A summit that is far too high

Assessing the annoyance caused by HS2 noise, part 4

In The challenge of the peaks (posted 22 Oct 2012) I explained why the World Health Organisation (WHO) advises that assessing the degree of annoyance that would be caused by railway noise should involve a measure of the noise caused by each individual train pass-by, using a parameter such as LAmax or SEL, in addition to the equivalent continuous sound level parameter.

The WHO advice does not explain how this should be done, but it seems obvious that the additional parameter should mirror the approach adopted for the equivalent continuous sound level, employing the four steps that I set out in How annoying is that? (posted 18 Oct 2012). So the threshold for the additional parameter should be set at a level that research has determined will just cause annoyance to the same proportion of the population as at the threshold level chosen for the equivalent continuous sound level parameter. Of course the levels for the two parameters, as expressed in decibels, are likely to be different.

Now it appears that HS2 Ltd may have made a move towards accepting this item of WHO advice – I say appears because HS2 Ltd has, as far as I can tell, offered no explanation of what it has done. When the first version of HS2 London to West Midlands EIA Scope and Methodology Report was published at the end of March 2012 the parameter LpAFmax had been introduced as an additional metric for determining the impacts of airborne sound and it survives in the September 2012 “final” revision (see paragraph 14.3.26 on page 153).

Since LpAFmax is just a more precise way of writing LAmax, hasn’t HS2 Ltd complied with the WHO suggestion? Well, it looks to me that the answer to this question is “no, not really”, and the reason is the threshold level that HS2 Ltd has specified for this parameter. This level is 85 dB and I commented in my blog Setting the bar too low (posted 23 May 2012) that for “HS2 Ltd to propose this as a threshold level is totally inappropriate”.

The reason for my comment is that 85 dB is a very high peak noise level. We know from an answer by HS2 Ltd to a Freedom of Information (FOI) request that the peak noise level from a current generation high speed train pass-by at 350 kph will be 95 dB LAeq,TP at 25 metres from the track. We also know that HS2 Ltd is assuming that HS2 trains will be 3 dB quieter than this, so the 85 dB threshold proposed is only 7 dB down on this. In order to experience this level you would need to be closer to the track than between about 60 and 100 metres (assuming level ground, track at grade and no additional noise mitigation).

The answer to the FOI request also tells us that 85 dB peak corresponds to 71 dB equivalent continuous sound level. This puts the noise nuisance at a level well above what would qualify you for a sound insulation grant for your property.

In contrast, the threshold that HS2 Ltd has stipulated for the equivalent continuous sound level is, at 50 dB LpAeq,16hr, a whole 15 dB below the sound insulation qualifying level.

So what is HS2 Ltd up to? It beats me – perhaps someone from HS2 Ltd will be kind enough to explain.

In Setting the bar too low I said that:

“What is required is a threshold that will work alongside the 50 dB LpAeq,16hr to give an alternative indication of the onset of annoyance, based upon maximum rather than equivalent continuous level.”

Clearly, the 85 dB threshold specified in HS2 London to West Midlands EIA Scope and Methodology Report is not doing that.

Footnotes:

  1. The acronym “SEL”, which is mentioned in this blog, refers to the Sound Exposure Level; the meaning of this term is explained in my blog Suffering from exposure (posted 18 Jun 2011).
  2. There is some ambiguity in HS2 London to West Midlands EIA Scope and Methodology Report about whether the levels specified are intended to be measured free-field, or with reflections from a building façade included; I have sought clarification from HS2 Ltd on this point. Depending upon the outcome of this clarification request, the threshold may be 3 dB lower than I have assumed in this blog. For further explanation of this point, please refer to my blog Adding to the confusion (posted 1 Nov 2011).
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One response to this post.

  1. Posted by chriseaglen on October 26, 2012 at 7:43 pm

    Suggest at such frequencies of average 8 minute spacing some locations will have numbers of passing trains in each direction peaking the noise annoyance. This has been found to occur with Javelins passing near Raiham in Essex where several per day pass opposite a block of flats by the conventional railway station. These have a different noise profile to the lower frequency Eurostars.

    Reply

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