INTRO: While railways can rightly claim many ’green’ credentials, noise reduction is one field where they need to make rapid progress. The European Rail Research Institute is tackling the issue urgently
BYLINE: Brian Hemsworth BSc, CEng, FIOA and William Bird MA, CEng, MIMechE, MCIT*
BYLINE:
* Brian Hemsworth is Manager, Noise & Vibration, at the European Rail Research Institute, and William Bird is Project Co-ordinator
RAILWAYS are widely accepted as most environmentally friendly transport mode, but their ’greener’ reputation is being increasingly scrutinised by policy makers and environmentalists, particularly with respect to noise. There is a perception that air and road transport have made great steps forward in reducing noise levels, but rail’s reputation in this area needs to be enhanced.
The EU Green Paper on Future Noise Policy has targeted railway noise as a prime area for legislation, and specialist railway engineers have long been aware of the implications. To avoid the possibility of punitive legislation that could seriously affect traffic levels, especially at night, it is important that concerted action continues to be taken at international level. The European Rail Research Institute is addressing noise issues head-on, in three main areas.
The background
ERRI was commissioned by the International Union of Railways to research noise and vibration through Specialist Committees of senior acoustic specialists drawn from European railways. The studies are complementary to those by individual railways, but a collaborative approach ensures that expert advice is exchanged in areas of common interest and avoids duplication of effort. Work has included noise from diesel locos, rolling noise, measurement standards and human responses to railway noise.
More recently, funding has become available through European Union initiatives. Consortia have been established with universities, research organisations and industry to bid for EU funding of up to 50% of project costs, with the UIC providing balancing finance. Member railways carry out aspeccts of the work, and in specific projects, have also committed funds. In all cases, ERRI manages UIC funding, and provides project management and technical support - either directly or via sub-contractors.
In parallel, ERRI is co-ordinator of UIC-supported innovative research in critical areas fundamental to the future development of quiet railway systems.
ERRI has established a unit to deal specifically with noise and vibration issues. Identifying political and environmental undercurrents, a programme has been developed for three key areas: rolling noise; innovative research; andmeasurement standards.
Rolling noise
An important contribution to the work has been the Twins software to simulate the generation of rolling noise (Fig 1). This was developed with help from Berlin Technical University, Keele University, the Netherlands Organisation for Applied Scientific Research (TNO), and noise consultants Bolt Beranek & Newmann and Vibratec.
Twins was derived from the Springboard software developed by BR Research and an earlier model by Paul Remington in which rolling noise generation is considered to be a forced vibration process with vibrating wheels and rails as the radiating elements. The force is generated in the contact zone and is a linear function of the combined roughness of the wheel and rail surfaces.
The consequence of this mechanism is well-known - disc-braked vehicles are significantly quieter than vehicles with cast iron tread-braked wheels because wheel running surfaces are smoother. Noise can also increase dramatically when rails become corrugated due to a significant increase in rail roughness at the wavelengths important in rolling noise generation.
Twins software is internationally acknowledged and has been validated for a wide range of railway operations. Validation experiments have demonstrated that at low frequencies, sleepers are a significant noise source. Twins simulations have provided the basis for several studies into low noise systems.
Acoustic specifications defined
The Optimised Freight WHeel And Track (Ofwhat) programme started in 1993 with the aim of defining optimised acoustic specifications for freight wheel and track components by combining Twins simulations with prototype testing. This was conceived as a first step in establishing a catalogue of noise reduction techniques for freight vehicles.
Component specifications were derived by Twins, competitive tendering was used to manufacture components, and running tests were carried out at Velim in 1995. These assessed the effect on rolling noise of rail pad stiffness and damping, rail absorbers, optimised wheel shape and wheel absorbers. A shape-optimised wheel fitted with dynamic absorbers was developed and tested (right).
The results confirmed the importance of rail pad stiffness in determining the level of generated noise. For minimum noise, a pad stiffness should be chosen where rail vertical noise and sleeper noise are equal. A reduction in track noise of 4 to 5dB(A) was achieved compared to a tested non-optimum pad with a commonly used value for its stiffness. Other treatments performed as predicted.
The study demonstrated that Twins could be used for theoretical assessments to derive specifications for low noise components. It also proved that a project could be completed on schedule and within budget, with the co-operation of several European railways and suppliers.
Silent Freight and Silent Track
Three projects are being sponsored by the EU Research Directorate as part of its Brite-Euram programme.
Silent Freight aims to reduce freight vehicle noise by examining parameters and developing innovative solutions for existing and future vehicles. Work began in February 1996 and is due for completion in December 1999. The key elements of the project are to:
