Silent night?

Environmental aspects of the 51m consultation response, part 19

In this blog I will continue my review of what Southdowns Environmental Consultants Ltd has said in Appendix 18 to the 51m alliance response to the public consultation (available here), which refers to criticisms that I have already made in earlier blogs.

In It’s on the level (posted 8 Jul) I accused HS2 Ltd of overlooking the effects of night-time noise in the AoS.  This is not strictly true; the impact of night-time noise has been discounted, not overlooked, “due to the predominantly daytime operation of HS2”.

I will consider what the Southdowns report says about this attempt to sweep this issue under the carpet in a moment, but first of all we need to calculate what the level of night time noise is likely to be. Table 1 on page 45 of Appendix 5 to the AoS (available here) gives the information that we require to do this; this table details the expected train pass-bys on the HS2 London-West Midlands link, per hour and per day. From this table it is a simple matter to derive that, for the initial phase of HS2, there will be 432 train pass-bys during the period 06:00 to 24:00 hrs that is used to calculate dB LAeq,18hr; this is simply the sum 120+9+312. For the night-time period of 23:00 to 07:00 hrs we need to use the “standard hour” total pass-bys of 22 (6+16) multiplied by the three operational hours during this period, yielding 66 pass-bys per night.

Since the equivalent continuous noise level (when expressed in its linear form) is proportional to the number of pass-bys divided by the period, the ratio of night-time to eighteen-hour noise, in decibels, is given by:


This yields -4.63 dB. So, in round figures, the equivalent continuous noise level for the night-time hours is 5 dB less than the similar level for the eighteen-hour period.

If you want some help with understanding the above calculation, then please refer to my blogs A pile of old logs (posted 2 Jun), dB or not dB, that is the question (posted 6 Jun), Dee-bee, eh? (posted 10 Jun), Getting the measure of things (posted 14 Jun), Suffering from exposure (posted 18 Jun) and Taking a longer-term view (posted 22 Jun).

Southdowns has obviously done this calculation, as its report states that “the 8-hour night-time reference level will fall below the 18-hour HS2 daytime reference level by around 5 dB”.

This 5 dB margin has proved to be convenient for HS2 Ltd, because it is reflected in the Noise Insulation Regulations (available here). The qualifying noise levels for insulation are specified in these regulations as 68 dB LAeq 06:00 – 24:00 at the façade (equivalent to 65 dB LAeq free field) and 63 dB LAeq 12:00 – 06:00 at the façade (equivalent to 60 dB LAeq free field). This has allowed the AoS to claim that:

“It is likely that all the properties which would be identified as eligible for noise insulation under the night time noise insulation criteria within the Noise Insulation Regulations have already been identified in the AoS as being eligible under the daytime noise insulation criteria.”

In fact the numbers are working even more in the favour of HS2 Ltd than I have stated, because the Noise Insulation Regulations definition of night time is only a six-hour period, which encompasses only one hour of HS2 operations (i.e. only 22 train pass-bys). If you put the numbers for this six-hour period into the above calculation, the difference between the day-time and night-time equivalent continuous noise levels increases to around 8 dB.

The response in the Southdowns report is to comment that “the Noise Insulation Regulations pre-date other and more contemporary guidelines for night-time noise limits which were published by the World Health Organization in 2009”, and I will examine these more recent guidelines in my next blog.

However I should also report that the AoS gives a second reason for discounting night-time noise, which is:

“It is unlikely that any further candidate areas for mitigation would arise as a result of a night time noise assessment using a high maximum noise level (e.g. 85 dB LAmax).”

The AoS does not provide any further elaboration of this statement and I have to confess that the origin of this 85 dB had me flummoxed. It would appear that Southdowns may have had a similar problem, because its report merely repeats the AoS claim without commenting upon its validity.

Acoustics expert Professor Colin Waters has provided a possible explanation of the figure used in the AoS in his contribution to the submission made to the public consultation by the Chiltern Countryside Group (CCG). This explanation may be found in Appendix A to Part 1 of the submission (here).

Professor Waters has unearthed, in a footnote to a table in Annex 1 of Planning Policy Guidance 24: Planning and Noise (available here), a case of a peak night-time noise level being specified as an alternative threshold to the equivalent continuous noise levels which are quoted as the primary noise thresholds. This level is 82 dB LAmax,S and Professor Waters points out that this is equivalent to a façade level of 85 dB, which may account for the AoS figure. He comments in paragraph 10.16.3 on page 57 of the CCG document:

“This should not conceivably be inferred to be an acceptable façade noise criterion of 85 dB LAmax,S and is not helpful in any way to assess the night time noise likely from HS2 operations. Leaving aside the fact that NEC considerations are not applicable to the assessment of new railway systems on existing dwellings, the adoption of 85 dB LAmax,S would be an extremely high noise level to use in this context.”

How silly of me not to have worked that one out for myself.


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