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RAILWAY NOISE AND VIBRATIONMECHANISMS, MODELLING AND MEANS OF CONTROL
By DAVID THOMPSON
ELSEVIERCopyright © 2009 Elsevier Ltd.
All right reserved.
1.1 THE NEED FOR NOISE AND VIBRATION CONTROL IN RAILWAYS
To some of us the sound of a passing train is music to the ears. Audio recordings of trains are sold; the sound of a steam engine labouring up a gradient or passing at speed may evoke a strong impression of its power or the nostalgia of a lost age. But to many people the noise from passing trains is unwanted and can be considered a disturbance.
It has always been so. The early railways were often subject to considerable opposition. The following was written in 1825, in a letter to the Leeds Intelligencer (quoted in [1.1]): 'Now judge, my friend, of my mortification, whilst I am sitting comfortably at breakfast with my family, enjoying the purity of the summer air, in a moment my dwelling ... is filled with dense smoke, .... Nothing is heard but the clanking iron, the blasphemous song, or the appalling curses of the directors of these infernal machines.' Nevertheless, although some objections such as this were attributed to environmental reasons including noise, most were based on economic or aesthetic arguments. An interesting example occurred as early as 1863, when the Manchester, Buxton, Matlock and Midlands Junction Railway (later to become part of the Midland Railway) in England was forced to build its line in a shallow cut-and-cover tunnel almost 1 km long so that it should not be visible from the Duke of Rutland's home at Haddon Hall. Today such schemes and changes in alignment are not uncommon to mitigate noise, but the idea is clearly not new.
Particularly since the 1960s, environmental noise has become an increasingly important issue. Noise is often identified as a source of dissatisfaction with the living environment by residents. As environmental noise levels have increased, the population has become increasingly aware of noise as a potential issue. This applies to railway noise in common with many other forms of environmental noise. Opposition to new railway lines is therefore now often focused on their potential noise impact. This may be because noise is quantifiable in a way that aesthetics are not, so that complaints about the railway as such become focused on the issue of noise. However, as noted by the Wilson Report of 1963: 'There is a considerable amount of evidence that, as living standards rise, people are less likely to tolerate noise.'
It was estimated in 1996 that 20% of the population of Western Europe lived in areas where the ambient noise level was over 65 dB and as many as 60% in areas where the noise level was over 55 dB. The major source of this noise is road traffic, which accounts for around 90% of the population exposed to levels of noise over 65 dB (i.e. 18 of the 20%). However, railways and aircraft are also important sources of noise in the community. Rail traffic accounts for noise levels over 65 dB for 1.7% of the population.
While not everyone reacts to noise in the same way, it is no surprise that in terms of annoyance, between 20 and 25% of the population are annoyed by road traffic noise and between 2 and 4% by railway noise. Nevertheless it has been found that for the same level of noise, railways are less annoying than road traffic, leading to a 'railway bonus' in some national standards, recommendations and guidelines, notably a 'bonus' of 5 dB in Germany.
The prevalence of high noise levels and increased public awareness of noise has led to the introduction of legislation to limit sound levels, both at receiver locations and at the source. European legislation has existed since the 1970s to control the sound emitted by road vehicles and aircraft. For road vehicles, reductions in the levels obtained during the drive-by test of 8–11 dB have been achieved between 1973 and 1996. However, it is widely recognized that this does not translate into equivalent reductions of noise in traffic, due to a mismatch between the test conditions and typical traffic conditions. In the former (low speed acceleration under high engine speed) engine noise dominates whereas in traffic usually tyre noise dominates for speeds of 50 km/h and above. Changes in the test procedure are proposed to overcome this. Rubber tyres are clearly not quiet, being responsible for much of the noise exposure due to transport (see also box on page 4).
By contrast aircraft noise has been reduced by stricter noise certification and by night-time flying bans and other operational measures. The introduction of high bypass ratio turbo-fan engines has reduced noise by 20–30 dB since the early 1970s, although the rapid increase in the number of flights means that the noise exposure has in many situations close to airports continued to rise or at least remained steady.
For railway noise, the difficulty of separating the influence of track and vehicle and the consequent difficulty of defining a unique source value for a particular vehicle have contributed to the long delay in the introduction of source limits. Legal limits on the noise emitted by individual rail vehicles have only been introduced in Europe since 2002. These have been achieved through the means of 'Technical Specifications for Interoperability' (TSI), which are intended primarily to allow interoperability of vehicles between different countries in Europe. Such limits have the potential to reduce railway noise in the long term.
Noise limits at receiver locations apply in many countries. These were mostly introduced initially to apply to new lines or altered situations, providing for mitigation such as noise barriers or secondary glazing where limits were exceeded. However, the recent introduction of the Environmental Noise Directive (END) has led to the requirement to produce noise maps of existing sources and to develop Action Plans to reduce noise in identified 'hot spots'. These, too, will mean that railway operators and infrastructure companies will have to consider how to minimize noise.
As well as noise, vibration from railways can cause annoyance. This may be due to feelable vibration, usually in the range 2 to 80 Hz, or due to the radiation of low frequency sound transmitted through the ground, usually in the range 30 to 250 Hz. Vibration may also cause objects to rattle, adding to the sensation.
Noise at a particular receiver location can be reduced by secondary measures, either in the transmission path such as noise barriers or at the receiver such as by installing windows with improved acoustic insulation. To a lesser extent vibration can also be dealt with by secondary measures, such as mounting sensitive buildings on isolation springs. Nevertheless, reduction of noise and vibration at the source is generally more cost effective. On the other hand, it is also generally true that noise control at source is more complex, as it requires a good knowledge of the mechanisms operating within the source. It is important that safe and economic operation of the equipment, in this case the railway, is not impeded by changes aimed at reducing noise. The railway is often seen as a conservative industry where there is reluctance to change the way things are done, particularly because of potential implications for safety or operational efficiency. Nevertheless, it is the author's belief and experience that significant noise reductions are possible by careful study of the sources, appropriate modelling, and use of those models for optimization, while taking into account the many non-acoustic factors.
Railways are generally acknowledged to be an environmentally-friendly means of transport with the potential to operate with considerably less pollution, energy use and CO2 emissions per passenger-km than road or air. High speed trains have been found to compete effectively with air transport on routes up to 3 hours or more, achieving large market shares on routes such as between London, Paris and Brussels. Mass transit systems hold the key to urban mobility. Rail freight is growing across Europe. In order to improve the market share of rail transport, and thereby improve sustainability, it is imperative that noise is reduced.
1.2 THE NEED FOR A SYSTEMATIC APPROACH TO NOISE CONTROL
The problem of reducing railway noise can be used to illustrate the classical approach to noise control. (Note that it is beyond the scope of this book to introduce the reader to the fundamentals of acoustics and vibration. For this, there are many good text books, e.g.. Familiarity is assumed, for example, with the decibel scale, frequency analysis and complex notation for harmonic motion.)
The first step in noise control is to identify the dominant source. There are many different sources of noise from a railway, and in different situations the dominant source may vary. Notably on North American freight railways, a major issue for environmental noise is related to locomotive warning signals. It is obligatory to sound the horn in an extended sequence on the approach to road crossings. There are many such crossings, especially in populated areas and so it is a major source of annoyance, particularly from operations at night. In other situations, such as stations in urban areas, the public address system may be the major source of noise in the immediate neighbourhood. However, the most important source of noise from railway operations is usually rolling noise caused by the interaction of wheel and rail during running on straight track. Other sources include curve squeal, bridge noise, traction noise and aerodynamic noise. Noise inside the vehicle also includes all of these sources, as well as others such as air-conditioning fan noise.
Having identified the dominant source, the next step is to quantify the various paths or contributions. Focusing on exterior rolling noise, the vibration of the wheel and the rail can be identified as potential sources. Early attempts to understand the problem tended to be polarized into attributing the noise solely to one or the other. More recently, however, it has become widely recognized that both wheels and rails usually form important sources which make similar contributions to the overall sound level. Prediction models allow their relative contributions to be quantified (measurement methods can also be used). Clearly, effective noise control requires both sources to be tackled. For example, in a situation where wheel and rail contribute equally to the overall level, a reduction of 10 dB in one of them, while the other is unchanged, will produce a reduction of only 2.5 dB in the total (see also box on p. 224).
The next step is to understand how each source can be influenced. Here, the theoretical models allow the sensitivity of the noise to various design parameters to be investigated (measurements alone do not). Noise control principles can be considered in terms of reduced excitation, increased damping, vibration isolation, acoustic shielding or absorption.
From these principles, actual designs can be developed and tested, first using the prediction model, then in laboratory tests and ultimately in practical tests on the operational railway. Tests should be carried out in a controlled situation; where possible not just the noise but intermediate parameters such as vibration should be measured.
It would, of course, be risky to proceed straight to this last step. The source or path that is treated may not be the dominant one, or the modification introduced may not influence the source as intended. Yet there are many examples in railway noise control where this has been done, often leading to the conclusion 'we've tried that and it doesn't work'.
Before noise control measures can be applied in normal operation, practical constraints have to be taken into account. The measures that have been developed in principle have to satisfy many other requirements of the operating environment. In the case of the railway these are particularly related to safety. At this point compromise is often required in the acoustic design. Ideally such constraints should be considered as early as possible in the design process, provided that they don't stifle innovation altogether.
The approach described in this book is based on the above principles, particularly the development of theoretical models with an appropriate level of detail to understand the source mechanisms and then the use of these models to develop and understand mitigation measures.
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