A very noisy bird, part 3

(… continued from A very noisy bird, part 2, posted on 26 Aug 2015).

In part 2 I promised to explain why acoustics expert Steve Summers had been wrong to claim that there was an error in exhibit P7494(2) during his cross-examination by Tim Mould QC, Lead Counsel for the Promoter (see footnote 1). This exhibit includes a bar chart which displays two sets of data: the results of some peak noise measurements taken in the interior of St Mary’s Church Wendover, indicated by red-coloured bars; and, the results to be expected if a similar exercise is carried out once HS2 is running, shown by green bars. So it is essentially two bar charts in one, and it will be easier to consider the red bars alone initially.

Tim Mould told the HS2 Select Committee that measurements had been taken inside the church “over a series of five-minute intervals during an evening” (see footnote 2). That it is standard practice for HS2 Ltd to set the sound level meter to record in five-minute intervals is confirmed by paragraph 1.3.14 of Annex B to Appendix SV-001-000 to Volume 5 of the HS2 Phase 1 Environmental Statement. This paragraph also confirms that the sound level meter logs the highest value of LpAF,max detected during each five-minute interval.

The exhibit provides additional explanation of how the data has been captured by labelling the y-axis of the bar chart “Number of events during 7 day survey in evening period (19:00-23:00)”. So we know that on each of these evenings the sound meter was running for four hours and so would have logged 48 five-minute measurement intervals. Useful confirmation that this is along the right lines may be gained from totalling the individual lengths of the red bars displayed on the chart, which comes to 48 events.

Of course, the measurements were taken over seven evenings, not just one. What I assume HS2 Ltd has done is to take the average of the number of events falling in each bin of the bar chart over the seven nights, which appears to be a sensible approach. So what is displayed by the red bars is a typical evening, based upon seven evenings’ measurements.

The bins are labelled with integer numbers representing the highest value of LpAF,max logged during each measurement interval. No information has been provided on the range of values set for each bin, e.g. 45 dB might mean within the range 44.5 to 45.5 dB, but this is not really essential information for current purposes.

So far so good, but now we have to consider the green bars that show the effect of adding noise that is predicted to result from HS2 train pass-bys to the sound environment inside the church.

If we take worst case, then HS2 trains will cause 36 short bursts of noise to be heard inside the church every hour (18 on the up line and 18 on the down line). There will be fewer bursts, perhaps a little over 20, when only Phase 1 is operational, and the number of pass-bys will tail off leading up to midnight. However, I think that it is a fair assumption that at least one HS2 noise burst will occur within each five-minute measurement interval over the four-hour evening period used for the chart.

Something that I should point out here is that only one such burst is required to be logged in each measurement interval for maximum effect; a second, or further subsequent, burst will not change the data logged by the sound level meter, as it is merely duplication. So all you actually need to see the maximum change to the bar chart is 12 equally-spaced pass-bys per hour (6 each way), or one per interval; any increase in traffic above this level will have no impact on the bar chart. This is surely a shortcoming of this measurement method.

Mr Summers estimated that the HS2 noise bursts would give rise to levels inside the church “to be in excess of 40 dB, probably up to 46 dB LA max” (see footnote 3). True to form HS2 Ltd has taken the level to be 40 dB, no more; well, they would say that, wouldn’t they.

So what is the effect of adding at least one noise burst of 40 dB LpAF,max to each of the red bars? Let’s construct the green bar chart.

If the red bar refers to a level of 41 dB or more, then the LpAF,max of the ambient sound event is greater than the HS2 sound event, and so it is the ambient sound event level that will be logged for the time interval. This means that all red bars to the right of the 40 dB bar will be unchanged by the introduction of the HS2 noise, and so the green bars will each be the same height as the red bars.

If the red bar refers to a level of less than 40 dB, then the LpAF,max of the HS2 sound event is greater than the ambient sound event, and so it is the HS2 sound event level that will be logged for the time interval. This means that all of the red bars to the left of the 40 dB bar will have no green bar counterparts, and the green bar for 40 dB will be equal in height to the sum of all of the red bars to the left of the 40 dB bar plus the height of the 40 dB bar. This total is 18, which purely by coincidence is the same number as the maximum one-way trains per hour on this section of the HS2 route; this accounts for Mr Summers’ confusion.

Having confirmed that the green bar chart has been drawn correctly, we are left with the question of whether it tells us very much about the impact that HS2 trains will have upon those taking part in events inside St Mary’s Church. Clearly, the sections of the two bar charts to the left of the 41 dB bar are very different; this accounts for 37 per cent of the time intervals and so the change is plainly significant. But, as I have already mentioned, the diagram is insensitive to the effects of increasing the frequency of noise bursts from HS2, and this is obviously an important factor in assessing the impact that HS2 would have. If we also take into account the “noisy bird” shortcomings of sound level meter measurements that I pointed out in part 2, then I don’t think that P7494(2) is particularly enlightening.

I don’t feel that Mr Mould’s defence, if a defence is what it was intended to be, that “the key change is going to be the frequency of existing events, which will be towards the lower end of the mid range of the noise levels that already form the existing and separate noise event picture within [the] church” (see footnote 4) is very robust. After all he also told the Select Committee that “we’re going to continue to work closely with the church and others to seek to mitigate at source and at receptor on this” (see footnote 5).

If HS2 Ltd is prepared to consider mitigation “at source and receptor” then you’d better believe that there is a problem, even if the HS2 noise will not “increase the peak noise experience” of audiences trying to concentrate on music being performed in the church.

Footnotes:

  1. Mr Mould’s cross-examination of Mr Summers took place during the public session held by the HS2 Select Committee on the afternoon of Tuesday 14thJuly 2015. The relevant passage is recorded from paragraph 190 of the transcript and may be viewed from 14:58 hrs in the video of the session.
  2. See paragraph 192 in the transcript of the afternoon session of the HS2 Select Committee held on Tuesday 14thJuly 2015.
  3. See paragraph 168 in the transcript of the afternoon session of the HS2 Select Committee held on Tuesday 14thJuly 2015.
  4. See paragraph 194 in the transcript of the afternoon session of the HS2 Select Committee held on Tuesday 14thJuly 2015.
  5. See paragraph 198 in the transcript of the afternoon session of the HS2 Select Committee held on Tuesday 14thJuly 2015.

Important Note: The record of the proceedings of the HS2 Select Committee from which the quotes reproduced in this blog are 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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