Mitigating railway noise

Science topics April 2014 City

Olivier Chiello - AME Department, LAE Laboratory

Living near a railway, a tramway or a Metro line can be testing for our ears. One way of reducing train passage noise is to install noise screens along the tracks or better insulate affected buildings. But the most direct, and in many cases the most effective, way is to take action at the source. But how?


Identifying sources of noise

At low speeds, most of the noise is caused by the drive units (engines, auxiliaries, ventilators). The rolling noise caused by contact Location and characterization of noise sources the passage of a tram by microphone antenna (Ifsttar credits). between the wheels and the rails begins to dominate at higher speeds. In the case of high speed trains, aerodynamic noise appears above 300 km/h. Other sources make a contribution in specific situations, for example screeching on tight bends or braking noises on arrival at a station. In the worst cases, noise levels near trains can exceed 100 dB(A) ! Last, vibrations that are transmitted by the soil, bridges or tunnels can pose problems in some situations, particularly in urban environments.
In order to localize and characterize sources, we frequently use microphone antennae. In the case of railways, a large proportion of the noise is due to the rails which can vibrate over a length of several tens of metres. However, conventional antennae are not appropriate for spatially extended sound sources of this type. In view of this, the LAE has conducted research with a view to developing alternative methods for identifying their contribution more accurately 1.

Understanding noise generation mechanisms

In the last 20 years a considerable amount of research has been conducted into rolling noise. We now know that it is caused by irregularities in the contact surfaces that set up vibrations in both the wheels and the track. In comparison, our understanding of the other mechanisms remains limited.
For example, explaining the origin of screeching noises remains a real challenge. In the framework of the AcouFren research programme2 the LAE is carried out research with this in mind in partnership with the Laboratory of Tribology and Systems Dynamics at the École Centrale de Lyon3. By implementing a strategy that combines modelling and experimentation, this work has made significant advances which have demonstrated that the screeching noises are caused by a complex phenomenon of vibrational instability affecting structures that are in frictional contact.

 

 

 

Animation : A numerical model of a railway disk braking system developed during the AcouFren project. When the disk rotates, the friction between the lining and the disk is unstable and the entire structure starts to vibrate, at what may be very high frequencies. This generates noise, particularly via the disk which acts as a loudspeaker.

 

Developing and testing solutions

We now have effective solutions to the problem of rolling noise: first, the interface between the wheel and the rail must be kept in good condition (by using composition brake shoes), vibrations must then be attenuated (by installing dynamic vibration absorbers on the rails). The challenge today is to optimize these solutions by taking account of the other constraints.
Research into ways of reducing the other sources of noise is frequently empirical. The development of optimized solutions therefore goes hand-in-hand with a better understanding of noise generation mechanisms. In the case of squeal noise caused by braking, for example, one part of the AcouFren project was to develop and test quieter brake linings. Software has been developed to test how the mechanical characteristics of linings affect noise emissions 4.

 

 


1Baldrik Faure, Caractérisation du rayonnement acoustique d'un rail à l'aide d'un réseau de microphones. Doctoral thesis, Université de Grenoble, 2011.
2 « AcouFren », Outils d'aide à la spécification et à la conception de freins à disques ferroviaires optimisés vis à vis du crissement. 2010-2014 research programme, financed by ADEME and managed by SNCF.
3 Andréa Loyer, Étude numérique et expérimentale du crissement des systèmes de freinage ferroviaires. Doctoral thesis, École Centrale de Lyon, 2012.
4 Chiello et al., Squeal noise generated by railway disc brakes: experiments and stability computations on large industrial models, Proceedings of the 21st International Congress on Acoustics, Montréal, 2013.