Adding to the confusion

Environmental aspects of the 51m consultation response, part 13

In this blog I will continue 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 middle or yellow dot rating, described by HS2 Ltd as “dwellings potentially qualifying for noise insulation under the Noise Insulation Regulations”.

The criterion set for this rating is specified in paragraph 6.1.2 on page 48 of Appendix 5 to the AoS (available here) as “greater than or equal to 68 dB LAeq,18hr at the building façade (i.e. a façade noise level)”.

Within these few words is a trap, of the sort that engineers love to set for the unwary, and I fell into it when I wrote Trust me, I’m an acoustic engineer (posted 4 Jul) and It’s on the level (posted 8 Jul). Fortunately the Southdowns report (available here) has alerted me to my error. The tricky bit is “at the building façade (i.e. a façade noise level)”, whilst the criterion for the red dot level was specified as a “free field noise level”.

As the name implies a “free field noise level” measurement is made away from any potentially reflecting surfaces, such as the façades of buildings. A measurement of “façade noise level” is required to be made one metre in front of the most exposed window or door in the façade of the building concerned. The hard surface of the building façade acts like a mirror and reflects the sound back, which reinforces the sound level in front of the façade. The effect is to produce a higher sound level measurement than would occur if the building was not there. According to the Southdowns report:

“… there is a difference of 3 dB between free-field and façade noise levels due to the enhancement of façade noise levels by surface reflections. The façade noise level of 68 dB LAeq,18hr set out in the Noise Insulation Regulations thus corresponds to a free field noise level of 65 dB LAeq,18hr.”

So, speaking in free field terms, the yellow band runs from 65 dBA to 73 dBA and the grey band from 50 dBA to 65 dBA.

The noise level limits from Planning Policy Guidance 24: Planning and Noise that I have used for comparison in It’s on the level relate to free field measurements. I stated in that blog that “The advice of Planning Policy Guidance 24 is that planning applications should “not normally be granted” when the noise level exceeds 66 dBA”. So even after making the 3 dB adjustment to the yellow dot range, most of the 8 dB spread of this band remains at levels above this planning stipulation.


2 responses to this post.

  1. what a shame you have lost,go a buy your ear plugs Now


    • I like the change of name “What a shame you lost”, but when you were “HS2 is right” you said “I will not be commenting again as unlike you I have work to do”. I’m glad that you have changed your mind, because I really like pointing out to you when you have got it wrong.

      And guess what, you have got it wrong yet again. I suggest that either you haven’t read the TSC report or that you haven’t read it very carefully. You have nothing to be triumphal about. What the TSC have endorsed is “high speed rail” and I don’t have any argument about that. However, the committee has been scathing about the HS2 proposal.

      If the DfT does what the TSC is asking and, before making its decision on HS2, reworks the business plan (including costing the environmental damage), reviews the design speed, looks at making more use of existing transport corridors, improves the Heathrow link, reviews the location of stations in London and justifies the track capacity (trains per hour) assumption, then there is no way that it will be able to make an announcement in December, as is currently planned. Also the reworked business plan will fall below Treasury BCR limits and will not be able to be given the go ahead.

      Contrary to what you might think, I’m seeing events turning very much in the “anti’s” favour. Even the dear old Labour Party has suggested that HS2 should take a different route.

      There is a very, very long way to go on this one, so, if you don’t mind, I won’t rush out to buy those ear plugs just yet.


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