#2024

@ JMD The 55dB level is a WHO recommendation that HMG work to. The issue isn’t so much that level as the quality of the survey that gave you a single figure of 72dB for the A303, without saying how far away from the road this was measured. Typically, it would be measured 10 metres away, but was it. We also know that traffic varies throughout each hour of each day, each day of the week and each week of the year. So you might expect them, if they had a years worth of data taken at a data rate of once every 200 milliseconds, to have at least 157 million data points for every site they had surveyed and you would have hoped they had already surveyed multiple sites on the existing route and collected background data on the two route options. So, a hell of a lot of data is needed, and they have offered you but a single data point.

Even if they had only sampled at a single point for 24 hours I would expect to see just over 400,000 data points. These would normally be given as a summary report that included:

The duration of the sampling (eg 24 hours, 1 year or whatever)
Frequency Weighting – you would expect it to be ANSI type A to represent the human auditory range – expressed as dB(A)
Time weighting – how often a reading was taken – this is either Fast (every 200mS) or Slow (every 500mS)
Average/Leq: – the Equivalent Continuous Sound Level
Min: the minimum time weighted recording
Max: the maximum time weighted recording
Peak: the maximum value reached without time weighting.

So 72dB means very little.

Moreover, you would expect to be shown a graph that captured the above with a level of granularity that showed hourly changes.

What we can do is get our own readings from a few distances – say 10 m and 100 m away from a fast dual section of the A303. If anyone is interested in helping do this, contact me (you will need an iPhone) – it’s vital information whichever route you look at. Then we can apply a simple rule of thumb used by HMG:

The sound level falls with increasing distance from the source. The principal reason is the wave front spreading and for a point source the “inverse square law” applies — doubling the distance from a point source produces a reduction in sound level of 6dB.

Of course, when we are looking at a road, that acts as a line source of sound and not a point source and in these circumstances a doubling of distance from the source only produces a 3dB reduction in the sound. So, knowing the sound level at 100m from a fast section of the A303 at peak times (and ideally night time as well), and knowing the distance HE are proposing to site the two routes from the nearest houses, we can get a rough and ready idea of the unmitigated noise levels. Of course, that assumes no wind, which is why further work involving tying that in is needed.

Another snippet of information from the same government publication will give a lot of food for thought:

Remember that the effect of trees as a noise barrier is often overestimated by non-acousticians. Some noise experts suggest that a tree belt must be more than 100m thick and very densely planted before any significant excess attenuation is achieved. However, trees may have a significant psychological effect by blocking the noise source from sight when in leaf. They may also provide some masking when rustling in the wind.