That’s a bit of an understatement

Environmental aspects of the 51m consultation response, part 14

In this blog I will complete my re-examination of the criteria used in the AoS to assess the number of dwellings at risk from noise annoyance from HS2, concentrating on the lowest or grey dot rating, described by HS2 Ltd as “dwellings potentially exposed to a noticeable noise increase”.

When I wrote Trust me, I’m an acoustic engineer (posted 4 Jul) and It’s on the level (posted 8 Jul) I deliberately simplified the way that the grey dot threshold is specified, in order to avoid overcomplicating what I wanted to say. However in order to appreciate the comments made by Southdowns in Appendix 18 to the 51m alliance response to the public consultation (available here), we need to revisit the definition of this threshold.

In search of what might reasonably be termed a “noticeable noise increase” Appendix 5 of the AoS (available here) draws upon two criteria. The first of these is paragraph 6.1.5 on page 49, which states:

“In terms of a railway noise change, 3 dB LAeq or more is generally considered as a noticeable change. For the AoS study, this has been taken as the difference in railway noise, with and without the presence of HS2.”

The second is in paragraph 6.1.6 on page 49:

“The World Health Organisation, in its 1999 Noise Guidelines report in 2000 on [sic] states ‘to protect the majority of people from being moderately annoyed during the daytime, the outdoor sound level should not exceed 50 dB LAeq’.”

Combining these two, the threshold is defined in paragraph 6.1.7 on page 49 of Appendix 5 to the AoS as follows:

“This been taken as an indicator of the onset of annoyance and, therefore, a Noticeable Noise Increase for HS2 AoS purposes is defined as having a total rail noise level of greater than or equal to 50 dB LAeq 06:00 – 24:00 with an increase in rail noise of at least 3 dB LAeq 06:00 – 24:00. At receiver locations where predicted existing rail noise levels are low or there is no rail traffic (assumed at 45 dB LAeq,18hr), a predicted HS2 noise level of 50 dB LAeq,18hr or above would result in a noticeable noise increase as per this definition.”

So what this means is, if the background noise level is, or is assumed to be, below 47 dBA then the grey dot range starts at 50 dBA. However, if the existing background noise is above 47 dBA, then the start threshold is increased by 1 dB for every decibel that the background noise exceeds 47 dBA. Perhaps you will see now why I simplified things back in July!

There is one further thing to take into account when revisiting what I wrote in July, which is that the upper limit of the grey range is 65 dBA, not 68 dBA as I assumed in the earlier blogs. This is because of the mixed use of “free field noise level” and “façade noise level” that I explained in my blog Adding to the confusion (posted 1 Nov).

In Trust me, I’m an acoustic engineer and It’s on the level my chief concern was the large range of noise levels encompassed by the grey dot classification. As you will read below, Southdowns shares this concern, but its report also takes issue with the application of the 50 dBA lower threshold in locations where the “predicted existing rail noise levels are low”. There are of course many such locations along the proposed HS2 route, as much of it passes through previously undeveloped countryside.

The Southdowns report points out that a “noticeable noise increase” of 3 dB on the assumed baseline noise level of 45 dBA at these “quiet” locations should dictate that the lower threshold for these locations be 48 dBA, not 50 dBA. Now this may sound like quibbling over a few insignificant decibels, but as I showed in It used to be really quiet ‘round here (posted 12 Oct) a couple of decibels change in the assumed noise levels can make a really significant difference to the estimate of the number of dwellings affected by noise.

The Southdowns report acknowledges that the 50 dBA cut-off used by HS2 Ltd is based upon the guideline values in chapter 4 of the World Health Organisation (WHO) Guidelines for Community Noise (available here), but contends that “The application of this lower cut-off level is at odds, however, with the stated intent of the WHO criterion level to protect the majority of people from moderate annoyance”.

The Southdowns report also refers to the DfT’s own guidance in the WebTAG Noise Sub-Objective (available here). Paragraph 1.4.8 states that a level of 45dB LAeq,18hr should be used “as the cut-off for both annoyance and valuation calculations”.

In addition, the Southdowns report is not totally happy that a 3 dB change is an appropriate measure of a “noticeable noise increase”:

“HS2 justification for the 3 dB minimal noise change is based on the argument that this is the just perceptible change in total noise over an assessment period. Such argument is understood to be based on the findings of cross sectional studies, where long term community response to noise from established sources has been studied. The findings of longitudinal studies where community response to step changes in traffic noise have been studied show, however, that significant adverse response may be associated with changes in noise exposure at below 3 dB.”

In support of this criticism, the Southdowns report refers to the draft guidelines for noise impact assessment drawn up by the Institute of Environmental Management and Assessment (IEMA) and the Institute of Acoustics (IOA) (available here). A table in paragraph 7.66 of these guidelines describes a noise change of between 0.1 and 2.9 dB as having a “slight impact”. Similarly an increase between 3.0 and 4.9 dB is described as of “moderate impact”, between 5.0 and 9.9 dB as having “substantial impact”, and over 10 dB as having a “severe impact”. However the draft guidelines also caution that these terms are only given as an example of how a noise change “might be categorised” and should not be used “to define the description of the noise change”.

Personally I think that Southdowns hasn’t really made a strong case on this one point; but then that is only my own personal view as an amateur and they are the experts.

However, the Southdowns report goes on to use the descriptions in the IEMA/IOA table to criticize, as I have done, the huge range encompassed by the grey dot classification:

“The HS2 noise appraisal criteria for Noticeable Noise Increase include a minimum noise change of 3 dB in ambient noise levels with and without the scheme but otherwise do not distinguish between the range of noise impacts that will be encountered along the proposed route. A predicted free-field train noise level of 64 dB LAeq,18hr would, for example, represent a noise change of 19 dB at receptors where the pre-existing ambient noise level is 45 dB LAeq,18h and hence be described as a severe noise impact using the above or similar scale. The magnitude of noise impact should be a key consideration in any consideration of the requirements for additional noise mitigation and this omission to distinguish between Noticeable Noise Increases between 3 and 19 dB or more represents a significant omission in the AoS approach to noise appraisal.”

The Southdowns 19 dB noise change is, in fact, 9 dB above the threshold of change that the IEMA/IOA deems as having a “severe impact” and, as I pointed out in It’s on the level, the top of this range is nearly four times as loud to the human ear as the bottom. Both Southdowns and I agree that this is far too great a span to be covered by a single noise nuisance classification as HS2 Ltd has done.


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