May I suggest a better option?

As I said in my blog Going over some old ground (posted 17 Apr 2012), I intend to devote this current blog and the next to reviewing the issue of selecting appropriate parameters to express the noise impact of HS2 meaningfully. Some of this will be a restatement of what I said in my blogs Taking a longer-term view (posted 22 Jun 2011) and That’s much louder than the average (posted 29 Nov 2011), but also some is new.

I believe that there are two issues here. The first is whether the parameter selected by HS2 Ltd, LAeq,18hr, is the most appropriate way of representing the equivalent continuous noise level for HS2. The second is whether the representation of the noise nuisance caused by HS2 solely by an equivalent continuous noise level parameter gives a true representation of the impact of that noise nuisance.

HS2 Ltd is right to claim, as I reported in Going over some old ground, that “in the UK and the EU, regulations, standards and policy all adopt [the equivalent continuous noise level] to describe railway noise for both conventional and high speed lines”. However, there is little uniformity between these various authorities as to the precise parameter to be used to express the equivalent continuous noise level.

In the absence of a consensus, it is suggested that HS2 Ltd should pay due regard to the requirements of the EU Environmental Noise Directive 2002/49/EC (here). This Directive employs two equivalent continuous noise level parameters, termed the “noise indicator for overall annoyance”, or Lden, and the “noise indicator for sleep disturbance”, or Lnight. The Directive requires that both of these indicators are employed to assess the nuisance level.

For those not familiar with these two parameters I should explain that they are both derived by considering the day of 24 hours being split into three periods: a day period of 12 hours, an evening period of 4 hours and a night period of 8 hours.

This allows Lnight to be defined as the equivalent continuous noise level (A-weighted), calculated over the 8 hour night period. If similar calculations are also made for the day period (Lday) and the evening period (Levening), then Lden (the day, evening, night level) is calculated from the logarithmic composite of the Lday, Levening and Lnight levels but with 5dB(A) being added to the Levening value and 10dB(A) being added to the Lnight value. This boosting of the evening and night values takes account of the increased annoyance that noise pollution causes during these periods.

Now I think that Lden is a pretty neat concept and I can’t understand why it is not used more widely in the UK or why HS2 Ltd is not promoting its use for HS2. I suspect that it may have something to do with the increased dBA levels that result from the application of weighting to the evening and night periods.

So the answer to the first question that I posed early on in this blog, about whether LAeq18hr is the most appropriate way of representing the equivalent continuous noise level for HS2, is probably no.

In the next blog I will consider the second question that I posed above.

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