Railway Noise & Vibration Management
Reading Time 5 minsby Prof. Dr. Andrea Bracciali
Noise and vibrations (N&V) are the most common pollution of railway systems. The approach to their management is manyfold and normally passes through the following steps:
- Measuring the current situation;
- Designing and implementing countermeasure;
- Assessing the results.
All these three phases must be tailored to the specific situation always considering that all railway systems (including in this definition also tramways, metros, light rail, etc.) are based on the steel-to-steel wheel-rail contact. While it ensures the best sustainability minimizing energy needed for rolling, the high stiffness (compared to road vehicles) of these components is the source of high dynamic forces that generate vibrations at the wheel-rail contact.
Tackling N&V may be challenging in some situations but for sure it requires a deep understanding of rails and wheels maintenance. If surfaces are damaged (worn or, generally speaking, uneven), many if not all countermeasures are potentially ineffective.
The first step to keep N&V under control is therefore the implementation of a preventive maintenance regime that keeps the touching surfaces in good shape (straight rails and round wheels). This requires some monitoring (e.g. by using sound level meters and/or accelerometers) and tools/machines that perform specific operations.
Understanding "Airborne" Railway Noise
“Airborne” railway noise is generated by vibrations of wheels and rails that turns into noise for a phenomenon called “fluid-structure coupling”. Roughly speaking, rails are the dominant source at low speeds and low frequency, while wheels are the dominant source at high speeds and high frequency. This leads to the obvious conclusion that different situations require different countermeasures or, in other words, that a particular solution may be good in a situation and completely useless in another.
Different strategies may (or must!) be adopted when dealing with airborne noise: first, vibrations should be limited by keeping wheels and rails in good shape as said above, second by using noise barriers and, if none of the previous measurements is applicable, by treating the receiver (e.g. changing the windows or installing air conditioning systems).
Understanding "Groundborne" Railway N&V
“Groundborne” railway N&V is more subtle because of the intrinsic variability of ground behaviour. Running on a rocky or on a sand terrain produces results in completely different propagation of vibrations. Vibrations up to 80 Hz are perceived as movements of objects within a house (bed, table, furniture in general, glasses in a sideboard) but typically without of with very low noise perceived.
When the frequency content of vibration is higher (up to 250 Hz), there is once again a significant coupling between the vibrating surfaces (walls, floor, ceiling) and the air and this generates variation in the sound pressure, i.e. noise. In this case noise is often called “groundborne noise” or “re-radiated noise”, to highlight the fact that it is not noise coming from outside, but it is generated withing the building.
Navigating Regulatory Frames and Measurement Tools
The use of the proper tools to assess a specific situation may differ not only from the technical point of view but also on the basis of local legislation. Different countries may have different regulatory frames and the physical quantities to be measured may differ. While peak noise is a clear indicator, equivalent levels during the reference period (i.e. day, evening and night) include also the different sensitivity or residence to incoming noise.
Similarly, the so-called “differential noise limit” is a measure of the difference between the peak level and the average level during a certain period. The same source may generate different levels of disturbance depending on the background noise or the moment of the day.
Implementing Effective Countermeasures
Once the specific noise environment has been fully characterized, countermeasures have to be selected and applied. They can be extremely variable, passing from the selection of different technical solutions (low noise wheels, rail damping, noise barriers, etc.) to operational adjustments (reducing the speed, diverting some traffic, etc.). It is fundamental to consider first measures at the source, as they may avoid the use of noise barriers, not always very good-looking, and modifications in the buildings. In this respect, the aforementioned implementation of a preventive surface quality maintenance programme is of paramount importance.
Navigating Operational Realities
Approaching a N&V is always difficult as in many cases people are highly annoyed and several complaints were already filed. The correct approach to N&V does not simply pass to the execution of a measuring campaign – there is much more, as a deep understanding of operations, maintenance, rolling stock, permanent way and local practices has to be considered, together with the investment capacity and the knowledge and training of managers and workers involved in the process. As communication has become fundamental today, local authorities and citizens should be constantly informed about the progress of the implementation of the N&V reduction strategy.
Ensuring Long-Term Solutions
It is therefore important that the supplier of such N&V reduction strategy has all the competencies and the right approach to avoid misunderstandings and false hopes. Objectives should be clearly stated, with a specific agenda agreed between the parties that includes the resources needed to close the circle.
Finding a good N&V measuring company, a wheel/track maintenance machines supplier and a civil engineering company implementing the measures is relatively easy, what’s difficult to find is a subject that understands the actual operation details and works closely with the management and crew of the administration where problems arise to correctly implement the plane.
Sustaining Solutions Over Time
Unfortunately solutions are rarely “once and for ever” or “fit and forget”: magic sticks don’t exist in acoustics and vibrations, so a constant attention is required along the years to guarantee that what was achieved immediately after the intervention plan is kept in the long term. An accurate planning for resources (human and financial) should always be part of a serious approach to N&V problems solution.