Or putting it another way

Impacts of aerodynamic noise on noise mitigation efficiency, part 4

In my blog It’s there for all to see (posted 5 Dec 2012) I presented noise maps prepared by SNCF researchers that appear to show that noise from aerodynamic effects generated by a HS2 train at maximum speed is likely to be of the same general magnitude as noise from the wheel/rail interface, and that these will be the two principal sources of noise on the train. The noise maps also show that these two noise sources differ in the region of the audio spectrum in which their sound energy is concentrated and in the position of the sources in the vertical plane. In respect of the latter consideration the main sources of aerodynamic noise are generally higher up the train, and so pose a challenge to the design of noise mitigation by barriers.

The paper in which the noise maps are presented is High Speed Trains external noise : a review of measurements and source models for the TGV case up to 360km/h, authored by PE Gautier, F Poisson and F Letourneaux.

In It’s there for all to see I also reported that at least one academic (Professor David Thompson) considered that such noise maps tend to underestimate the noise generated by the track/wheel interface. Whilst we need to bear this reservation in mind, I think that the results of the array measurements made by the SNCF researchers, as reported in their paper, merit further consideration.

Take for example Figure 9 in the paper, which depicts a three-dimensional histogram comparing the energy of the various identified train noise sources (reproduced below).

Noise source power estimation for TGV pass-by (Source: Gautier, et al – SNCF)

The four rows, identified by different colours, each relate to a different pass-by speed: yellow is 200 kph, green is 250 kph, blue is 300 kph, and red is 350 kph. The columns each relate to one of fourteen identified noise sources, which I have numbered 1-14 for convenience.

I have two reservations about this diagram. The first is that the energy levels shown are not totals across the audio frequency band, but each relate to the “most energetic third octave band of the source”; I’m not sure what impact, if any, this may have on the comparative levels. The second reservation is that, whilst the yellow, green and blue rows appear to be relatively consistent with each other, the red row looks anomalous. The text of the paper advises that there were some practical difficulties in making the measurements on which the histogram is based and, more particularly, that “a different method was developed” for the red (350 kph) row. I therefore suggest that we should treat the red row with a little suspicion.

These reservations apart, what does the histogram appear to be telling us about high speed train noise sources?

Setting aside the red row, which I am sceptical about, the row representing the next highest pass-by speed is the blue row (300 kph). This row exhibits six dominant noise sources: “first bogie – first power car” (1), “front wind screen” (2), “louvers (sic) front power car” (3), “up louvers (sic) front power car” (5), “auxiliaries – first power car” (6), and “pantograph” (10).

In the red row (350 kph) the relationship between sources 2, 3, 5 and 6 is approximately maintained, but the pantograph (10) increase is greater; this is, more or less, what the theory predicts. What does seem unexplained is the even greater increase in first bogie noise (1), which makes this source clearly the dominant one. A similar effect is seen on “last bogie – rear power car” (11), which gets promoted to second most dominant. This may be a prediction error, or may be correct and due to rolling and aerodynamic noise effects combining in that area; it is impossible to say and the authors of the paper don’t shed any light on this.

The red row also shows a very big increase in “intercar gap – first power car/first coach” (7). An appreciable increase in this noise source may be anticipated, it being an aerodynamic noise source, and the difference between the blue and the red bars is approximately the same as between the green and the blue. However, this is in contrast to the “pantograph” bar (10), where the difference between red and blue is less than between blue and green.

If however we put any doubts about the credibility of the red row aside, what the SNCF histogram shows is that the loudest noise generated by a 350 kph train pass-by will come from a source low down (the front bogie), with a secondary source about 3 dB down (the last bogie of the rear power car), which will also be low down on the train. The third loudest source will be the pantograph, which is about 4 dB down on the front bogie noise source; this will of course be high up on the train.

If we take Professor Thompson’s comment that microphone array measurements tend “to give greater prominence to the wheels and less to the track than the theoretical models” into account, we may suspect that the noise from the base of the train will be more prominent than the histogram suggests. Unfortunately, although Professor Thompson indicates in this paper that the underestimation of the rail noise component can be appreciable at the higher frequencies, I for one find it difficult to use this information to estimate the likely extent of any error in the histogram in this respect.

In document Noise Source Height of High Speed Trains for the Appraisal of Sustainability, which was an attachment to HS2 Ltd’s response to FOI request 11/327, we are told that a 3 metre absorptive barrier “provides a noise reduction of approximately 10 dBA”. Such a barrier can be expected to attenuate the noise emanating from the bogie areas but, due to its limited height, will have no impact on the noise from the pantograph. So the net effect could be to promote the pantograph to be effectively the loudest noise source and one which will undermine the mitigation offered by the barrier.

In view of the reservations that I have expressed about the histogram and the comment by Professor Thompson about the accuracy of microphone array measurements, it is difficult to be too dogmatic about the problem that pantograph noise may pose. However, there is enough evidence here that a potential problem has been identified and that HS2 Ltd would be, perhaps, wrong to dismiss it lightly. What is needed is a good deal more work to ensure that the likely impacts of pantograph noise are understood; I hope that HS2 Ltd is prepared to undertake this work, before finalising noise mitigation proposals.

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