Have you heard?

Measuring noise, part 11

Well he did warn us! I reported in my blog of 25 May that the Transport Secretary had said when commenting upon the ‘sound booth’ noise simulation devised to wow the crowds at the HS2 Ltd roadshows: “I’m expecting that the predominant reaction when people hear the sound simulations will be ‘we don’t believe you’ …”.

Well you were spot on there Mr Hammond. I spoke to many people after they had left the Cubbington roadshow and virtually all were incredulous about what they had heard (or not heard).

My own reaction to viewing the simulation for Greatworth Northamptonshire, a village close to Brackley, is typical. There on the screen you could see the train speeding across the landscape, only four hundred metres or so from the outskirts of the village. The train appeared to be on a similar level to the land around, possibly even slightly elevated, and there was no higher ground or other natural noise shielding obstacle between the train and the observation point. The train noise was noticeable, but hardly as horrific as you might have imagined, and was nowhere near as noisy as the bus that passed on the road by which we, the imaginary observers, were standing.

So why don’t we believe it? The simulation was built by Arup, which is a very reputable consultancy firm, so nothing can be wrong there. It would obviously help though  if HS2 Ltd were to publish details of the methodology and mathematics that lie behind the simulation and then we could all pick over them and satisfy ourselves (or not) about whether all is fair. I managed to confirm with the Arup personnel at the roadshows that the two “tweeks” that I identified in my blog of 25 May have been employed and that this saves at least 5 dB on the noise level, but that is not enough to account for the credibility gap.

No, the answer lies, I think, in a single word: “mitigation”. You can see it on the video of the Greatworth simulation; alongside the track is a wall of noise-absorbing panels which shields most of the train from view, although you can still see the top of the train as it speeds past (the panels are 3 metres high). The man from Arup told me that it had been assumed that these panels will reduce the train noise by around 20 dB. Now that is a lot of mitigation. Take the panels away and the noise level jumps by 20 dB, which the ear will hear as a four-fold increase in the noise volume.

I suggested to the man from Arup that perhaps it would have been a good idea to have run the simulation with and without mitigation. He said that this was how they tried it at first, but the public reaction was so bad that they abandoned the idea. Generally, he said, that once people had heard the noise simulation without the mitigation they were not very receptive to the mitigated example.

So mitigation will be very important, but it is essential to note two important caveats in this respect. The first is that you can’t take mitigation for granted. It adds cost, is visually intrusive and is unlikely to be popular with passengers, since it blocks the view out of the train. So if you live on the outskirts of a village, are surrounded by neighbours and will be affected by high noise levels without mitigation, then I think that you have a reasonable chance that HS2 Ltd will provide some form of noise absorbing barrier. However, if you are in an isolated farmhouse close to the line, then I would venture that you have far less (probably zero) chance of having mitigation measures put in place.

An indication of how much (or little) sound mitigation we may expect may be achieved by looking at the “Residential Noise Appraisal Maps” in section 3.5 of Volume 2 of the Appraisal of Sustainability Main Report(here). These maps have been annotated with “cartouches” (elongated ovals) at intervals along the route. These are meant to indicate “primary candidate areas for mitigation”; in other words these are the locations which have been identified by HS2 Ltd at this stage as the ones most likely to be provided with some noise mitigation measures. If you are not in an area so marked, then I guess that you should worry that you may be subjected to the full unmitigated noise that they didn’t want you to hear at the roadshows.

The second caveat is whether sound mitigation will work as well as HS2 Ltd assumes it will. I expressed doubts in my blog of 17 May about whether 3 metre high barriers will do the job, based upon advice from the United States. Since it is vital that the proposed noise mitigation works, I have raised this issue with personnel from HS2 Ltd,Temple and Arups on various occasions. The reaction has always been the same; I get a response that amounts to the equivalent of “there, there” and a pat on the head and am told that they know what they are doing. No one has ever attempted to try to explain why train noise apparently behaves differently in the UK to in the US.

One final point is worth noting. In my blog of 17 May I noted that the materials used in noise-absorbent barrier construction are more effective at attenuating higher frequencies, rather than the lower frequencies that dominate with high speed trains. I am pleased to report that HS2 Ltd and Arups agree with this and have, I was told, included this effect in the model used to build the simulations.

You may have noticed that the sound of the train that you heard in the sound booth was very “boomy”, i.e. it had a high proportion of lower-frequency energy in its noise spectrum. This is a feature of aerodynamic noise, but this bass bias is accentuated by the absorbent barriers, since they cut the higher frequencies more efficiently than the bass frequencies. This works like a bass tone control on an amplifier and makes the sound even boomier. My reaction to the simulation was that this feature of the train noise made it more irritating and, possibly, more penetrating, but that is only my subjective view.

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