Reducing noise from light rail systems by dxi20863


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                                                                                                                                                NOISE REDUCTION

          lightrailsystems                                                                                                                                                      ]

          .:. James Abbott, Technical Editor                                                                                                                                    i

          Light rail systemoperators makingstrenuous                                   anti-slip/lock mechanisms, which help in avoiding
          effortsto reduce                     d
                          noisepollution.Careful esignof                               roughness of the surfaces and damaging 'flats' on
          vehiclesand track, and effectivemaintenance                                  the steel tyres of the wheels (caused by skidding on
          policies,cancut noiseto a minimum.                                           crushed leaves or other debris on the rail). Regular
                                                                                       use of wheel lathes in the depot and/or grinding
                   A        dvocates of light rail are keen to stress the
                           mode's environmentally sound credentials.
                            By drawing motorists out of private cars,
                   light rail can reduce the antisocial effects of traffic
                                                                                       machines on the track could be expected to help
                                                                                       reduce roughness of wheel and rail. Hans van
                                                                                       Leeuwen estimates that reducing roughness can
                                                                                       give a 3 to 10 dB noise reduction. Optimising the
                   - principally noise and air pollution as well as    -               wheel and rail profile can be very effective in
                   helping the local economy by reducing                               reducing squeal noise.
                   congestion and thus cutting time wasted in traffic                      Design and maintenance of the vehicle can be
                  jams. As most light rail vehicles use electric                       important, as regards not only the wheel shape but
                  propulsion, there is no generation of air pollution                  also the question of damping material between
                  around the vehicle. But light rail vehicles can                      elements of the vehicle. Hans van Leeuwen found
                  generate noise, so to emphasise those 'green'                        such measures can give noise reductions of
                  credentials engineers are striving to reduce noise to
                  as low levels as possible.
                       A study by Hans van Leeuwen of DGMR
                  Consulting Engineers in the Hague, The
                  Netherlands, presented to the Euronoise
                  conference in Naples in 2003, identified the sources
                  of noise from light rail systems. The study found
                  that the noise emission of a light rail vehicle is
                  comparable to the emission of 11 cars. But to carry
                   100 people (assuming 66 cars are required for this
                  number) light rail generates less noise: a light rail
                  vehicle generates around 78 dB (A), compared to
                  86 dB (A) for the car alternative.                                   between 3 and 6 dB, while for squeal noise
                      The principal noise from light rail vehicleswas                  reductions could be up to 20 dB, effectively
                  identified as coming from the interaction between the                eliminating squeal noise. But he cautioned that
                  wheel and the rail. The noise from electrical                        a poorly investigated or dimensioned measure
                   machinery   -    compressors, ventilation systems, the              might not be effective, or might even have a
     ,/            choppersystemand so on wasminorby  -                                negative effect.
                  companson.                                                               The DGMR study also found that rail and
                     The steel-on-steel noise from the wheels and                      track construction is relevant for noise and
                  rails is of three types: rolling noise, impulse noise                vibration emission. Reductions of up to 4-6 dB can
                  and curve squeal noise. The noise is released both                   be achieved by shape optimisation, the addition of
                  into the air and, through vibration, into the ground                 a damping material and optimisation of rail pads.
                  and adjacent buildings.                                              Again, these measures can help in the reduction of
                      Hans van Leeuwen advocated reducing the                          squeal noise.
                  roughness between the wheel and rail as the most
                  effective mechanism for reducing noise. Most                         Track design
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      emissions to a minimum will pay close attention to               emerged. They found that there are a number of
      the design of the track. The Parsons Brinckerhoff                acceptable styles, but there are also design issues
      consultancy was awarded a contract by the                        that need resolution.
      Transportation Cooperative Research Program in                        Electrical isolation of the rail as a means to
      the US to research best practice from light rail                 mitigate the effects of stray current corrosion is still a
      systems around the world, with a view to                         major factor in design of the track system. The rails
      producing a guide to designers of new systems. A                 act as the negative return current conductor, with
      study team led by Parsons Brinckerhoff's Gordon                  electrical current running through the rails to the
      Martyn researched light rail systems in Belgium,                 substation to complete the circuit. Stray current
      France, Germany and the US. Three types of track                 problems can arise with little or no insulation of the
      were identified, as follows:                                     rails because some traction current could leak into
                                                                       the structure and ground to travel back to the
      Embedded track, as found on street-running   sections            substation. Buried metallic water and gas pipes or
      The team found that designs of embedded track                    cables provide a lower resistance path for stray
      have improved as new light rail systems have                     currents. When the currents leavethe buried metal
                                                                       lines, corrosion of the utility pipes occurs. This
       ALH Rail Coatings Limited 'Series Six' pre                      corrosion can be catastrophic. Newer insulating
       coated rail system is the choice of clients,                    materials and an understanding of stray current
       designers and contractors for UK tram systems.                  control requirements are helping to resolve this issue.
       Midland Metro (Centro), Manchester Metro                             Another area of concern is failure of the paved
       Phase II, Nottingham Transit Line 1 (NET) and                   surface: clearly, even if a light rail vehicle is emitting
       Dublin's LUAS all chose this system..                           little noise, if a poor road surface caused by the
           Factory pre coating ensures very high levels                light rail track is increasing road vehicle-generated
       of rail cleanliness by grit blasting and immediate              noise, the whole system can be reckoned to have
       priming; in an ideal environment. Combined                      failed. Embedded track observed by the Parsons
       these measures ensure that 'Series Six'                         Brinckerhoff team was in a variety of pavement
       polyurethane polymer is fully bonded to rails                   sections, ranging from bituminous asphalt and
       thus providing the highest levels of electrical                 flexible cobblestones to very rigid concrete, all of
       insulation, preventing stray currents.                          which are relatively successful. The pavement type
           Granite chippings or bauxite, both chemically               that works in a warm, dry climate, however, may
       treated with an adhesion promoter can be                        not work well in a cold, wet area with freezing and
       incorporated in 'Series Six' polymer, providing                 roadway salting, both of which affect pavement and
       increased skid resistance at the road surface,                  track life span. In these colder and wetter climates,
       another important safety benefit of the ALH                     special consideration must be given to surface
       Rail Coatings Limited system.                                   pavement design, especially surface and internal
                                                                       drainage, to mitigate freeze/thaw-cycle damage.
                                                                            Another issue is excessiverail wear and
                                                                       corrugation (the formation of undulations on top of
                                                                       the rail head that result in rough and noisy operation).
                                                                       As Hans van Leeuwenfound, the interface between
                                                                       the wheel and the rail is a critical area.

                                                                       Directfixation(or slab)track, where the track is
                                                                       embeddedin concrete
           This system can incorporate drainage boxes
       and rail bonds within the 'Series Six' polymer                  This design has become more prominent in the
       and is completed with stray current collection                  past two decades as an economical application for
       mat, reinforcing steel, concrete and a finishing                tunnels and aerial structures. The light rail system
       layer of asphalt, paving blocks or concrete which               managers visited by the Parsons Brinckerhoff study
       may be coloured to enhance appearance.                          team found that direct fixation helps to reduce
           Working with Hyperlast's technologists plus                 maintenance and increases electrical isolation.
       rail and tram partners ALH Rail Coatings Limited
                                                                       Conventional ballasted track
       continues to develop products and systemswhich
       include materials for in situ pouring, installation             This is the predominant track type for locations
       on sleepers,sleeperboots and grass track for an                 within an exclusive right-of-way. In general, it
       environmentally friendly solution.                              performs well. There are many different types
                                                                       of sleepers and fastenings in use. The need for
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                                                                60                                                                  'I

                                                                                                      NOISE REDUCTION

electrical isolation to prevent stray current               that are visually intrusive and very expensive.The
corrosion has had an impact on the designs                  system has undergone extensive trials on a full-scale
of fastening systems.                                       test track in the Czech Republic and has been
                                                            installed at two sites in The Netherlands and one in
Corrugation                                                 France. Trials have shown that a reduction of about
Corrugation, says Gordon Martyn, remains a                  six decibels can be expected.
serious concern in maintaining rail on any type of              Corus' Silent Track tuned rail dampers can be
system. Light rail systems appear to suffer from            either installed 'on track' onto rail that is already in
short wave (roaring rail) corrugation, which causes         service, or in factory conditions onto rail that is to
substantial noise. Corrugation appears most                 be used in renewal or new-build programmes. Corus
frequently at steep grades, sharp curves and                and Volker Stevin Rail & Traffic have developed a
stations. While changes in rail and wheel metallurgy        technique for installation that minimises possession
may impede the propagation of corrugation, it               times and ensures the integrity of the installed
appears that a regular programme of grinding can            product. Factory fitting of Corus tuned rail
keep this problem under control. The track designer         dampers can be achieved at a location convenient to
needs to factor this maintenance operation into the         the client, for example onto long-welded rail strings
design of embedded track, where grinding wheels             at a welding facility, or at one of Corus' own sites.
may strike high pavement.

Track design items
Noise generation has been an issue for some
urban rail systems in North America, for
instance in San Francisco and Washington,
and also in Europe. 'The use of direct fixation
fasteners, ballast mats, and floating slabs are
some of the solutions that are being used to
make light rail an unobtrusive neighbour' says
Gordon Martyn.
    A number of companies have developed
products to respond to this demand.
    ALH Rail Coatings Ltd, a joint venture between
Hyperlast Ltd and GrantRail Ltd in the UK, supplies
polymer pre-coated rail systems.The company's
patented 'Series-Six'rail encapsulation is a two-               Corus' Silent Track system has the ability to be
component modified polyurethane system providing            'tuned' to absorb vibrations specific to a particular
acoustic dampening of frequenciesthat generate rail         track form. It has a multi frequency response over
noise; this technology is particularly beneficialin         the important acoustic range for rail noise and offers
urban areas. Hyperlast's polyurethane elastomer             noise reductions of 5-7 dB, depending on track form
technology has been used to develop a product with          and condition. There are no maintenance
high flexibility,long-term resistance to abrasion, tear,    requirements and no effects on other elements of the
water, acids and alkalis, as well as electricalinsulation   rail infrastructure, the product has the same life as
and vibration absorption properties.                        the rail itself, and there is no interference with other
    Anglo-Dutch steel company Corus has                     track maintenance activities,such as tamping. The
developed the Silent Track noise reduction rail             system is visually unobtrusive and eliminates or
system, which significantly reduces noise generated         reduces the need for acoustic barriers.
by railway traffic. The system reduces noise                                                                                 I
generated by rails as train wheels pass over them by        Acoustic barriers
employing 'tuned dampers', developed in                     While the bulk of this article has concerned the
conjunction with Southampton University's                   reduction or elimination at source of noise from                  "
Institute of Sound and Vibration Research, which            light rail systems, the other possibility is to contain
are attached to either side of the rail. These dampers      the noise close to the track by erecting acoustic
efficiently absorb vibrations and hence prevent noise       barriers in between the track and adjacent
from being generated by the rail. The tuned damper          buildings. Hans van Leeuwen of DGMR estimates
can be fitted to a variety of standard rail sections,       that separationsin the propagationpaths - through
which makes it a significantly more attractive option       open air or vibrations in the ground - can effect
 than noise containment measures such ~s h~rri"'rs          r",clllrt;on"of lln to 1"irlR .

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