Noise reduction measures on Istanbul urban rail systems

Noise is unwanted sound and is among the most pervasive pollutants today. The problem with noise is not only that it is unwanted, but also that it negatively affects human health and well-being. Problems related to noise include hearing loss, stress, high blood pressure, sleep loss, distraction, productivity loss and a general reduction in the quality of life and opportunities for tranquility.

Sound is a pressure fluctuation in the air; the magnitude of the sound describes the physical sound in the air. The response of human beings to sound depends strongly on the frequency of sound. In general, people are less sensitive to sounds of low frequency, such as 100 Hz (Hertz) than to sounds at 1000 Hz; also at high frequencies such as 8000 Hz, sensitivity decreases.

Railway noise probelms

There have been important advances in the theories concerning wheel-rail noise generation during the 1970s and 1980s, and research in the area is continuing – especially with respect to high-speed rail. The theory of wheel-rail noise generation, though highly developed, is incomplete.

Five alternatives can be said as possible mechanisms for railway noise generation:

• Traction Noise: For diesel engines, exhaust noise, engine and transmission vibrations. Electric power units are much quieter, though noise is emitted from the traction motor and extra cooling fans. Pantograph noise is significant at high speeds.
• Rail/Wheel Noise: Rail and wheel are set into vibration. This produces external and internal noise. Main sources are poorly aligned track joints and the roughness of the wheels and the track.
• Auxiliary Equipment Noise: compressors, ventilation and brake systems.
• Aerodynamic Noise: Produced by passage of the train through the air. Its contribution to the total noise level increases with speed.

There are several descriptive terms for various types of wheel-rail noise, including:

• The tangent track noise (or rolling noise) is due to wheel and rail roughness, which may include rail corrugation and random surface defects (pits of rail, rail joints, wheel flats, etc.)
• The impact noise is generated by rail imperfections, joints and more significantly, special track works. Special track work includes switches and crossover diamonds.
• Wheel squeal noise is generated at curves due to a stick slip phenomenon involving nonlinear interaction of the wheel with rail.

Equivalent noise level standards

For a large number of situations, measurements have been carried out to determine the emission of trams and light-rail vehicles. The measurements have been done according to ISO 3095 ‘Measurements of noise emitted by railroad vehicles’.

Each railway authority provides design guidelines for noise produced by trains. These guidelines are comprehensive and they include limits for vehicle interior, exterior and the station platform. Although railways have some advantages over roadway in respect of noise, an increase in speed necessitates some regulations on vehicle, track and the environment.

The regulating authority in Turkey is the Ministry of Environment and Forest. The noise reception limits are stated in the ministerial order ‘Assessment and Management Regulation of Environmental Noise’ publication No 25862 and date 01.07.2005. The limits stated are energy equivalent sound pressure levels separated over daytime (06.00-22.00) and night-time periods (22.00-06.00), measured on real conditions together with other transportation noise sources. The limits stated are used in connection with new town planning of residential areas and in connection with planning of new railway lines and substantial upgrading of railway lines in residential areas and also in connection with existing railways. According to our regulation, it should be equivalent noise level of 65 dB(A) during daytime and 55 dB(A) during night-time for railway. Also, the same limits are accepted in France, Germany and England. Average equivalent noise levels of European countries are 61 dB(A) during daytime and 53 dB(A) during night-time1.

Causes of noise in Istanbul urban rail systems

Because urban rail systems pass very near historical buildings and dwellings in Istanbul, some measures shall be implemented to reduce noise. The possible reasons of noise and control measures were investigated in Istanbul Aksaray-Airport light-rail metro and Zeytinburnu-Bağcılar tram line. As a result, the following main reasons were determined for railway noise:

• Rail corrugations in light-rail system
• Wheel squeal noise at the narrow curves in tram line

Corrugation is known as the untreated problem in railway engineering, and it causes extra dynamic loads and deteriorates for both the train and also the track. The phenomenon of rail corrugation, cyclic wear patterns on the rail head with typical wavelengths of 30-80mm, is apparent on a significant proportion of track on most railway administrations. The presence of corrugations, especially of high amplitude (higher than 0,1mm), and the resulting high oscillatory forces in the track, can reduce component life, lead to premature failure and increases maintenance cost. Another effect of corrugation is to increase vehicle rolling noise as a result of the higher combined wheel and rail surface roughness at the contact patch (noise is hearable when amplitude exceed to 0,05mm)2. Severe corrugation can increase rolling noise from disc-braked vehicles, i.e. those with comparatively smooth running surfaces, by up to 20 dB(A) (A-weighted decibels), which is perceived as an approximate four times increase in loudness. Therefore, the presence of corrugations can lead to a significant deterioration in the trackside environment.

Some measurements have been done on Aksaray-Airport light-rail metro line and the results are:

• On alignment and in gentle curves (R>600m), short wave corrugations with 20-80mm wavelength are on both of rails
• In tight curves (R<600m), corrugations with 80-150mm wavelength are only on the inside rail, but the amplitude is higher
• The corrugations are not seen on tunnels. It is thought to be related with rigid substructure, concrete under ballast layer

Wheel squeal is one of the most noticeable types of noise produced by rail transit systems, and is generated when a rail car rounds a curve of tight radius. Wheel squeal originates from frictional instability in curves between the wheel and rail. Three types of stick-slip motion have been postulated for producing wheel squeal noise:

• Longitudinal stick slip
• Flange contact with gauge face
• Stick-slip due to lateral creep across the rail head caused by nonzero angle of attack of the wheel3.

There are some tight curves on the Zeytinburnu-Bagcılar tram line, and when a train passes the curves near houses, many complaints are lodged.

Noise control measures on Istanbul urban railway systems

It is not possible to prevent corrugation causing noise completely. The most effective approach to controlling rail corrugation is to grind the rail running surface periodically. Railroads grind rail preventively to reduce rail wear, control rail surface and sub-surface fatigue, control rail surface plastic deformation to improve truck steering, improve the dynamic stability of rolling stock and improve rolling stock wheel life. This is achieved by implementing preventive grinding cycles based on track curvature and traffic tonnage to remove a thin surface layer of metal from the rail.

Generally, every railway authority determines grinding intervals depending on operation conditions and possibilities. But, it is generally accepted to grind when the depth of corrugation is approximately 0,1mm.

Normally, the main reason for grinding is to reduce the dynamics loads, and so to reduce train and track deformations. But, German Railways (DB AG) has determined grinding criteria only to prevent noise. If the depth of the corrugation with wavelength 0-100mm is approximately 0,05-0,07mm, noise grinding shall be done. This is because, it was accepted that the equivalent noise level should be below 48 dBA at the 25m distance from the track axle. Noise grinding has reduced significant reduction exceeding 12 dBA. In addition, it was calculated that the cost ratio of the noise grinding to noise barrier is 0,1 in 25 years service life4.

Yearly, total traffic load was calculated 10,5 MGT in Istanbul Aksaray-Airport LRTS, and according to our experiences and general implementations of other railway organisations, preventive grinding should be done every year in curves R<600m and every two years in curves R>600m .

The rail grinding was done to clear corrugations and to reduce noise in Aksaray-Airport LRTS in 2006. As a result, a significant noise reduction was supplied. To evaluate noise reduction, noise measurements were done in the ‘bedroom’ near the Davutpasa metro station before and after rail grinding. Sound level measurements were carried out using a Bruel&Kjaer 2260 Sound Level Meter, set for ‘A’ weighting. To provide a good comparison, measurements were done at the same hours (23.00-24:00) and at the same train speed (80km/h).

Equivalent noise level was measured 60,1 dBA before grinding and 44,0 dBA after grinding. That is 16,1 dBA noise reduction was supplied by eliminating corrugations. The all noise complaints were prevented by rail grinding.

To prevent wheel squeal noise, a friction modifier was implemented on top of the rails by a stationary lubrication pump. While grease lubrication has been used successfully by some properties, concerns exist regarding impacts on vehicle traction and safety which have prevented implementation by some transit agencies. Compilation of grease lubricant experience is needed to assess the legitimacy of these concerns. The effectiveness of these lubricants in controlling squeal, and the operational circumstances in which a particular design would be appropriate, should be investigated. In addition to a rigorous evaluation of the performance of these products, an examination of the practical impacts on operations, safety, and secondary impacts (e.g., soil contamination by the lubricant) is needed. After long evaluation, a friction modifier preventing wheel squeal noise was approved and decided to install at the all narrow curves in urban rail systems.

Normally, dry top-of-rail and gauge face have a 0.4-0,6 coefficient of friction. Wayside lubrication at the lubricator site will produce a 0.30-0,35 COF top-of-rail and 0,25 gauge-face. If coefficient of friction is below 0.3, it causes braking distance and contact fatigue on rail and wheel. The both of rail should be lubricated, because the different coefficient of friction on rails causes some difficulties in turning of bogie and increases loads on rails5.

Lubrication products may by applied continuously or intermittently. For transit applications, continuous application is feasible since train operation is consistent in terms of frequency and weight-on-rail. How much lubricant is needed is dependent on track curvature and grade, as well as train speed and load.

Some researches and tests under control have been done in a depot area for two years, and then the optimum lubrication system has been accepted. During tests, more lubrication have caused lower coefficient of friction and some braking and skid problems, and less lubrication has not reduced noise level enough. So, by arranging lubrication quantity and pumping intervals, suitable lubrication systems were installed in the beginning of two tight and opposite curves (curve radius is 30m and the total length of curves is 150meters) in Zeytinburnu-Bağcılar tram line. On this curve, especially at night hours, there were too many complaints about tram noise. Lubrication product (Lubcon Sintono Terra HLK) has been satisfactory to provide a good noise reduction without any braking problems.

On top of both rails, four holes (Ø4mm) with 35cm intervals have been drilled on the gauge corner of rails. So, the top of rail and gauge face have been lubricated simultaneously, and also it has reduced lateral rail wear. A sensor has been installed under the rail head at a distance of 100m and it counts the number of wheels when a train passes on sensor and transmits it to and electronic control unit in the box. The electronic control unit commands the pumping of lubricant according to the number of wheel specified before. After tests, it has decided to lubricate one time (1,4 gr lubricant) for every three train. And there has been observed not to be any braking or skip problem and lubricant material to spread along 150 metre track.

Sound level measurements were carried out using a SVAN 947 Sound Level Meter, set for “A” weighting at noon (11:00-12:00). The microphone was handheld on the outside (high rail side) of the test curve. The sound metre was held 3 meters from the center of the track, with a microphone 1.2 meters above the height of the rail. Sound measurements were made in the centre of the curve. The sound level meter was programmed for event recording, enabling the instrument to automatically measure and stores the event data. The data reported is the averaged Laeq values for each of five trains under particular conditions evaluated. Equivalent noise levels was measured 97,47 dBA before lubrication and 76,4 dBA after lubrication. Also, background noise levels were measured 74 dBA without train. That is, 19 dBA noise reduction was supplied. All noise complaints were prevented by rail lubrication.

As a result, it was decided to install stationary lubrication pumps on tight curves (R<100m) to reduce wheel squeal noise and lateral rail wear in tram lines and LRTS. For this reason, it was decided to purchase 15 stationary pumps in near time for tram line and LRTS.

Conclusion

The most significant effect of urban rail systems on environment is noise. Railway authorities shall take some measures to lower noise levels which discomfort the passengers in vehicles and residents near the railway line. And it is aimed to provide legal noise levels after the implementation of these control measures.

In Istanbul urban rail systems, the main noise reasons related to track are rail corrugations in light rail system and wheel squeal noise at the narrow curves in tram line. To prevent noise, preventive grinding should be done every year in curves R<600m and every two years in curves R>600m in Istanbul Aksaray-Airport LRTS and stationary lubrication pumps shall be installed on every tight curve (R<100m).

References

1. A Study of European Priorities and Strategies for Railway Noise Abatement
2. UIC International Eisenbahnverband, 1979: Katalog der Schienenfehler, Utrecht:UIC.
3. TCRP Report 23, Wheel/Rail Noise Control Manual
4. Track Compendium, Bernhard Lichtberger, February 2005
5. Donald T.Eadie, Marco Santoro, Kelsan Technologies Corp., June 2003