How annoying is that?

Assessing the annoyance caused by HS2 noise, part 2

In Not quite measuring up (posted 14 Oct 2012) I said that relating decibels to annoyance is by no means a simple task. The problem is that noise nuisance is largely subjective and, accordingly, defies objective measurement. Nethertheless, what we expect from HS2 Ltd is some sort of numerical indication of how bad the noise nuisance will be in our own particular neighbourhood.

Not surprisingly, a great deal of thought and research has gone into how the annoyance resulting from unwanted noise can be predicted. Perhaps the leading authority in this field is the World Health Organisation (WHO), which has published a useful handbook on this topic called Guidelines for Community Noise. In this handbook (in paragraph 2.3.6 on pages 10 and 11) the WHO tackles the subjectivity issue:

“Different people will respond quite differently to the same noise stimulus. These individual differences can be quite large and it is often most useful to consider the average response of groups of people exposed to the same sound pressure levels. In annoyance studies the percentage of highly annoyed individuals is usually considered, because it correlates better with measured sound pressure levels.”

So a summary of the WHO approach is:

  • Decide on a noise parameter – let’s assume that this is LAeq for our purposes.
  • Do some research to determine how what proportion of people in a sample react with annoyance to noise at different values of that noise parameter (or crib the results of someone else’s research).
  • Decide upon the acceptable proportion of the population to be annoyed.
  • Set a noise threshold for the chosen noise parameter at the level that will just cause annoyance to the selected proportion of the population.

And this is just what HS2 Ltd is proposing to do, using LAeq,16hr as its chosen principal noise parameter.

However, the Guidelines for Community Noise warns us (in paragraph 4.2.7 on page 42) that:

“The annoyance response to noise is affected by several factors, including the equivalent sound pressure level and the highest sound pressure level of the noise, the number of such events, and the time of day.”

In the same paragraph we are also warned about “noise with low-frequency components” needing to be lower than levels acceptable for noise without this characteristic.

The problem is that the parameter than HS2 Ltd has settled on, LAeq,18hr, has no ability to reflect these different characteristics of the noise and some modifications to the HS2 Ltd methodology appear to be required. What I aim to do in the current blog series is, as far as I am able, suggest what these modifications should be.

For the remainder of this current blog I will consider the way that impact of the “time of day” might be taken into account. The WHO says (in the same paragraph 4.2.7) that, for the same annoyance level to be experienced “sound pressure levels during the evening and night should be 5–10 dB lower than during the day”. HS2 Ltd has, as far as I can see, totally refused to acknowledge this factor in the work done to date. I find this inexplicable, because there is a readily-available variant of the equivalent continuous sound level parameter that accommodates this evening and night lift in the annoyance level.

This “off the shelf” parameter is the day-evening-night equivalent continuous sound level, which is given the symbol Lden. I introduced this parameter in my blog May I suggest a better option? (posted 21 Apr 2012). It is a simple variant of the basic equivalent continuous sound level parameter, LAeq, which sums the energy over the full 24 hour day, but split into three periods: a day period of 12 hours, an evening period of 4 hours and a night period of 8 hours. Before the energy sums from each of these three periods are combined to produce a single decibel value for Lden, a manipulation is carried out to boost the evening energy sum by 5 dB and the night energy sum by 10 dB. This rather neat trick allows the additional annoyance value of noise in the evening and night periods noted by the WHO to be reflected.

The use of Ldenhas a good pedigree; it is the preferred parameter to represent the “noise indicator for overall annoyance” adopted by the EU Environmental Noise Directive 2002/49/EC (here). Whilst this Directive tolerates the use of “existing national indicators”, which includes LAeq,T, as an interim measure, it is quite clear that the EU intends that Lden will become obligatory on EU members in due course for the purpose of “the preparation and revision of strategic noise mapping”.

In paragraph 2.1.1 on page 11 of the September 2012 version of the HS2 Ltd document HS2 London to West Midlands EIA Scope and Methodology Report we are promised that the environmental impact assessment will employ “current best practice”. It appears to me that, at the current state of knowledge, Lden represents best practice for assessing the annoyance caused by noise.

I consider it quite inexcusable that, given the backing by the EU for Lden, HS2 Ltd has not adopted it as its chosen principal noise parameter, in place of LAeq,16hr. I wonder if this may have anything to do with the expectation for HS2 that, for any given receptor location, Lden will always have a larger decibel value than LAeq?

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