Checking the small print

In the postscript to my blog I only just noticed that (posted 25 Apr 2012) I noted another policy change between the Appraisal of Sustainability (AoS) and the recently published document HS2 London to West Midlands EIA Scope and Methodology Report (here). In this postscript I identified this change as adding a train pass-by peak noise parameter to the metrics that will be used to identify HS2 noise impacts for the environmental impact assessment (EIA). However, I also said there that the threshold value set for the EIA was specified so high that this parameter would have no effect.

I suggest that it might be helpful to devote a couple of blogs to expanding on this point.

The threshold level for this parameter is specified in paragraph 13.3.24, on page 119, of the HS2 London to West Midlands EIA Scope and Methodology Report; there it is prescribed as “85 dB or greater at the façade of the receptor”. I take this to be a true “façade” measurement, so it is equivalent to 82 dB(A) free-space (I explained this in my blog Adding to the confusion, which I posted on 1 Nov 2011).

The units given for this parameter in paragraph 13.3.24 are “LpAF,max”. Now precise usage of terms varies from document to document, and I have been guilty of overlooking these differences when they do not signify any change in the parameter itself. However, I think it might be useful at this stage to digress slightly in order to examine what each of the constituents of “LpAF,max” signifies.

We came across the use of “L” as long ago as 14 Jun 2011 in my blog Getting the measure of things; it indicates that the parameter refers to the level of sound on a dB scale.

The use of “p” has only crept in to my blogs relatively recently, and when we first encountered its use in the noise limits for the Shinkansen train (in That’s much louder than the average, posted 29 Nov 2011) I did not comment on it. All that the “p” signifies is that the level is relative to the threshold of hearing; in other words 0 dB is set as the threshold of hearing. I explained this in my blog Dee-bee, eh?, which I posted on 10 Jun 2011. HS2 Ltd didn’t bother to use the “p” in the AoS – it was taken for granted that all levels were expressed relative to the threshold of hearing – but the HS2 London to West Midlands EIA Scope and Methodology Report is more precise on this point.

The “A” signifies that A-weighting has been applied; I explained what this was and why it is used in Dee-bee, eh?.

The “F” component hasn’t cropped up in any other context in my blogs before, and so hasn’t been explained. It means that, when a sound meter is used to measure the parameter, the meter should be switched to its “fast” time weighting setting. This means that the meter can respond to fast changes in the sound level and, therefore, make a true maximum measurement. For those of you who like to know such things, the “fast” setting has a time constant of 125 ms. Unsurprisingly, there is also a “slow” setting, which has a time constant of 1 second. If this is specified the “F” is replaced by “S”, as is the case in the parameter LpASmax used to specify the maximum noise for the Shinkansen train (as noted in That’s much louder than the average).

Finally, there is “max”. We first encountered this in my blog Getting the measure of things (posted 14 Jun 2011). It simply means that the parameter defines the maximum sound level of an event. If we replace it by “eq”, as in LpAeq,8hr, then we have our old favourite the equivalent continuous sound level.

So that’s the “small print” sorted and we are ready to get back to what HS2 Ltd is proposing for a train pass-by peak noise threshold, but I think that we had better leave that until the next blog.

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