The challenge of the peaks

Assessing the annoyance caused by HS2 noise, part 3

In this blog I shall continue to explain how I think HS2 Ltd should improve how it is proposing to estimate the annoyance that noise from HS2 will cause.

Let’s consider further the diagram that I borrowed from a Civil Aviation Authority website to indicate how the equivalent continuous sound level is calculated in my blog Not quite measuring up (posted 14 Oct 2012).

Equivalent continuous sound level (source: CAA)

In Not quite measuring up I queried, in passing, just how good a characterisation of a varying waveform the equivalent continuous sound level is for the purposes of indicating the annoyance value of that waveform. This question gains validity when you consider that there are an infinite number of wiggly waveforms that could be superimposed upon the diagram, each having the same value of equivalent continuous sound level. It is inconceivable surely that all these possible waveforms would cause an identical degree of annoyance.

The noise waveform for HS2 is a rather extreme example of a wiggly waveform. It will consist of a series of very similar noise energy bursts, a few seconds in duration and with a high peak level of energy that is well above the equivalent continuous sound level. These bursts will be experienced about once every 90 seconds, on average, and during the periods in between the railway noise level will be, for practical purposes, zero.

We know from an answer by HS2 Ltd to a Freedom of Information request that the peak noise of a pass-by of a HS2 train will be around 14 dB above the level of LAeq,18hr. This is a big difference, and I mused about the impact that this might have in my blog I only just noticed that (posted 25 Apr 2012).

The problem of characterising such “difficult” waveforms has not escaped the notice of the World Health Authority. Paragraph 2.3.5 on page 10 of its document Guidelines for Community Noise states:

“A number of studies have concluded that equal levels of different noise types lead to different annoyance. For example, equal LAeq,T levels of aircraft noise and road traffic noise will not lead to the same mean annoyance in groups of people exposed to these noises. This may indicate that the LAeq,T measure is not a completely satisfactory description of these noises and perhaps does not completely reflect the characteristics of these noises that lead to annoyance.”

In Guidelines for Community Noise the WHO identifies a number of characteristics of the noise source which may contribute to increased annoyance, and these include the rate and magnitude of level variations, the frequency content, whether the noise is continuous or is a series of discrete events, and even the psychological effects caused by certain noise sources. In the light of this it seems an impossible task to assess the annoyance that any particular noise source will cause. The WHO offers the following advice (in paragraph 2.3.1 on page 8):

“Thus, current practice is to reduce the assessment of environmental noise to a small number of quite simple quantities that are known to be reasonably well related to the effects of noise on people (LAeq,T for continuing sounds and LAmax or SEL where there are a small number of distinct noise events). These simple measures have the distinct advantage that they are relatively easy and inexpensive to obtain and hence are more likely to be widely adopted.”

However, the WHO does make a specific recommendation (in paragraph 2.1.5 on page 5):

“LAeq,T should be used to measure continuing sounds such as road traffic noise, many types of industrial noises and noise from ventilation systems in buildings. When there are distinct events to the noise such as with aircraft or railway noise, measures of the individual events should be obtained (using, for example, LAmax or SEL), in addition to LAeq,T measurements.”

So the advice of the WHO to HS2 Ltd is clear. It is insufficient just to use the equivalent continuous sound level to assess annoyance from HS2 noise; either the peak level of a train pass-by, or the energy of a single train pass-by, should also be employed for the evaluation.

In the next blog I will look at whether HS2 Ltd has heeded this advice.

Footnote: The acronym “SEL”, which is mentioned in this blog, refers to the Sound Exposure Level; the meaning of this term is explained in my blog Suffering from exposure (posted 18 Jun 2011).

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