Even though Switzerland plays a leading role in combating railway noise emissions, there still remains much to be done in this country. On this point at least, all those attending the Empa Science Apéro were in agreement, from greenhorn researchers to scientific old foxes!

Rossano Stefanelli, of the ETH Zurich, conducts research into the source of high pitched noises emitted by trains and trams. He tortured the audience’s ears with a few examples of the screeching and squealing that he deals with, just to get into the swing of things. But the serious question is what causes these unpleasant noises? And why is it that one rail carriage can screech like a banshee, while another of exactly the same type and construction runs as quiet as a mouse? The answer is complicated, but basically lies in the minute differences that occur between individual components.

What causes squealing in curves?

What is certain is that the source of high frequency noise emissions lies in the interaction between the sliding and sticking processes at the point where wheel and rail contact. But temperature, humidity and the condition of the rail also play a role, as do other “external factors”. To investigate the problem scientifically, these factors and the effects they have on noise emissions from various sections of railway were measured. Following this the ETH researchers used specially developed simulation software to computer-model the performance of the rail vehicle. Finally they compared the measurements with the results of the simulation. After many cycles of fine-tuning the program to improve the model, the scientists were finally able to simulate the squealing-curve phenomenon and suggest counter-measures. These include sound absorbers on the wheels, rail dampers, rubber tires and noise abatement walls. But even despite this progress, the researchers still have their hands full. Unanswered questions include why does screeching often occur spontaneously and unexpectedly, only to disappear equally unexpectedly?

Combating noise emissions at source

One advantage of rail traffic is that it uses energy very efficiently. This environmental friendliness is, however, counteracted by high levels of acoustic emission. Exceeding noise emission thresholds by 20 Decibels (dB) or more, the rail network is the noisiest method of transport. The worst offenders are goods trains operating at night, which are often equipped with brake blocks made of gray cast iron – an outdated technology which produces high levels of rolling noise. Passenger trains, on the other hand, have been fitted for years with lower noise (but dearer) disk brakes.

Prof. Markus Hecht, Head of the Rail Vehicle Section of the Institute for Land and Sea Transport at the Technical University of Berlin, assured the audience that based on an innovative new product, quieter goods trains were possible,. «Leila», the newly developed goods wagon bogey produced by a German-Swiss consortium, is a full 18 dB quieter than conventional bogeys fitted with gray cast iron brake blocks. It is also lighter, more resistant to wear and offers reduced down-time. It boasts such technically advanced features as electronically regulated disk brakes and an electronic maintenance and diagnostic system. Existing goods wagons can be relatively easily retro-fitted with “Leila”, allowing the currently rather high unproductive holding time to be reduced, so increasing the profitability of the rail network. This is a competitive advantage which should not be underestimated in the battle between rail and road transport operators. The first goods wagons will be converted as soon as “Leila” has been licensed for public use, according to Prof. Hecht. In two years it will be possible to use the newly developed bogey all across Europe.

Implementation of Noise Abatement Ordinance behind schedule

Acoustic emissions in Switzerland fall under the jurisdiction of the Noise Abatement Ordinance (LSV), which in 1986 defined the permissible upper noise limit for railways. In contrast to similar legislation abroad, the LSV also applies to existing railway lines in Switzerland. The remediation work necessary to meet the levels laid down in the LSV is unfortunately way behind schedule, explained Jean-Marc Wunderli, Empa noise abatement expert and environmental engineer, to the audience at the Science Apéro. The necessary financial resources were only promised as part of the Financing of Public Transport Ordinance (FinöV), leading to the hope that most cases of excessive noise will be remediated by the year 2015. The replacement of rolling stock will cost around CHF 820 million, and will result in noise emissions falling below threshold limits in 38% of problem cases. A further 31% of excessive emissions can be remediated by constructing noise barriers. These are very effective because they can be located very close to the sources of acoustic emission. To alleviate the problem of the remaining noise emissions requires alternative measures such as installing noise-proof windows.

Jean-Marc Wunderli also drew the audience’s attention to the need to update «Semibel», the noise simulation software developed in the 1980’s but still used both in the development of planning criteria and in evaluating the status quo. Comparisons made by the Empa with newer computer models show that Semibel suffers from systematic errors both when calculating noise levels at acoustic sources and noise propagation.

More attention must also be devoted to research into the influence of noise on human health. Although, for example, railway noise has undergone significant change over the years, there have been no studies on this topic since the LSV came into effect. Dr. Wunderli also commented that the threshold values and evaluation methods should not be thought of as chiseled in stone, but were to be reconsidered and adapted as necessary.

Further Information:

Jean-Marc Wunderli,. Empa Acoustics Lab., jean-marc.wunderli@empa.ch, Tel. +41 44 823 47 48.

What is a Science Apéro?

The EMPA Academy provides a forum for debating current scientific and socially relevant issues through its Science Apéros. Held at regular intervals, these usually involve three or four speakers with backgrounds in research, politics and commerce, who present results and trends relevant to the selected topic seen from their particular point of view. After the round of presentations, a lively discussion ensues involving the audience. This continues during the aperitif after the formal proceedings come to a close.

Science Apéros are open to the public and expert alike; entrance is free of charge.

The current calendar of events can be viewed at: www.empa-akademie.ch/veranstaltungen