Not a precise science, part 2

(… continued from Not a precise science, part 1, posted on 5 Oct 2015).

It is hardly surprising that the propagation modelling carried out by HS2 Ltd does not produce noise level predictions which are, in the words of acoustician Graham Parry, “absolute certainties”; there are so many variables that have to be taken into account. These variables divide themselves into two classes, which can be conveniently characterised by terms coined by the former US Secretary of Defense, Donald Rumsfeld (see footnote 1).

The easiest variables to model are the “known knowns”. These are effectively constants that define the path between the noise source and the receptor location, such as the distance between the two, the terrain profile and any surface features such as buildings and other structures or natural surface elements (including mitigation features) that might screen or reflect noise. The essential nature of these features can be captured in data sets and a computer model used to predict propagation along that path. The mechanisms involved are well understood and there is a wealth of experience in developing and employing such models, so they tend to accurately reflect the effect of these known knowns.

There are also known knowns that are variables, such as the speed of the train pass-by, the length of the train and, where the equivalent continuous sound level is being computed, the frequency of train pass-bys. These can be taken into account as user-specified inputs into the programme.

Far more problematic for the developers and users of propagation models is a whole host of “known unknowns”. These are all variables where the values for any given source to receptor path may vary over time, or be different for separate train pass-bys.

In his evidence to the Select Committee, Graham Parry singled out one particular area where known unknowns are important, which he identified as “differences in trains”. He explained (see footnote 2):

“If they are all engineered and maintained exactly the same, particularly if the wheels and rails are properly maintained, one would expect that spread of data to come in, so we would have a much tighter spread and more certainty about predictions, but to date none of the data that has come forward shows that you do not get that spread.”

What you might term “meteorological conditions” are also important unknown factors. These include wind strength and direction, atmospheric temperature gradients and the proportion of moisture in the air and the ground.

When faced with known unknowns it is necessary to make an estimate of the likely value, which takes account of the possible excursions from typical values. Good practice is to adopt a precautionary approach and to consider the effects of reasonably worst-case conditions. The evidence of the Environmental Statement is that HS2 Ltd has not been entirely consistent in this; whilst a degree of precaution appears to have been applied in accounting for meteorological variations, the approach towards train noise performance could be assessed as somewhat optimistic (see footnote 3).

One particular meteorological variable that has cropped up time and time again in evidence given by petitioners to the Select Committee is wind strength and direction; people are acutely aware, in the light of their own experience, just how much louder sound from distant sources can be when the observer is downwind. The ES concurs that “wind speed and direction … affect sound propagation” (see footnote 4).

The ES asserts that, for wind direction at least, a precautionary approach has been taken in developing the propagation model (see footnote 5):

“For the purpose of developing the prediction methods, to err generally towards a worst case, only sound level data for which the receiver was downwind of the source was used. This means that the method is representative of downwind conditions (i.e. forecasting high noise levels at distance from the route).”

Mr Parry referred to this as “positive wind” (see footnote 6) and some empirical data is provided in the ES as evidence that a positive wind approach has indeed been taken, which should provide reassurance for those petitioners who live downwind of the HS2 trackline when the prevailing wind direction is considered. I will review this evidence in part 3 of this series.

(To be continued …)


  1. Mr Rumsfeld famously said, when responding to a question in 2002 about there being a lack of evidence for Iraq possessing weapons of mass destruction:

“Reports that say that something hasn’t happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – the ones we don’t know we don’t know. And if one looks throughout the history of our country and other free countries, it is the latter category that tend to be the difficult ones.”

  1. See paragraph 238 in the transcript of the afternoon session of the HS2 Select Committee that was held on Monday 7th September 2015.
  2. I will examine some aspects of HS2 Ltd’s treatment of train noise source levels in part 4 of this blog series.
  3. See paragraph 1.3.21 in Annex D2 to Appendix SV-001-000 of the ES.
  4. See paragraph 1.3.22 in Annex D2 to Appendix SV-001-000 of the ES.
  5. See paragraph 238 in the transcript of the afternoon session of the HS2 Select Committee that was held on Monday 7th September 2015.

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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