A rude awakening

Environmental aspects of the 51m consultation response, part 21

In this blog I wish to take a brief diversion from what Southdowns Environmental Consultants Ltd has said in Appendix 18 to the 51m alliance response to the public consultation (available here), and look at another issue associated with night-time noise.

In my blog Nessun dorma (posted 7 Dec) I introduced the 2009 WHO document Night Noise Guidelines for Europe (available here). This documentation recommends maximum night-time noise levels using the parameter Lnight,outside; as I explained in Nessun dorma this parameter is the free-field equivalent continuous noise level incident upon the most exposed façade of the building in which the subject is sleeping, determined over the eight hour night period 23:00 to 07:00 hrs.

The reason given for this parameter being chosen by the WHO is to “be consistent with existing practices in the legislation”, Lnight having been adopted as the night-time noise indicator by the EU Environmental Noise Directive 2002/49/EC. Whilst using a single parameter to characterise night-time noise has its advantages, it does carry the risk of over-simplifying the complex effects that noise, and in particular the nature of the noise, can have on sleep and sleep-related health problems. Of particular concern here is that noise that is discontinuous with loud single events, such as that caused by aircraft and high speed trains, may be more damaging than more constant noise patterns, such as machinery and some traffic noise.

Whilst promoting the parameter Lnight,outside, the Night Noise Guidelines for Europe acknowledges that the peak noise level may also be a useful indicator for some effects of sleep disturbance:

“Long-term effects such as cardiovascular disorders are more correlated with indicators summarizing the acoustic situation over a long time period, such as yearly average of night noise level outside at the facade (Lnight, outside), while instantaneous effects such as sleep disturbance are better with the maximum level per event (LAmax), such as passage of a lorry, aeroplane or train.”

Despite the title of this blog, a single noise event does not have to be loud enough to wake you up to cause sleep disturbance. The Night Noise Guidelines for Europe comments that:

“Noise disturbs sleep by a number of direct and indirect pathways. Even at very low levels physiological reactions (increase in heart rate, body movements and arousals) can be reliably measured. Also, it was shown that awakening reactions are relatively rare, occurring at a much higher level than the physiological reactions.”

The Night Noise Guidelines for Europe updates previous advice on night-time noise effects in the 1999 WHO document Guidelines for Community Noise (available here); I have referred to the 1999 guidelines in a number of previous blogs. This earlier WHO document differs from the 2009 guidelines in recommending both equivalent continuous and peak noise level limits; as it explains:

“It is not enough to characterize the noise environment in terms of noise measures or indices based only on energy summation (e.g., LAeq), because different critical health effects require different descriptions. It is equally important to display the maximum values of the noise fluctuations, preferably combined with a measure of the number of noise events.”

In Table 1 on page xv of Guidelines for Community Noise the WHO recommends a guideline value for LAmax of 45 dB, measured within a bedroom, in order to avoid sleep disturbance. The 1999 recommendation is quoted in the 2009 guidelines:

“If the noise is not continuous, sleep disturbance correlates best with LAmax and effects have been observed at 45 dB or less. This is particularly true if the background level is low. Noise events exceeding 45 dBA should therefore be limited if possible.”

The 2009 document adds that “thresholds are now known to be lower than LAmax of 45 dB for a number of effects”. This comment is reinforced by the data presented in Table 1 on page xiii of  Night Noise Guidelines for Europe, which indicates that physiological effects may be observed at peak noise levels as low as 32 dB LAmax,inside.

Let’s look at some numbers to try and flesh out this discussion. If we assume a dwelling that just creeps into the bottom end of the grey dot rating, the eighteen-hour equivalent continuous noise level, LAeq,18hr, will be 50 dB. I calculated in Silent night? (posted 25 Nov) that the night-time equivalent continuous noise level, LAeq,8hr or Lnight, outside will be 5 dB less, i.e. 45 dB Lnight, outside.

The Southdowns report informs us that:

“Instantaneous train pass-by noise levels will exceed average 18-hour daytime noise levels by up to around 15 dB at speeds of up to 360kph on the basis of HS2 information provided outside the AoS.”

So at our sample grey dot dwelling the peak noise level will be 65 dB LAmax (50+15); this level will not change day or night. To get the value of LAmax within the bedroom all we have to do is decrease the level by an amount to take account of the losses through the building walls. The magnitude of these loses will depend upon building design and whether the bedroom window is open; the WHO 2009 guidelines suggests that a value of 21 dB be used. So an estimate for LAmax within the bedroom is 44 dB, indicating that worrying night-time peak levels, even by the 1999 standards, may be encountered throughout all three coloured dot ranges.

Another observation that may be made from the above calculation is that, since the peak level is the same day or night, the ratio of the peak to equivalent continuous noise levels will be 5 dB greater at night, i.e. 20 dB.

It is no wonder that Southdowns comments in its report that the impact of peak noise levels warrants “further consideration for the purposes of public consultation and the additional consideration of sleep disturbance”.

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