Are you taking this seriously?

Impacts of aerodynamic noise on noise mitigation efficiency, part 1

I recently posted a short series of blogs explaining where I think that HS2 Ltd is going wrong on its approach to estimating the annoyance that HS2 noise will cause; the intention was to do this as succinctly as possible and to avoid jargon and other technicalities as far as was feasible. I now hope to do the same for the issue of whether the higher planned speeds of HS2 will, by generating greater levels of aerodynamic noise, dictate additional requirements on the design of noise mitigation measures; this blog is number one in that series.

I have discussed this matter previously in my blogs:

  • In A little bit of magic (posted 17 May 2011) I reviewed what the US Federal Railroad Administration manual says about aerodynamic noise and contrasted this with HS2 Ltd’s proposals, as set out in the Appraisal of Sustainability.
  • In Going over the top (posted 9 Nov 2011) I reported on what Southdowns Environmental Consultants Ltd had said about this issue in the 51m response to the public consultation and commented the potential for reducing the level of aerodynamic noise at source.
  • In Going over some old ground (posted 17 Apr 2012), amongst other things, I reported on the response by HS2 Ltd to queries raised about aerodynamic noise by respondents to the public consultation.

In Going over some old ground I promised to revisit this whole issue in “a future blog”; that is what I now propose to do, although I fear that it is going to take a short series of blogs rather than just one.

The problem is that, unlike slower trains, ones like HS2 that are designed to run at speeds well over 300 kph generate noise, arising from air turbulence effects (aerodynamic noise), from points high up on the train, including the roof and the pantograph (the power pick-up device). This noise is in addition to the “conventional” sources of train noise – the propulsion machinery and cooling fans contribute to this, but the dominant source for a travelling train is the wheel/rail interface. Whilst experience has shown that the conventional noise sources, being lower down on the train, can be controlled by relatively simple measures, such as low noise barriers, the control measures that will be effective for ultra high speed trains, such as HS2, are by no means certain.

This is not an issue of which HS2 Ltd is unaware. In paragraph 6.3.3 on page 50 of Appendix 5 of HS2 London to the West Midlands: Appraisal of Sustainability HS2 Ltd provides an excellent summary of the issue:

“To mitigate potential impacts in areas of high operating speeds, there is a need to control aerodynamic noise through advanced rolling stock design. Without first mitigating the source of aerodynamic noise, wayside noise barriers are not likely be as effective or feasible, due to the required increase in barrier height, to provide shielding to the entire train.”

The problem with this opening gambit is that there is no discussion in the Appraisal of Sustainability (AoS) about how aerodynamic noise might be controlled at source, or what the likelihood is that such control is feasible. Instead, the AoS appears to take the ability to realise such control for granted, and assumes the use of “3m high noise barriers (or bund) at all the preliminary candidate areas for mitigation or, at viaducts, by 2m high barriers” (see paragraph 6.3.5 on page 50 of Appendix 5). Since such low barriers will not “provide shielding to the entire train” they will offer no mitigation to aerodynamic noise from sources high on the train.

The apparent state of denial exhibited by the author(s) of the AoS is also carried through into the HS2 Ltd document Review of HS2 London to West Midlands Appraisal of Sustainability (here). Paragraph 7.2.5 on page 27 comments on the responses to the public consultation that mention this issue:

“The issue of the assessment of noise from the train pantograph was also raised, in particular with reference to the height of the source compared with noise barriers. The pantograph is a consideration in the assessment of the noise impact and the AoS took this into account by making appropriate changes to the relative height of train source and barrier when considering mitigation. While noise from the pantograph does need to be considered, and will be reviewed at the time of the EIA, its significance is often overstated. The wheel-rail interface will remain the most significant part of the noise from the train, even at high speed.”

This promise to review this issue as a part of the environmental impact assessment (EIA) work is also acknowledged in paragraph 14.3.20 on page 150 of HS2 London to West Midlands EIA Scope and Methodology Report (here).

“The airborne sound generated by rail operations associated with the Proposed Scheme, both mainlines and connecting chords, and classic lines will be calculated using the calculation method developed and validated initially for the environmental assessment, and then the design, of HS1. The method is empirical, developed from over a thousand measurements. The method has been further tested and verified since HS1; and calculates maximum sound levels for each train, as well as equivalent continuous sound levels. The method has been further refined for the Proposed Scheme to allow for aerodynamic sound sources at speeds over 300 kph.”

This, in my view, is a wholly inadequate statement of intent. HS2 Ltd remains silent about just how the calculation method has been “further refined … to allow for aerodynamic sound sources at speeds over 300 kph” or what it is proposing to do about noise barrier heights. In short, this paragraph gives no assurance whatsoever that this problem is being taken seriously by HS2 Ltd.

This impression has been further reinforced by a presentation that we were given at my local community forum where we were told by the HS2 Ltd noise expert that our worries about the impacts of aerodynamic noise were due to “misunderstandings”, by which I presume that he meant ours, not his.

Personally, I prefer to believe the expert acousticians at Southdowns Environmental Consultants Ltd, who said in the response that they wrote to the public consultation on behalf of the 51m group:

“The contribution of aerodynamic noise to overall train noise levels at higher speeds requires further and more detailed consideration …” (paragraph 6.1.6 on page 23)

And:

“The noise criteria and assumptions incorporated into the HS2 noise model represent a significant project risk in the event that significant noise effects have been underestimated and engineering alignment options are constrained to the published alignment. Options for additional mitigation will then be limited to the installation of higher noise barriers at 4- 5m height or more above ground and/or long term speed restrictions.” (paragraph 6.1.10 on page 24)

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