A research team from Empa address the levels of noise pollution in Swiss cities and intend to reduce it—starting with railways.
How do scientists intend to reduce noise pollution?
Railway noise is considered ‘unhealthy,’ which is why hundreds of millions of Swiss francs have already been invested in noise barriers, quieter braking systems and other measures with the goal to protect at least 80% of the Swiss population from noise pollution by the year 2025 – but because railway traffic will continue to increase, it has been noted that a lot remains to be done.
Thus, to further reduce noise pollution, researchers at Empa and the Vaud School of Economics and Engineering, under the leadership of colleagues from EPFL, are relying on an inconspicuous component of the rail system: ‘rail pads.’
Rail pads are made of elastic plastic, which are inserted between the rails and concrete sleepers. They serve to protect the highly stressed track made of compacted ballast and concrete sleepers by allowing the rails to move just slightly. However, it is precisely this minute movement of the rails that makes them ‘sound’ louder – and this noise is the decisive factor at frequent speeds of between 60 and 160 km/h.
How will the rail pad material be altered?
In Switzerland, rail pads are typically constructed of the hard polymer ethylene vinyl acetate (EVA). It is true that a softer material would better protect the tracks – but at the price of higher noise pollution.
This noise is a dilemma that the team commissioned by the Federal Office for the Environment (FOEN) intends to solve with a composite material. The concept is a material with a hard shell and soft core.
More precisely: a shell made of EVA and a core made of the soft material polyisobutylene, whose damping is precisely tuned to the frequency range from about 200 to 2,000 Hertz, at which vibrations are particularly noise-intensive.
How did scientists create this material?
This type of material has never been designed before so, in order to prepare for any possibility, the experts designed dozens of variants: sandwich structures made of flat layers – with and without a ‘lid’ made of EVA. Zigzag-shaped PIB fillings, surfaces with incisions and other methods. But, in order to find out in the laboratory what impact each type of construction has, extensive preliminary work was necessary.
The complex interaction between rails, sleepers and ballast was simulated by a ‘three-sleeper unit cell,’ which is a piece of track, just under two metres long, equipped with a ‘shaker’ that generates defined frequencies and a probe that measures the sound intensity.
Although this measuring cell does not reproduce the real behaviour of a railroad track, it does allow precise comparisons to be made under varying conditions.
Additionally, researchers led by Bart van Damme of Empa’s Acoustics and Noise Control lab developed a simulation of the system, by employing finite element methods that matched the results of the experiments very closely, which was the basis for eventually extrapolating the behaviour to a longer railroad track.
With these elements in mind, scientists were able to consider their rail pad designs. As a result, it was observed that sandwich structures, which can be easily bent thanks to indentations, were unsuitable for protecting the track bed and reducing noise at the same time, van Damme said.
Zigzag-shaped infill made of PIB also did not bring any advantages. The best solution proved to be a PIB content of over 50%, inserted in a ‘shell’ made of the harder EVA.
When will this new structure be tested on railways?
Experiments with this structure will take place on a rail track in Nottwil starting next March. “These rail pads are easy to manufacture. We will need almost 400 of them on the 100-metre track,” explained van Damme, which is why a company is on board to manufacture the patented components.
Measurements of noise, vibration, deformation, and other characteristics will show how the rail pads perform. “We hope that they will cause audibly less noise and at the same time protect the ballast better than the conventional hard intermediate layers,” added van Damme.
In any case, there is optimism among the researchers. “The models developed in the project allow targeted optimisation of the sometimes contradictory requirements,” concluded Empa lab head Jean-Marc Wunderli. “Since no significant additional costs are expected for the production of the intermediate layers, I hope they will be used on a large scale and thus make a significant contribution to reducing railway noise.”
