I can hear you

Impacts of aerodynamic noise on noise mitigation efficiency, part 5

In my blog It’s there for all to see (posted 5 Dec 2012) I reported that members of the Infrastructure and Engineering Department of SNCF (French Railways) have been working for a number of years on a programme of field measurement and computer modelling of high speed train noise sources and mitigation methods. This team has produced a number of papers arising from this research.

One of these papers casts some interesting light on the design of noise barriers and their efficiency. We have already come across one of the authors, Frank Poisson, in It’s there for all to see.  The other two authors are Pascal Belingard (first named) and Selim Bellaj, and the paper’s title is Experimental Study of Noise Barriers for High-Speed Trains. The paper was presented at the Tenth International Workshop on Railway Noise, held in Japan in 2010. Unfortunately it was published in an expensive proceedings volume and I have been unable to find a free copy that can be downloaded. I had to get mine from the British Library, before I found out that HS2 Ltd had made copies available.

The paper is full of interesting results, but it is Section 4.2 of the paper that I would really like to bring to your attention. This section looks at the measured performance of a plain reflective barrier, located approximately 5 metres from the centreline of the nearest track. Although a 3 metre high barrier was used, the effective height was only 2.1 metres due to the ground on which the barrier was built being lower than the head of the rails.

The paper reports results for two different train pass-by speeds: 320 kph, and 375 kph. We are told that the overall noise reduction, across the audio band, which the barrier offered was 6 dB(A) at 320 kph and 5 dB(A) at 375 kph.

However, the researchers also measured the performance of the barrier in each separate third octave band across the audio band (see footnote 1) and display the results in a graph that is presented as Figure 6, and which I have reproduced below.

Noise barrier performance variation with frequency (source: Belingard, et al - SNCF)

Noise barrier performance variation with frequency (source: Belingard, et al – SNCF)

In this graph the noise reduction provided by the barrier (confusingly referred to as “gain” in the diagram) is plotted for each third octave band, identified by its centre frequency, for the two train pass-by speeds; 320 kph is the blue line and 375 kph is shown in red. I have added a thick red line to highlight a section of the 375 kph plot; the reason for this addition will become apparent.

Unsurprisingly, the barrier performance is not consistent across the whole audio band. Below 160 Hz the performance is somewhat erratic; this is explained by the researchers as due to interaction between the train and the barrier as the “distance between the train and the barrier is around the wavelength of the noise source at this frequency”.

Above 600 Hz the traces for the two different pass-by speeds are reasonably similar, both rising to a peak noise reduction at around 3,150 Hz. At this peak the noise reduction at 320 kph is around 10 dB, falling to around 8 dB at 375 kph.

What I find most revealing, however, is the region between 160 Hz and 600 Hz since, as you may recall from my blog It’s there for all to see, the noise energy generated by the pantograph is concentrated in this frequency band. In this region the graph for 320 kph exhibits a fairly smooth climb from about 2 dB to about 4 dB. The section of the 375 kph graph that I have highlighted in thick red is, in contrast, fairly level, at around 1.5 dB, before rising rapidly to join the 320 kph trace at around 500 Hz. So the barrier is very inefficient at stopping noise energy at frequencies around 300 Hz and is providing hardly any mitigation in this area.

My interpretation of what the figure tells us is that noise energy at around 300 Hz is escaping over the barrier, resulting on the barrier performance in this region being very poor at 375 kph. I regard the red-highlighted section as the “signature” of the aerodynamic noise high up on the train body.

The researchers seem to agree that aerodynamic noise is making its presence felt at 375 kph, suggesting that the reduced performance of the barrier at the higher speed is “probably due to the increased contribution of aerodynamic noise with the train speed, especially around the pantograph and its cavity that are higher than the noise barrier”.

The SNCF researchers also found that adding 1 metre of absorbent material to the top of the simple reflective barrier improved the performance by about 5 dB(A). What I would like to have seen is a graph of performance across the audio band for this barrier; unfortunately, the researchers have failed to provide this. What I think such a graph would show is that the dip in performance at around 300 Hz is much more pronounced, since the absorbent material will have no impact on the noise from high up on the train.

The researchers conclude that “aerodynamic noise sources located on the roof of the train cannot be neglected” at higher running speeds. I hope that Hs2 Ltd is not intending to do this, but fear that the signs are that they are planning to do this very thing.

On a more positive note, my hopes were raised by a photograph that was shown by HS2 Ltd during the presentation on noise that I refer to in my blog Are you taking this seriously? (posted 27 Nov 2012). It was intended to illustrate the use of noise barriers. I have traced the source of this photograph; it is a view of the HS1 route. Just zoom in on the front of the train – how high do you think the noise barrier is? It looks to me that the barrier rises to at least the height of the power catenary. What do you think? So perhaps there is some hope that HS2 Ltd will specify adequate noise barriers.

However, my confidence is not boosted by another slide that was shown during the presentation, and which I have reproduced below. It shows “low level” noise barriers being employed for noise mitigation on HS1 viaducts (see footnote 2).

HS1 noise mitigation on viaducts (Souce: HS2 Ltd)

I wonder how much mitigation against pantograph noise would be offered by that puny thing!

Footnotes:

  1. 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 Organization for Standardization and a tabulation of bottom, centre and top frequencies may be found here.
  2. Preliminary design drawings issued for the proposed viaduct at Balsall Common show a 1.4 metre high noise barrier being deployed, which appears to provide an effective height of about 1.7 metres above the rail head.
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