So that’s why you ignored it

In A step in the right direction (posted 7 May 2012) I promised that I would look at the impact that adopting the World Heath Organisation (WHO) night noise guideline (NNG) might have on the estimated count of the number of properties affected by HS2 noise; so how might we do this?

Well the first thing to take into account is that, although HS2 trains operating at night will be no quieter than those running during the day, the “equivalent continuous sound level” method employed by HS2 Ltd to calculate noise will yield a lower sound level for the eight night hours between 23:00 and 07:00 than for the sixteen day hours between 07:00 and 23:00. This is because, whilst the contribution to the total noise power over the time period used to calculate the equivalent continuous sound level is the same for each train pass-by (day or night), there will simply be fewer train movements at night.

In my blog Silent night? (posted 3 Dec 2011) I showed how the difference in the day and night time equivalent continuous sound levels could be calculated based upon the train movement data in Table 1 on page 45 of Appendix 5.4 to the Appraisal of Sustainability (here). In that blog I came up with the answer that the night equivalent continuous sound level would be about 5 dB(A) lower than the daytime equivalent.

Now there are two caveats that I should apply to this result. The first is that train timetabling for HS2 is, as might be expected at this stage of the project, a bit of a wet finger in the air activity, and so the data in the AoS table is not definitive. The second is that the goal posts for the daytime equivalent continuous sound level have been moved slightly; the period employed for the AoS was 06:00 to 24:00 and it is now proposed for the EIA that the period 07:00 to 23:00 is used to be consistent with the adoption of a 23:00 to 07:00 night-time period. Both of these caveats can affect the calculation, but I would expect any differences to be small and am confident that we can disregard them for the purposes of this blog.

This calculation allows us to draw a significant inference. At a location where the night noise is just at the NNG threshold of 40 dB(A), the equivalent daytime noise level will be (40+5) or 45 dB(A). Now this is a significant 5 dB(A) below the “grey dot” daytime threshold of 50 dB(A) that was used to calculate the “estimated number of dwellings potentially impacted by operational noise” given in Table 3 and Table 4 on pages 51 and 52 of Appendix 5.4 of the AoS; this estimate is approximately 24,300 dwellings without the Y extensions and 33,600 with the Y extensions (both without additional mitigation). So I think that we can expect this count of dwellings to go up somewhat, due to locations that exceed the NNG but did not exceed the day threshold employed for the AoS.

It is difficult to estimate just how much the count of affected dwellings will increase due to the inclusion of those above the night-time threshold only; we will have to wait for HS2 Ltd to do the detailed calculations. We can get some idea from the observation made by Southdowns Environmental Consultants Ltd in paragraph 2.3.2 in Appendix 18 to the 51m alliance response to the public consultation (here) that a 1.25 dB(A) difference (due to inclusion of the Y extension traffic) is all that was responsible for the increase in the count of affected dwellings (without mitigation) from approximately 24,300 to approximately 33,600 (around a 40% enlargement).

Now we have to be careful about drawing a direct comparison, because (as I stated in A step in the right direction) the night threshold will probably have most impact in rural areas where property densities are low. There have also been changes to the route design since the AoS was issued. However, it looks likely that the tally of affected properties could go up significantly (perhaps even doubling) when night noise impacts are taken into account.

I think that we can see now why HS2 Ltd preferred to ignore night noise in the AoS.


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