When it comes to expanding rail networks in dense urban areas, tunnels are often seen as the best solution.
If space is at a premium, tunnels can unlock extra capacity while avoiding many of the challenges that come with above-ground rail.
A key benefit of using tunnels is minimising noise pollution and disruption to communities that live along the tracks.
Unfortunately, that creates a different problem to deal with – that of noise and vibration inside the tunnel.
This is where Schrey & Veit (S&V) – a German manufacturer specialising in advanced noise mitigation solutions for railway infrastructure – can help.
Dr Christoph Gramowski, Head of Research and Development at S&V, tells us more.
Why tunnels are noisier
Rolling noise is three to six decibels louder in tunnels compared to open track.
This is partly because the noise, which mostly comes from the wheels of a train and the rails, is primarily radiated horizontally.
“With open track, this issue predominantly affects people living near the rail lines,” explained Gramowski.
“However, with a tunnel you have ‘acoustically hard’ concrete side walls, which means most of the sound is redirected back towards the train. This doubles – or more than doubles – the noise inside the train when it is running through tunnels.”
Another factor that contributes to the noise is the use of slab track rather than ballast.
Tunnels with a smaller diameter are cheaper to build, so slab track systems are often chosen as they save valuable vertical space.
Well-designed mass-spring systems for slab track can limit vibrations transmitted into the ground, reducing secondary structure-borne noise in nearby buildings. However, slab track doesn’t have the same elasticity that ballast track has, so it needs more flexible rail fastening systems to protect from high static forces and excessive wear.
This increased flexibility allows more rolling noise to be generated, as rails can vibrate more easily.
“The higher noise and vibration levels can create an uncomfortable experience for passengers,” Gramowski said.
“Past examples of line openings where excessive interior noise only became apparent during passenger service highlight the consequences of insufficient attention to acoustic engineering and reliable noise predictions during planning.
“There are also contractual requirements that must be met regarding vehicle interior noise – so whoever is delivering the tunnel sections must find a way to decrease that noise below the specified limits.”
A smart solution
According to Gramowski, rail dampers are the best way to mitigate noise in tunnels.
S&V’s rail dampers use a mass spring system to combat the vibrations created when trains pass over rails, targeting the noise directly at the source.
“We recommend installing dampers both on the rail and on the wheels of the train, working together to reduce noise,” he said.
Other options for reducing noise in tunnels include absorber mats, but these are less effective as they can only muffle sound after it has already been generated. They also require large surface areas and regular cleaning.
“Another disadvantage of absorber mats is that they cover the concrete surface of the tunnel,” Gramowski added.
“When a surface is covered like this, it makes it harder for building and civil engineers to inspect the tunnel and they cannot easily see cracks or leaks.”
Tried and tested
S&V rail dampers have already been installed in metro tunnels across Australia, Asia and the Middle East, with noise reductions of between six and 15 decibels recorded as a result. Gramowski spoke about one trial in Perth where the dampers decreased noise by eight decibels.
“We were very happy with the results,” Gramowski said.
“Eight decibels is a significant noise reduction, especially when you consider that the trial was on a section of tunnel that was only 100 metres long.
“If the test section had been extended, there would most likely have been an increase in noise reduction.
“We have video footage from before and after installing the dampers and the difference is clearly audible.”
Additional benefits
Schrey and Veit’s rail dampers bring many other benefits for operators.
Easily and rapidly installed using a clip-on system, the dampers require no glue or structural anchoring and little to no maintenance.
Many of S&V’s customers have also noticed a reduction in rail corrugation thanks to the dampers. As a result, rail grinding is required less often – saving operators time and money.
In some cases, the drop in rail wear and tear has been dramatic, especially on curved sections of track.
“We have one client in Malaysia who went from having to carry out rail grinding 27 times per year, to just four times per year after installing the dampers,” Gramowski said.
“They had a return on their investment within two years from the reduced maintenance, and they also avoided the need to buy a second rail grinder.
“This was not even foreseen before the project – it was just an extra benefit of the dampers.”
Another customer in the Far East has installed S&V’s dampers specifically to reduce the high level of corrosion they were experiencing.
“It’s too early to see the results on the corrosion for this customer, but they’ve had a remarkable reduction in vibration and noise.
“The dampers were installed overnight and the next day they recorded a 15 decibel noise reduction.”
When maintenance teams do need to carry out rail grinding or tamping, the dampers don’t get in the way.
And when it comes time to replace rails, the dampers can easily be detached and reattached to the new rails.
S&V’s dampers also contain a low proportion of combustible material, minimising fire load.
“No rail damper anywhere in the world can be produced without a small amount of rubber or plastic,” said Gramowski.
“However, we’ve had an independent assessment from experts in Germany, and they found that the volume of rubber in our dampers is so low – and covered with steel plates – that it is not a significant issue.”
Lower costs, excellent effectiveness, and zero maintenance make rail dampers an essential solution for serious tunnel noise control and improved passenger comfort.
S&V’s rail dampers are homologated for use across all kinds of rail networks, from metro to heavy haul.
They can even be adjusted to reduce specific frequencies, with custom vibration control, and they’re robust enough to withstand the extremes of the Australian climate.
“Our dampers are setting a new standard in rail track performance,” said Gramowski.
“They not only promise quieter and smoother operations, but also ensure that rail systems remain efficient and reliable – meeting the demands of the future.”
