NOISE AND VIBRATION BACKGROUND
‘Noise and vibration’ is a term used as though they are one and the same thing. But they are not, and this article aims to give a very brief overview of how they differ. We will also show how a wide range of Pandrol Track Systems group products have a role to play in solving railway noise problems, railway vibration problems, and in situations where both railway noise and railway vibration are both an issue at the same time. Understanding the issues leads to making the best choices.
Let’s look first at transmission of vibration into the ground on which the railway track is built. From there, these ground borne vibrations may find their way into adjacent buildings and cause nuisance and annoyance. They may lead to parts of the building (often windows and doors) and its contents shaking and giving off noise. But the aim here has to be to control that vibration and prevent it from getting into the building in the first place. Do that, and any resulting ground borne noise will also be eliminated. Now, the vibrations that we want to control originate at the hard and uncompromising steel-steel wheel-rail contact, as a result of the roughness on the wheel and the rail. We can reduce the roughness by making sure that the wheels and rails are as smooth as possible. But if we need more, then the most commonly employed method of vibration control is to reduce the stiffness of the track or add to the mass of that part of the track that is resiliently supported. The basic physics is that there will be a ‘natural frequency’ or resonance at which the mass bounces on the stiffness, with a high level of vibration. At frequencies that are significantly higher than this, it’s not possible to transmit much vibration energy through the track and into the ground. So as we increase track mass or reduce stiffness, the natural frequency reduces, and so does the frequency above which vibration control becomes effective. We eliminate the higher frequencies from our building, because they can’t be transmitted through the track structure. We’re off to a good start in applying this method, because most trains have quite a high unsprung mass (below the suspension), which plays a part in the system and is useful in reducing the natural frequency. So we can get quite a long way just by making our track softer, and using the unsprung train mass that we already have at our disposal. The softest track fastenings that Pandrol can supply can be very effective in controlling ground vibration. In many cases, this is all that is needed to effectively eliminate ground vibration from buildings. But in some special circumstances, for example near theatres and concert halls, we may need to do more, and will need to begin adding mass to the track to reduce the natural frequency further. Small additions of mass will have a limited effect, but resiliently mounted blocks and sleepers have a role to play. These can be mounted on resilient materials supplied by the Pandrol Group company Pandrol CDM Track.
For very low natural frequencies and the best performance in controlling ground vibration, we need large heavy concrete slabs, sitting on resilient elastomer bearing. These are ‘floating track slabs’, which can also be designed and supplied by PCT. So far, so (relatively) simple. The more mass that is added and the softer the track, the better the reduction in ground vibration. But there are some constraints. Firstly, of course, all this extra mass and resilience has to be designed in such a way as we still have a safe, maintainable operating railway. Secondly, adding mass and resilience tends to increase cost. And thirdly, that extra material and lower stiffness implies that the track itself may contain more material, which is vibrating at a higher level. And that leads to extra noise being emitted directly from the track itself. Now in some circumstances, that may not be too much of a problem. For example, in a tunnel, the noise from the track won’t escape from the tunnel (unlike vibration, which will — unless controlled). As long as the trains are well sound proofed, the extra noise from the track is a problem we can live with. Here we can focus entirely on solving the vibration problem, without concerning ourselves too much with the effects of noise from the track.
But suppose the track is mounted on a structure, and in open air? Then any ‘airborne’ noise from the track itself has a direct path to anyone standing or living nearby. That adds to noise from the structure, which is what we were aiming to control by reducing track stiffness and adding track mass.
Generally, and without going into detail that is beyond the scope of this article, railway airborne noise tends to be controlled by applying the measures that are the opposite of those used to control railway vibration — that is by increasing rather than reducing track stiffness. So to get the best overall solution, with the lowest total of noise from the track plus noise from the structure, we may need to compromise on track stiffness. Pandrol has a whole range of fastening products at different stiffness levels that allow the best solutions to be adopted in each circumstance. The above is an example in which both ‘secondary noise’ from a structure and ‘airborne’ noise from the track (and train) are present. Where tracks run at grade rather than on structures, secondary noise becomes insignificant and airborne noise is likely to be the biggest issue. Here, an important consideration is how far along the track vibrations are transmitted, and therefore over what length of rail significant airborne noise is emitted. Stiffer track, giving a stiffer connection to the ground, reduces the length of vibrating rail. To reduce airborne noise, tracks with stiffer fastenings and stiffer resilient elements may therefore be required, and Pandrol can provide these. However even here, a compromise in track stiffness will often be required, since the levels of dynamic force generated in the track and transmitted into the sleepers and ballast increase as track stiffness increases. Excessively stiff track can lead to increased rates of development of the roughness and irregularity that leads to noise and vibration in the first place.
Selecting the optimum stiffness for the rail fastenings is an important consideration for standard ballasted track with sleepers. It is also possible to provide additional resilience below the sleepers (under sleeper pads) or below the ballast (ballast mats), which in different ways and to different extents protect the ballast from degradation and affect the overall noise emission from and vibration transmission through the track. Where track is constructed without ballast (as slab track or non-ballasted track), an important source of resilience and damping is no longer available, and the track fastenings plus any additional resilient elements specified have an even more important role to play in determining the overall performance of the track with respect to noise and to vibration.