Track & trackside
frequencies, the renewed track is noisier than the un-renewed track.
rubber rail pads. These relatively soft pads are now installed as standard on modern track to reduce damage to the sleeper and ballast from impact loading. In contrast, the un-renewed track had thinner and stiffer EVA pads. The rolling stock on the line is predominantly 4-car Class 377 and 3-car Class 313 Electric Multiple Units. The rail roughness was measured with a corrugation analysis trolley (CAT) trolley [Figure 1]; this is a device that is pushed along the track at walking speed and uses the signal from an accelerometer to derive the unevenness of the track.
registers the rotation of the wheel. Notably, at all wavelengths the wheel was found to be much smoother than both tracks showing that the influence of the wheel roughness on the overall excitation was minimal.
The length of track excited when a train runs over it depends on the attenuation of the track with distance (decay rate). To measure this requires fixing an accelerometer to the rail head, and making a series of measurements of frequency response to a force impulse at different distances along the rail head [Figure 2].
Renewed track noisier
The device is capable of measuring wavelengths between around 3 mm and 0.5 m - at typical train speeds on the line (115 km/h) this corresponds to frequencies between around 64 Hz and 10 kHz. As expected, the renewed track was found to be somewhat smoother than the un-renewed track. The roughness of 64 Class-378 wheels was measured; these wheels are the same wheels used on Class 377’s and undergo a similar maintenance schedule. The roughness was measured using a TriTops device– with this a displacement transducer (LVDT) measures the undulations of the wheel surface as it is turned while a tachometer
The decay rate varies with frequency - typically at low frequencies (<400 Hz) decay rates are high, whereas above around 500 Hz waves begin to propagate freely in the rail and the decay rate decreases, before increasing again to a peak at around 5 kHz. When the pad stiffness is high, as is the case for the un-renewed track at Fishbourne, the coupling between the rail and sleeper increases, which increases the sleeper contribution to noise. It also increases the decay rates, reducing the rail component of noise. The result of this is that at most
track and 7 dB less on the renewed track. Typical of most modern passenger rolling stock in the UK, the wheels of the Class 377 trains are relatively small and the fact they are disc-braked allows the web to be straight - both of these factors reduce the noise radiation.
Rail dampers attractive
The results of this study suggest that in many situations typical of the UK, rolling noise is likely to increase somewhat after a track renewal. Furthermore, it is suggested that for modern UK track and rolling stock, the track component of noise will often be of much greater importance than the wheel. This can make technologies such as rail dampers attractive to reduce the noise.
•
Institute of Sound and Vibration Research, University of Southampton, Southampton, SO17 1BJ.
www.isvr.soton.ac.uk
February 2014 Page 85
In summary it was found that there were two counteracting effects on the noise due to the track renewal: a reduction in roughness (quieter) and a decrease in decay rates (noisier). Noise was measured 7.5 m away from both tracks during 37 train passbys over a two day period. These measurements showed that that on average the renewed track was 4 dB noisier than the un- renewed track [Figure 3]. Subsequently, the noise was predicted using the TWINS model, on the basis of the measured roughness levels and decay rates. The wheel was modelled using a finite element model from which the modes shapes and frequencies were extracted. The predictions of noise both in terms of the overall levels and the spectra were found to be in good agreement with the measured values [Figure 3]. It can be seen that the sleepers, rails and wheels all contribute to the overall noise level on the unrenewed track [Figure 4]. After renewal the rail component was much more dominant [Figure 5]. This was due to the reduction in track decay rates which more than offset the small reduction in rail roughness. The component of noise produced by the wheels was about 4 dB less than the track (sleeper plus rail) on the un-renewed
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