10 dB reduction, but also grinding the wheels will not give another 10 dB. However, it will surely have a positive effect, and it might also increase the life time of the rail grinding since the roughness tends to spread between wheel and rail. The reduction potential of local measures such as increasing the fac¸ade insulation (nor-
22 VTI rapport 559A mally by changing windows) is about 7–10 dB(A) on the indoor level at best. Screening with sound barriers can achieve between 5 dB(A) for low screens and up to 15 dB(A) for very high screens, but the effect is local. Measures affecting the source is a global solu- tion and they also have an impact where building screens might not be prioritised, such as for parks and other recreational areas. For high speed trains that travel over 300 km/h the aerodynamic sources become impor- tant. If they are not addressed other measures have little or no effect since the aeroacous- tic sources will dominate. In [21] examples are given for pantograph and boogie noise where reductions of about 5–10 dB seem possible by careful design taking fluid dynam- ics into account.
VTI rapport 559A 23 4 Determining rail vehicle noise emission
4.1 Sound power and sound pressure The human ear perceives the small pressure changes in the atmosphere as sound, so the quantity sound pressure level is what is relevant for the receiver. It describes the strength of the noise at a certain receiver position. On the other hand the sound pressure level varies with different distance and direction to a rail vehicle. Therefore sound pressure level is not suitable for describing the acoustic source strength. Instead the sound power level is used. It describes the source strength and is sometimes given in the unit B (Bell) instead of dB to avoid confusing it with the sound pressure level. In many cases the sound power level is circumvented when specifying noise emission limits. In [25] there are examples of noise limits from Europe and all are given as the sound pressure level measured at a certain distance from the track (25 or 7.5 m) at a cer- tain speed. It would make more sense to specify the sound power level when the source is addressed, i.e. emission limits, and use the sound pressure level when the receiver is addressed, i.e. noise exposure limits, see Fig. 4.1.
Receiver Source Sound pressure level Sound power level
L p
L w Figure 4.1 Noise emission vs. exposure and sound power level Lw vs. sound pressure level Lp.
On the other hand the sound power is indirectly specified together with a directivity when the limit is based on one microphone position at a certain distance. The directiv- ity would have to be handled in an approach where sound power level is used anyway. A train that radiates most of its noise upwards will have to be handled in a different way compared to a train that radiates noise uniformly.
4.2 Maximum and equivalent level The quantity sound pressure level is normally a function of time and space. When a rail- bound vehicle passes a microphone close to the track the sound pressure level will vary both as a function of time and of the microphone position. To get a single metric for one microphone position some kind of averaging or data selection must be performed. The two most common approaches are to take the equivalent level (the average of the effec- tive value squared of the sound pressure) or the maximum level. In both cases the A- weighted level is applied to weight together the level at different frequencies, a compact description of the details can be found in [11, ch.6]. When very close to the track the maximum level for one passage is determined by the
24 VTI rapport 559A loudest source, i.e. noisiest wheel or boogie if rolling noise is dominant. At medium distances the maximum level is determined by the loudest combination of sources, and for long distances all sources on the vehicle are important. This effect is illustrated in Fig. 4.2.
Figure 4.2 Illustration of sources on a train that contribute to the maximum sound pressure level at different distances.
Determining the maximum level involves more random variations than determining the equivalent level. The maximum level varies more, and when measuring a number of ve- hicles, only one single vehicle will determine the maximum level. The equivalent level will have contributions from all measured vehicles. In the same way the maximum level is more sensitive to disturbances under the measurement period. A single disturbance can set the maximum level, but for the equivalent level the disturbance is averaged out as the number of measured vehicles increases.
4.3 ISO 3095 The standard ISO 3095 [24] was approved 19 May 2005, and represents a brake-through for specifying and measuring the noise emission from rail vehicles. The standard is in- tended for type testing and monitoring of noise emitted by rail-bound vehicles. Apart from describing how to measure and evaluate sound levels and the rail roughness, it also includes a procedure for qualifying a section of a test track in terms of acceptable rough- ness. The most important quantities measured in the standard is the A-weighted level equiv- alent level during the passage LpAeq,Tp and the transit exposure level Transit Exposure Level (TEL) for constant speed. For accelerating and decelerating vehicles the FAST- weighted (and A-weighted) maximum level LpAFmax is the main result. Fig. 4.3 is taken from the standard and shows the sound pressure level as a function of time. The pass-by time Tp is defined as the length of the vehicle divided by its speed.
The equivalent level during this time is the LpAeq,Tp . The measurement time interval T is the time where the level is at least 10 dB lower. The transit exposure level TEL is the equivalent level during the measurement time in- terval plus a correction for the length of this time compared to the pass-by time, which can be expressed as TEL = LpAeq,T + 10 log(T /Tp). (4.1) Note that the acronym TEL does not imply “energy level”, it is an adjusted equivalent level. As an example a 100 m long uniform train would get about the same TEL as a 200 m long train of the same type, apart from the minor corrections at the onset and
VTI rapport 559A 25 Figure 4.3 Sound pressure level as a function of time during train passage. Taken from [24]. departure. On the other hand the single event level SEL is an energy level and would increase by 3 dB if the train length was doubled. For more details see the definitions in [24]. The roughness measurements are specified to be carried out in a number of lines de- pending on the width of the running band (width of the contact patch). These lines must be at least 1 m long at six positions along the test track. A rail roughness limit spectrum is given in the report, and a special procedure is defined on how to verify that the track falls within those limits. Note that no standardised roughness measurement equipment is readily available, there- fore the apparatus used must be described in the report for each measurement. The acous- tical measurement is specified in a simple manner; a type 1 sound level meter is to be used. For the roughness the acceptable measurement uncertainties and dynamic ranges needed for the roughness length and amplitude are specified instead.
4.4 TWINS The Track Wheel Interaction Noise Software (TWINS) is a software that has been de- veloped, validated and used within a number of European rail research projects [26, 27]. The software uses the finite element method (FEM) to calculate receptances (force to vibration response functions) of the track and the wheel separately, and then a contact model and roughness data to couple them together. The contact model is needed since the receptances do not include the mechanical coupling between the wheel and the rail as opposed to the geometry and material data of the wheel and rail which are included. Noise control measures such as damping materials or tuned absorbers can be included, which are handled by the FEM calculations provided that their material data is available. The contact model takes care of the coupling between the wheel and rail. It includes the effect of filtering by the contact patch (see section 2.2), but its main task is to deter-
26 VTI rapport 559A mine the dynamic contact force between the wheel and rail. This is achieved by apply- ing contact mechanics on a fine mesh of points for a given wheel and rail roughness, and then summing up the total force. Finally the radiated sound pressure is calculated via a boundary element approach or a simplified Rayleigh integral method, see Fig. 4.4. The program is distributed via UIC.
Figure 4.4 Flow chart of the TWINS model.
VTI rapport 559A 27 5 European limits, targets and calculation methods
5.1 Noise emission Almost all European countries have legislation protecting the inhabitants from high noise levels from railways, but only Austria, Finland, Italy and Germany have limits on the noise emission from vehicles on their national networks [25]. For road vehicles all European countries (and many other) have legislation limiting the emission based on measurements using the ISO 362 standard [15, ch.14], even though those limits have had little or no effect on reducing the general exposure [28]. This unbalance between the exposure limits and the emission limits is unfortunate. In- stead of concentrating on improving the vehicles and tracks the focus is shifted towards building barriers and increasing fac¸ade insulation, see Fig 5.1. Work on reducing the emission is voluntary or regulated between the vehicle manufacturer and buyer.
Emission Exposure
Unregulated The only available Limits given in legislation noise control measure? Figure 5.1 Illustration of emission and exposure.
For the part of the rolling stock which is international there are limits throughout Europe based on the Technical Specification of Interoperability (TSI) issued by the European Commission [19,29,30]. This document has been issued for high speed trains and just recently for regular passenger and freight trains [31]. The limits are expressed as a max- imum allowable transit exposure level according to ISO 3095 [24] for different speeds. The limits are also proposed to be lowered after some time to increase the pressure on vehicle manufacturers and operators to take action. The same approach of lowering the emission limits is used in the national programmes. Germany plans to reduce all limits by 8 dB(A) after a ten year period, Italy will lower the levels by 2 dB(A) in 2012 and Austria has already lowered the limits for freight wag- ons by 10 dB(A) [25]. There is also a possibility of continuing work on national legislation in Sweden, but there is a risk that strict legislation will be seen as an obstacle for free trade and inter- operability.
5.2 Noise exposure Although the main focus of this report is on emission, it is relevant to briefly look at the exposure limits and metrics to understand how they influence the complex of problems
28 VTI rapport 559A related to noise and railway traffic. Most European countries operate with two limits on the equivalent level, one for the daytime and one for the nighttime. Some countries also use a morning and evening time period [25]. In Sweden there is also a limit on the maximum level in addition to the limits on the equivalent level. The maximum level together with information on how many times loud events occur at night is the most relevant metric for sleep disturbance effects. In an ef- fort to include both general annoyance and sleep disturbance in one metric, the level day-evening-night Lden was developed [4]. It is defined as a weighting of three equiva- lent levels for three time periods. A 12 h day period Ld, a 4 h evening period Le and an 8 h night period Ln. The Lden is calculated from the above levels as 12 4 8 L = 10 log 100.1Ld + 100.1(Le+5) + 100.1(Ln+10) . (5.1) den 24 24 24
This means that the same vehicle passage will contribute to the Lden as 5 or 10 dB(A) louder if it occurs during the evening or night period, respectively.
The main benefit with the Lden is that it is very simple to use, a single metric. As a con- sequence there is a risk of over simplification, that many important aspects are lost. However, in the European Commission directive 2002/49/EG [32] it is selected as the noise indicator to be used for mapping and general handling of noise exposure issues in the future. There is a lot of work to be done in order to harmonise all the national limits and regula- tions according to the European noise directive. For Sweden a summary of the transition to Lden is given in [33]. Apart from taking the typical traffic flow at different time pe- riods into account, the directive also demands that the Lden is evaluated for a full year. This means that typical weather variations must be taken into account, as well as other factors that vary with the season.
5.3 Standardised calculation methods Apart from measuring the level by conventional methods it is possible to calculate the noise levels in question. Since it is difficult and expensive to measure the sound level in many cases the standardised calculation methods traditionally have a strong position when dealing with railway noise. The benefits are many, the calculations are not sen- sitive to weather or temporary traffic situations, and they can predict the level in future scenarios. The noise emission of different rail vehicles is the input data from which the calculations start, and then the effect of the propagation distance, weather and so on is applied to get the noise level at a certain receiver position. Measuring the emission is therefore intimately connected to the source model and inner workings of the calculation method. Again the contribution to the sound emission from the rail itself (roughness, pads and so on) makes things complicated. It is not enough to know the vehicle type and speed to predict the noise level, data on the rail quality is also needed.
5.3.1 Nordic methods The Nordic Calculation Method for Noise From Rail-Bound Traffic [34] was revised in 1996, and is often denoted NMT96. The method predicts the maximum and the equiva- lent level in octave bands from input data on geometry and traffic distribution. A small database with emission values for vehicles from the Nordic countries is included, and it has been updated with new Swedish types taken into service after 1996 [35].
VTI rapport 559A 29 Weather is included by assuming a standard weak downwind condition (slightly higher noise levels than in a quiet atmosphere) in all directions. Screens and barriers are in- cluded, and reflections in buildings can be taken into account to some extent. The method is unique in that it is common for all the Nordic countries. Other European methods in use are national initiatives even if they share some similar principles. Nord2000 is a more sophisticated method developed in cooperation between the Nordic countries [36,37]. It is similar in principle to NMT96, but calculates the levels in third octave bands and is more accurate in many situations. Weather effects are also included, and apart for the source data it is a common method for road, rail and industrial noise. Nord2000 is expected to be ready for widespread use and to replace NMT96 during 2006 [33].
5.3.2 European methods The Harmonoise project [38] started 2001 and was completed in 2005. A total of 21 partners, both research institutes and consultancies, worked with a new outdoor sound propagation model for the European Union. The model has two levels of detail, one en- gineering grade method which is the main result, and one detailed reference method, see Fig. 5.2. The method is intended to replace the national methods used in the EU, but in a transition period Harmonoise will coexist with the national methods.
Figure 5.2 Harmonoise project structure. From [38].
30 VTI rapport 559A 6 Discussion
6.1 Increasing traffic volumes and noise exposure When the limits on exposure were discussed in the Nordic countries one or two decades ago a lower noise exposure was expected as a result of general technological and tech- nical development lowering the emission [39]. Instead the opposite has happened. As a result both of the lack of regulation on emission and as a function of increased traffic volumes the exposure has increased. So how can this trend be stopped and reversed, so that the noise exposure decreases in- stead of increases? The answer is not likely to be higher sound screens and better fac¸ade insulation. The answer lies in reducing the noise emission. But that work is international and progressing slowly, so for the most exposed areas waiting for silent trains and tracks is not feasible. Can national initiatives on reducing the noise emission be a way towards lowering the exposure? Many European nations already have such programmes, so of course it is pos- sible. But the interoperability and trade barrier issue must be considered, and for the rolling stock which is already international it is more difficult.
6.2 Rail access charges One important instrument for emission control is the rail access charges, the fee the op- erator pays the infrastructure manager for using the railway system. They already vary quite a lot between different countries [40,41] so the trade barrier argument should not pose a problem. If this fee is lower for silent vehicles it would create a market pressure on the vehicle manufacturers to implement low noise solutions, and a possibility for op- erators to invest in low noise technology. In Sweden a noise emission fee for trains as an emission control measure was proposed already in 1993 [39]. Within the research programme PINA 1 the issue has been raised, and a preliminary investigation was reported to Banverket (the Swedish rail infrastruc- ture manager) in December 2005 [42, 43]. Two of the conclusions from this study are that the freight traffic would be affected more than the passenger traffic if the fee is dif- ferentiated between them, and that more investigations are needed on the monetary eval- uation of railway noise. An interesting comparison to make is with the European Union Greenhouse Gas Emis- sion Trading Scheme (EU ETS)2. This programme (and other similar approaches around the world) makes it possible to trade gas emission rights. Would it be possible to trade noise emission rights?
1PINA is a research programme aimed at estimating the cost of using infrastructure based on the marginal cost principle, http://www.vti.se/tek. 2http://europa.eu.int/comm/environment/climat/emission.htm
VTI rapport 559A 31 6.3 Who is responsible for the rolling noise? Rolling noise is only generated when a vehicle is moving on the track, so without the ve- hicle movement there is no source. On the other hand not only does the rail roughness contribute to the sound radiation from the wheel, the rail radiates a substantial part of the sound itself. If the pads are poorly optimised and the rail very rough a vehicle manufac- turer may argue that the poor condition of the rail is responsible for the sound radiation, not the design of the vehicle. According to Tab. 3.1 measures taken on the track (such as regular rail grinding) have an influence of the same order of magnitude as measures taken on the train wheel. This problem is at least partly solved by the updated standard ISO 3095 [24], where an acceptable track condition is specified at least in terms of roughness. Both the vehicle manufacturer and the buyer can then rely on an internationally accepted measurement procedure, and it might also create some pressure on the infrastructure managers to re- duce rail roughness. There is still potential for confusion though. What if an infrastructure manager imposes limits or charges on noise emission, and then fails to keep its tracks in condition so that some or all vehicles on the track emit more noise? What if an operator uses low grade equipment that increases rail roughness rapidly, affecting other operators on the same track section? A successful future European rail network demands deliberate and successful actions that reduce the noise exposure of those living close to railroads. This in turn makes it imperative to reduce the emissions, which would be a challenge even without substan- tially increased traffic volumes in the near future.
32 VTI rapport 559A Glossary A-weighted level Sound pressure level which is weighted to mimic the sensitivity for different fre- quencies of an average human ear.
The European Rail Research Advisory Council (ERRAC) European organisation aimed at guiding research and increasing innovation in the rail sector. interoperability Infrastructure and vehicle compatibility across Europe.
Level Day Evening Night (Lden) Equivalent A-weighted sound pressure level where events at evening or nighttime gets a penalty of 5 or 10 dB(A) respectively. pantograph The structure on a railway vehicle that contacts the overhead electrical wire.
Transit Exposure Level (TEL) Adjusted A-weighted equivalent sound pressure level for train pass-by measure- ments from ISO 3095 [24]. Technical Specification of Interoperability (TSI) Document that specifies the technical requirements of a train used across borders in Europe. Track Wheel Interaction Noise Software (TWINS) Software for noise emission calculations.
The International Union of Railways (UIC) Worldwide organisation for railway cooperation.
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VTI rapport 559A 37