Trust me, I’m an acoustic engineer

Measuring noise, part 9

(For the reason that is explained in Adding to the confusion, which was posted on 1 Nov, the bottom threshold for the yellow dot range should be 65 dBA, not 68 dBA as is stated below. Accordingly the range covered by the grey dot classification is reduced by 3 dB to 15 dB.)

In this blog I am going to examine the methodology that HS2 Ltd has used to produce some figures for the Appraisal of Sustainability for the number of properties that will be affected by noise from HS2.

Before I get my teeth into that topic however, I want to point out the potential for one further shortcoming in the approach that HS2 is taking to noise predictions. There is an interesting admission in paragraphs 5.6.7 to 5.6.11 of Appendix 5.4 to the Appraisal of Sustainability (available here). In these paragraphs the noise modelling methodology employed by HS2 Ltd is identified as Calculation of Railway Noise 1995 (CRN). This is stated to be the official model for assessing eligibility for sound insulation under England and Wales Noise Insulation Regulations for Railways and the model typically used for the environmental impact assessment of railway projects.

This would be very reassuring, except that it is admitted that the model would need to be adapted for speeds above 300 kph, but that this has not been done because “the research basis for this change in calculation methodology is not currently available.” Instead HS2 Ltd has “bodged” the model to achieve results that are consistent when compared with SNCF (French National Railways) data for speeds in excess of 300 kph.

This is a rather worrying admission since, as I explained in my blog of 17 May, aerodynamic noise (which dominates above 300 kph) is a rather different animal from the mechanical noise associated with conventional trains.

So we have another complaint about the way HS2 Ltd is tackling noise predictions, to add to a growing list; but perhaps we should return to the main subject of this blog, which is how HS2 Ltd has identified dwellings that will be adversely affected by noise from HS2.

In order to do this HS2 Ltd has defined three noise pollution threshold levels, using the “average level” parameter dBA Leq, 18 hours (which I will abbreviate to dBA for the rest of this blog).

  • The highest threshold is 73 dBA and dwellings that HS2 Ltd predict will suffer this, or a higher level, of noise pollution have been marked by red dots on the Residential Airborne Noise Appraisal Maps in section 3.5 of Volume 2 of the Appraisal of Sustainability (available here). This threshold has been derived from Department for Environment, Food and Rural Affairs (Defra) work on mapping noise for aircraft, road, rail and industrial noise in England (Defra Noise Mapping). Defra mark places that experience noise pollution of this magnitude as “Important Areas”; clearly Defra regards noise pollution that is this high with some concern. HS2 Ltd terms this, with its usually tendency for understatement, as “high average noise levels”.
  • The next lowest threshold is 68 dBA and dwellings that are at or above this level, but below the highest threshold level, have been marked by yellow dots on the maps. This threshold has been taken from the Noise Insulation (Railways and Other Guided Transport Systems) Regulations 1996. It is the minimum noise level at which dwellings qualify for noise insulation.
  • The lowest threshold is 50 dBA and dwellings that are at or above this level, but below the middle threshold level have been marked by grey dots on the maps. This threshold is defined as where dwellings exposed to HS2 noise “could have a noticeable (although not necessarily significant) increase” in existing noise levels.

The maps have been drawn in two versions. The first version assumes that no additional noise mitigation has been employed and the second assumes “additional indicative mitigation” has been employed, although there is no guarantee of where HS2 Ltd will fit noise mitigation in its final design.

My reaction to all of this is that red dot locations will be absolute hell, yellow dot locations will only be marginally better and grey dots cover an extremely wide noise range of up to 18 dB. The difference between a grey dot location at the lowest level and one at the highest will be nearly four times as loud.

The postulated impact on dwellings along the whole of the London to Birmingham route has been summarised by HS2 Ltd in Table 4 on page 99 of Volume 1 of the Appraisal of Sustainability Main Report(available here). This table reveals that there are approximately 10 red-dotted dwellings, but that this would rise to about 70 is no noise mitigation was employed. Also approximately 150 dwellings qualify for a yellow dot (rising to about 1,400 without mitigation) and 4,700 dwellings are rated grey dot (rising to about 24.300 without mitigation).

I think that we are meant to be reassured by these figures; personally I would prefer to see a proper noise contour map rather than this “dotty” substitute. What the figures do illustrate, however, is how important effective noise mitigation will be and I expressed doubts about just how effective the proposals in the Appraisal of Sustainability will be in my blog of 17 May.


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