Originally published on HS2 and the Environment:
(… continued from Facing a brick wall, part 1, posted on 27 Sep 2015).
In part 1 I mentioned that I had raised the issue of the mix of free-field and façade sound level measurement methodologies in the HS2 Ltd assessments when I found myself presenting my petition to the HS2 Select Committee early on this year. It was a fairly chaotic session; I was thrown into some disarray, due the intervention of one Member of the Committee, and I must confess that I am still smarting from the treatment that I received (see footnote 1). As a result of the tactics that were employed – yes I feel that “tactics” is an appropriate description –I fear that I may not have presented my evidence in the most convincing way. I was also faced with a fairly relentless refusal by the Promoter’s expert witness, Rupert Thornely-Taylor, to concede anything that might possibly be to the disadvantage of his client; I really felt that it was me that had been placed in front of a brick wall, rather than the sound level meter, and that the brick wall wasn’t the only façade that was being erected!
Mr Thornely-Taylor’s response to my query about the free-field/façade confusion deserves reproducing in full in order that you may fully admire the sheer effrontery of his defence of the HS2 Ltd position (see footnote 2):
“The Lmax is to be determined at the façade, which means there has to be a numerical correction between the general ES approach predicting free field noise levels, but façade corrections are dependent on the façade, whether it is facing the source or away from the source. You cannot plot contours of façade LAmax with precision. You have to plot free field contours and I think that would cause more confusion than assistance because of the need for the inexpert reader to understand about façade corrections and you do see all the LA maxes in the detailed ES volumes for each assessment location.”
Overall, the excuse appears to be that finding a satisfactory solution to the current muddle is filed in the “too hard to do” box. On the positive side, his first sentence does appear to concur with much that I said in part 1, and we also both agree that you cannot determine the façade level “with precision”. Notwithstanding, there is a generally-agreed method of deriving a representative façade level from the free-field measurement – the increasing of the level by a fixed number of decibels, which I referred to in part 1 by reference to PPG24 – but he makes no acknowledgement of that approach. In making his references to the plotting of “contours of façade LAmax” and “free field contours [of LAmax]” he seems to have forgotten that HS2 Ltd has steadfastly refused to provide any plots of LpAFMax whatsoever, whether façade or free-field, and his concern that clearing up the matter “would cause more confusion than assistance” is barely credible; what could be more confusing than the situation that we have at present, which appears to have tripped up even one of the expert witnesses that we have heard (see footnote 3).
As I pointed out to the Committee (see footnote 4), Mr Thornely-Taylor’s position that façade corrections are too confusing for the likes of us appears to be significantly undermined by the ES, which describes how a free-field value of baseline noise was determined for the ES Volume 5 tables when the measurement location did not allow a free-field measurement to be made directly. The ES explains:
“Where baseline data has been obtained from measurement positions that are not free‐field (taken to be within 3.5m from all reflecting surfaces other than the ground), measurements have been undertaken at a distance of 1m from the reflecting surface and a correction of ‐1.5dB for LpAeq,Tand ‐2.5dB for LpAFmax applied to the data where railway noise is the dominant source, or a correction of ‐3dB applied where all other sources are dominant.”
Applying the inverse of this process, it is clear that the free-field peak level predictions of operational airborne noise that are listed in the tables in Appendix 5 to the ES may be “converted” to façade levels by simply adding 2.5dB, and it is these increased levels that should be compared with the LOAEL threshold of 60dB LpAFMax. This is clearly important as it means that the peak noise predictions, with respect to LOAEL, are 2.5dB worse than might be first thought.
After a bit of a verbal tussle with Mr Thornely-Taylor, during which I expect that we managed to lose most, if not all, Members of the Select Committee, he finally conceded that the façade levels would be higher than the free-field figures reproduced in the ES (see footnote 5).
So what might this mean? How, for example, does it affect the plot of train maximum pass-by contours that acoustician Steve Summers prepared and presented in his evidence to the Select Committee and that I reproduce on the original article? [see Peter’s blog for the map]
As I mention in footnote 3, it is my contention that Mr Summers constructed this plot using free-field levels. If I am right about this, then 2.5dB should be added to every value if we want the 60dB contour to coincide with LOAEL. Doing this will shift the green line further to the left, meaning that many more properties in Wendover will fall the wrong side of the LOAEL contour. The 70dB contour that Mr Summers has shown in red will also shift to the left, encroaching further into the town.
So please bear this in mind when giving evidence to the Select Committee. Ensure that you quote façade levels if you are comparing maximum train pass-by levels with the LOAEL threshold and make sure that the Promoter does the same.
Footnotes:
- The hearing for my individual petition was held on the morning of Wednesday 21stJanuary 2015, and I am first on in the video of that session. To get a feel for the way that I was “invited” to present my case refer to paragraph 5 and paragraph 9 to paragraph 36 of thetranscript.
- See paragraph 94 in the transcript of the morning session of the HS2 Select Committee held on Wednesday 21stJanuary 2015.
- Steve Summers appeared to be quoting free-field levels and made no comment about the significance of the LOAEL threshold being specified as a facade level when he gave his evidence to the HS2 Select Committee on the afternoon of Tuesday 14thJuly 2015.
- See paragraph 110 in the transcript of the morning session of the HS2 Select Committee held on Wednesday 21st January 2015. The reference to the ES is paragraph 1.3.8 in SV-001-000Annex B (page 3).
- See paragraphs 152 to 156 in the transcript of the morning session of the HS2 Select Committee held on Wednesday 21st January 2015.