Not a precise science, part 4

(… continued from Not a precise science, part 3, posted on 13 Oct 2015).

In this part I will consider the area of known unknowns that contribute to uncertainty in the prediction of operational airborne noise levels that acoustics expert, Graham Parry, tagged as “differences in trains” in the evidence that he gave to the HS2 Select Committee during the afternoon session held on Monday 7th September 2015.

In assessing what the strength and equivalent location of the noise sources on a passing HS2 train will be, HS2 Ltd has used as a baseline the European Commission’s Technical Specifications for Interoperability (TSI), together with a working assumption that the generation of train design that will operate on HS2 will show an overall 3dB improvement in noise emissions compared with current TSI-specified trains. So HS2 Ltd is relying on the limits set by a “type approval” specification, which requires a sample train running in closely-defined conditions to meet a maximum emitted noise specification, to define the noise output to be expected from every HS2 train that passes. Is this a safe assumption?

Mr Parry’s evidence gave cause to doubt it. He told the Select Committee that the experience of the noise tests that had been carried out on the TGV was that there is a “spread of data” and that “none of the data that has come forward shows that you do not get that spread” (see footnote 1).

He added (see footnote 2):

“Most of [the testing] was done on the same railway line, but quite often you still have this spread of data when measuring in exactly the same location. While they might all be TGVA trains, they were all different; they had different acoustic properties.”

So we are very much in “known unknowns” territory and, to be perfectly honest, it is impossible to judge from the information in the Environmental Statement, just how much slack there is in HS2 Ltd’s calculation methodology to cater for this inevitable variation. However, one area of variability that I have been able to get confirmation on indicates that the likelihood is that the answer to that question is “hardly any, perhaps none”.

The source of noise to which I refer is the wheel/track interface, where imperfections in the metal to metal rolling contact at each such interface on the train give rise to vibration and, as a result, noise emission. I have visited this mechanism before, in my blog It could be a bumpy ride (posted 24 Oct 2011) and expressed concern that no allowance appeared to have been made for trains in operation performing worse than the noise limits set by the TSI. In that blog I cited empirical evidence from SNCF, no less, that track work can cause appreciable degradation in track roughness. I also quoted the advice in the TSI documentation that “the actual track” quality should be taken into account and reported the view of the acoustic consultants to the 51m consortium that “an acoustic track quality correction … should be applied to calculated noise levels” – a pragmatic engineering solution that is commonly adopted to counter tolerances in calculations.

So what is HS2 Ltd’s reaction to all of this evidence and advice (see footnote 3)?

“As already noted the rolling sound component from a TSI‐compliant train running on in‐service track can be higher than that measured on a TSI reference track. This can be due to in‐service growth of wheel and rail roughness, and a track system that radiates more sound than a TSI reference track. However it is assumed that wheel and rail roughness will be controlled via an appropriate maintenance regime, and a low‐noise track will be specified, thereby ensuring that sound emissions from TSI compliant trains running on HS2 infrastructure will not exceed the TSI noise limits.”

So stick your fingers in your ears and go “la, la, la” and the problem will go away.

It is not as if there has not been a precedent for adopting a safety margin in a UK railway project. In evidence (see footnote 4) that the Promoter’s expert witness on noise matters, Rupert Thornely-Taylor, gave the Lords Select Committee that deliberated on Crossrail – yes hasn’t he done well out of the taxpayer – he told their Lordships that 5dB had been added to groundborne noise predictions to cover uncertainty in the computer model.

Why should HS2 Ltd policy on HS2 train noise performance ignore precedent and run so counter to what appears to be plain common sense? I suspect that a decision was taken that, faced with a choice of crossing the fingers or suffering the public relations fall-out that would be the result of an increase in already sensitive noise levels, the PR men held sway over the engineers.

(To be continued …)


  1. See paragraph 238 in the transcript of the afternoon session of the HS2 Select Committee that was held on Monday 7thSeptember 2015.
  2. See paragraph 240 in the transcript of the afternoon session of the HS2 Select Committee that was held on Monday 7thSeptember 2015.
  3. See paragraph 1.1.35 in Annex D2 to Appendix SV-001-000 of the ES.
  4. See paragraph 216 in the transcript for 19thFebruary 2008 in Volume II of the 1st Special Report of Session 2007–08 by the House of Lords Select Committee on the Crossrail Bill.

Important Note: The record of the proceedings of the HS2 Select Committee from which the quotes reproduced in this blog have been taken is an uncorrected transcript of evidence, which is not yet an approved formal record. Neither witnesses nor Members have had the opportunity to correct the record in such instances, and it may therefore be subject to changes being made in the light of any such corrections being requested.


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