Rock and rail ain’t no noise pollution

We all know how noisy trains can be. The wheels, carriages, brakes, and incessant whine of the buffet car attendant all contribute towards a cacophony that clouds your thoughts and gives you a headache.

This is why it’s particularly impressive that engineers from European rail company Lucchini have evaluated the design of a new wheel – the Syope – by distilling its sound from the overall racket of an ETR470 Pendolino train at full tilt.

In what is believed to be one of its first industrial applications, Lucchini used a phased microphone array method to separate and measure the noise produced by individual wheels moving at over 220km/h. Collected from a linear array of 21 microphones, the signal was analysed using progressively varying time delays to eliminate non-wheel noise, correct for the Doppler shift and separately track the noise from each individual wheel and bogey. The results showed the new wheels to be 3 to 4dB quieter than standard wheels.

A microphone array approach uses the same principle as human hearing to track a moving noise, but the use of 21 widely-spaced microphones provides far greater sensitivity than two ears.

Lucchini asked engineers from LMS to implement the technique. In general, off-centre sources that are averaged over time tend to interfere and cancel each other out, while a coherent source, in this case the train wheels, is enhanced. Varying the delay between the microphones for each time sample makes it possible to point the array at a nominal target as it moves down the track. It is then just a matter of repeating the analysis for each set of wheels and for each each frequency line in the bandwidth of interest. The Doppler effect is removed by an adaptive resampling method – at each step in the analysis, the digital filters are reallocated to account for the change in frequency caused by the Doppler shift.

The microphone array was positioned 3.3m from the track, and an optical barrier was used to trigger the acquisition. A time history from each microphone was recorded onto disc, where it was analysed using an LMS software application.

LMS International Tel: +32 16 384 200; Fax: +32 16 384 350

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