Jumping to conclusions

As I was studying the data for noise receptor ID213490 in table 1 in Report SV-002-017 in Volume 5 of the Environmental Statement (ES), a couple of lines from a song that was around in my student days were running through my head; it’s funny how you remember these things. These lines concerned the dangers of jumping to conclusions – refer to the acknowledgements at the foot of this blog for details – and I couldn’t help wondering if the ES has been guilty of doing this in making an assessment of the “existing baseline sound level” for noise receptor ID213490.

The particular example of possible injudiciousness to which I refer is whether the ES is justified in assuming that the soundscapes of the measurement location used to derive the source data and the receptor location where that source data has been utilised to derive levels of existing baseline noise are sufficiently similar to make that process tenable. In order to help further consideration of this matter, I have reintroduced below the map that I first reproduced in my blog Putting us on the map (posted 28 Feb 2014).

Noise prediction and measurement locations in east Cubbington (Source: HS2 Ltd)

Noise prediction and measurement locations in east Cubbington (Source: HS2 Ltd)

As I explained in my blogs Putting us on the map and This might lead to annoyance (posted 4 Mar 2014), the noise receptor ID213490 is indicated by the blue letter “A” on the above map, and the measurement location by the red letter “B”. Both of these locations have one similarity; they are adjacent to the B4453, which is the main existing source of noise.

However, there are also, I believe, important differences in the soundscapes of the two locations. The receptor is in a built-up area with only the odd tree, where there are no obvious sources of noise other than the road. The measurement location is within South Cubbington Wood – the wind rustling leaves on trees can be a significant source of sound – and there is a commercial wood yard opposite – a likely source of industrial noise. Possibly even more significant is that the measurement location is on a 50mph stretch of road, whereas the receptor is within a 30mph section; as far as I can see no correction has been made for a reduction in speed of the traffic generating the noise.

That no account has been taken of these putative differences in soundscape is evident if the existing baseline sound levels for the receptor indicated on the above map by the purple letter “D” are considered. This receptor has been allocated the reference ID234564; it is about the same distance from the B4453 as ID213490 and has been assigned the same data source coding string in table 1 in Report SV-002-017 (“1,C,i,c”). Despite the possible soundscape differences of the two receptor locations however, the values of day and night existing baseline sound levels quoted in table 1 are exactly the same for both locations.

Further data are available that might be more relevant to ID213490, but this has not been presented in the ES, as far as I can see, in a way that helps the assessment of the existing baseline sound levels at that receptor. This data was gathered at the measuring location that is identified by green letter “C” on the above map, and has the identity code CN111S. This site is described in the ES as a “short-term attended satellite monitoring position, where monitoring has been undertaken simultaneously with that at a long-term monitoring location” (refer to paragraph 1.3.15 in ES Volume 5 Appendix SV-001-000).

The ES does not tell us which long-term monitoring site the short-term site was linked to, but if it was the one at the red letter “B” (CN028L) then this could have provided a useful check on the appropriateness of using the data from that long-term site for the receptor at blue letter “A”. Now it is the case that the short-term monitoring site is further from the centreline of the B4453 than the long-term one, but a correction for this could easily have been made. Whatever, the ES remains silent on whether this check was carried out and what the results were if it was.

There is one further operation that has been carried out on the day and night LAeq levels reported in SV-002-017 before transferring them to the “Do nothing (Opening year baseline)” columns table 1 in SV-004-017. This is to add the correction factors given in table 3 in SV-002-017 that take account of the extra noise arising from predicted increased traffic on the B4453 between 2012/132 and the planned HS2 opening year of 2026. For the day level this predicted increase is 0.9dB, making the “do nothing” figure 59.7dB (58.8+0.9) or 59dB rounded down. For the night level the figure is 47.0dB (46.7+0.3).

There is one further main column heading in table 1 in SV-004-017, which is “Do something (Opening year baseline + Year 15 traffic)”. As the heading implies, the figures in the two columns to which it applies are simply the addition of the “Proposed Scheme only (Year 15 traffic)” and the “Do nothing (Opening year baseline)” levels. Of course, if you have read my blog dB or not dB, that is the question (posted 6 Jun 2011), you will realise that adding two sound powers expressed in dBs is not a straightforward matter, and involves converting back to linear quantities before making the addition and reconverting to dBs. Fortunately there is a really simple calculator tool available on the web that does this for you.

For receptor ID213490 the two decibel power estimates for the day are 50dB and 59dB, and the calculator gives the answer 59dB (rounded down) when these are added, which is the figure that has been entered in the relevant “Do something (Opening year baseline + Year 15 traffic)” column in table 1. The two night figures are 43dB and 47dB, yielding an answer from the calculator of 48dB, whereas the level that has been entered in table 1 is 47dB. I can think of two reasons that may account for this small discrepancy, examples of which can also be found in other entries in table 1:

  • Rounding error, as the predicted levels from HS2 are only presented in the ES as whole numbers.
  • A note in paragraph 4.3.4 of Report SV-004-017 that where HS2 has the effect of “modifying an existing source”, e.g. by realigning a road, the “do something” predicted level “has been corrected so as to not double count the sound associated with the road or railway on its new and existing alignment”.

Finally we need to look at the two columns sharing the heading “Change”, which are really the whole point of all the calculations that I have reviewed so far. The first column records the dB change between the day noise level predicted with HS2 operational (“do something”) and the prediction if HS2 is not built (“do nothing”). In the case of receptor ID213490 the two figures are the same at 59dB, and so the change in noise due to HS2 is, according to the ES, 0dB. So, despite the day noise from HS2 being above the 50dB threshold set by HS2 Ltd, according to the ES HS2 has “no significant effect” at receptor ID213490, since the train noise is effectively swamped by the noise from the B4453. Consideration of the night noise yields a similar result.

This example underlines why making a correct assessment of the existing noise levels is critical in assessing the impact that HS2 will have, and why I have examined this topic in considerable depth.

I suggest that you check out what the ES predicts for your own local receptors. For my part, I am going to contact the environmental health officer at my local council to seek a second opinion on whether what HS2 Ltd has done is reasonable.

Acknowledgements: The song that was “running through my head” is My Enemies Have Sweet Voices by Al Stewart and Peter Morgan; it is on Al’s Zero She Flies album. The reference to conclusion jumping is in the third verse.

The Ordinance Survey mapping upon which the noise contours, route design and receptor locations are overlaid has been reproduced in accordance with the principles of fair dealing as set out in the Copyright, Designs and Patents Act 1988.  On this basis, this mapping is:

Reproduced by permission of Ordnance Survey on behalf of HMSO.

© Crown Copyright. All rights reserved.


One response to this post.

  1. Posted by chriseaglen on March 8, 2014 at 8:18 am

    the levels are well below those likely in practice. But annoyance is not only about levels. The cock crowing, the crows, the cows at feed times compared with the jet taking off. Modulations impact disturbances. Salford and npl can both understand from wind turbines modulations. The work is helpful. Hs2 is not on the science mission but the distortion mission. It is possible the people were not all experienced acoustic engineers but environmental consultants. Ask the nao to also validate the process please. Best now help the average petitioner make their point in the next 10 weeks please.


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

%d bloggers like this: