It’s there for all to see

Impacts of aerodynamic noise on noise mitigation efficiency, part 3

Members of the Infrastructure and Engineering Department of SNCF (French Railways) have been engaged, over a number of years, on a programme of field measurement and computer modelling of high speed train noise sources and mitigation methods. This research has progressed to the stage where at least initial conclusions may be drawn.

A number of papers have been published by this team. The one that I will use for the purposes of this blog was presented at the Eighth World Congress on Railway Research, held in South Korea in 2008. The authors of the paper are Pierre-Etienne Gautier, Frank Poisson and Fabien Letourneaux, and the title is High Speed Trains external noise: a review of measurements and source models for the TGV case up to 360km/h.

The authors add their voices to the general consensus that aerodynamic effects make a significant contribution to noise levels at the planned HS2 operating speeds:

“… the contribution of the rolling noise, which is the main noise source for conventional speeds, remains high up to speeds around 360 km/h for a TGV train set which complies with the TSI [EU Technical Specification for Interoperability] limits at 300 km/h. Then, a significant reduction of the pass-by noise of a TGV train set running at commercial speed (~320 kph) can only be reached by acting both on the aerodynamic sources and the rolling noise sources …”

One of the most interesting features of the paper is that it allows us to “see” the sources of noise on a moving train, by presenting a series of “noise maps”. These are based upon acoustic array measurements made on actual train passes; these arrays allow the frequency characteristics of the noise sources to be identified, by separating the measurements into specific third octave bands (see footnote).

A small section from one of these noise maps (Figure 6 in the paper) is reproduced below. This shows the noise map for the front of a train passing at 320 kph, concentrating on the third octave bands centred on 250 Hz (upper diagram) and 315 Hz (lower diagram). The noise levels are indicated by colour, with red being the highest level, through orange, yellow and green, to blue, which is the lowest level. Noise below the blue range is not shown.

Noise map of TGV for low frequencies (Source: Gautier, et al – SNCF)

The dominant noise source in the two low-frequency third octave bands is clearly the pantograph at the rear of the lead power car. However, the situation looks different in the higher frequency third octave bands, centred on 2,000 Hz and 2,500 Hz, shown in the following section from Figure 8 in the SNCF paper.

Noise map of TGV for high frequencies (Source: Gautier, et al – SNCF)

In this band the influence of the pantograph is hardly noticeable and it is the bogies that are generating the most noise. These two diagrams just underline the view of most world authorities that noise mitigation should be designed to reduce both bogie and aerodynamic noise at the maximum operating speeds of HS2, a point that is made by the SNCF team in the comment that I have quoted earlier in this blog.

Contrast this with a slide that was shown during a presentation on noise given by HS2 Ltd at the September round of community forums.

HS2 Ltd train noise map (Source: HS2 Ltd)

A footnote to this slide advises that it is “based on SNCF 1/3 octave noise map at 360 km/hr”. The note doesn’t say which third octave band has been selected – the two examples of the actual SNCF diagrams above illustrate that this choice is critical to how dominant the pantograph will look as a source of noise. The speed is 360 kph, compared with 320 kph in the two SNCF diagrams, which should make the pantograph more dominant.

What is most striking about the HS2 Ltd diagram however is that it looks like someone has taken a red crayon and drawn a line along the bottom of the train. What is this, I hear you say; it isn’t something that is in the SNCF diagrams?

The footnote on the slide also explains this; the diagram has been “modified to represent LpAequsing [the] output from TWINS modelling”. According to a product leaflet “TWINS is currently the most comprehensive and widely used calculation model for assessing the acoustic effects of wheel and track design on railway rolling noise”. So this software has presumably been used to assess the level of rolling noise, although no explanation was provided of the why or wherefore by the presenter at the talk that I attended.

However I have been able to find the reason for this “doctoring” of the SNCF noise maps in a more technically orientated presentation given to the noise sub-groups of the Planning Forums, where the audience included local government environmental health professionals and the like; an audience deemed by HS2 Ltd to be capable of understand the technicalities. The PowerPoint file for this presentation was kindly made available to me by HS2 Ltd.

The justification for the “red crayon” line is observations made by Professor David Thompson of the Institute of Sound and Vibration Research at the University of Southampton in a paper that he presented to the Fourteenth International Congress on Sound and Vibration in July 2007.

In section 4 of this paper, Professor Thompson identifies the rolling noise produced by a train as the summation of noise generated by vibration of the wheels and the noise generated by the vibration of the rails and the sleepers. In section 5 he comments that noise maps derived from microphone array measurements tend “to give greater prominence to the wheels and less to the track than the theoretical models”. It is this noted shortcoming of the array measurement methodology that HS2 Ltd has attempted to address using the TWINS model.

So what the HS2 Ltd diagram is telling us is that the noise generated by the track/wheel interface is underestimated by the SNCF noise maps. What the slide that was shown next in the Planning Forum sub-group presentation appears to illustrate – of course I only have the picture, I don’t know what was said by the presenter – is that this problem may be easily overcome by the installation of trackside noise barriers. Interestingly, this slide also shows the pantograph as a noise source that is not shielded by the noise barriers.

This is, as I recall it, the first time that HS2 Ltd has sought to characterise noise expected to emanate from HS2 as being worse than general expectations. Of course, the motive for doing this is probably to convince us that the pantograph noise, being much less than the wheel/track interface noise, may be disregarded as a potential problem. I don’t buy into this scenario. The pantograph noise problem stands alone and is not diminished by any expectation of higher noise levels from the base of the train and the track.

Footnote: A one-third (or third) octave band is a segment of the frequency band defined by the centre frequency of the band. The upper band-edge frequency of such a band is the cube root of two times the lower band frequency. The set of centre frequencies is specified by the International Standards Organisation and a tabulation of bottom, centre and top frequencies may be found here.

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