Another thing worth ignoring

The effects of speed, part 4

All noise sources from trains tend to get louder the faster you go. It stands to reason from what I said in my previous blog (17 May) that, since the aerodynamic noise is fairly low in conventional trains and not only dominates in high speed trains but increases rapidly in volume as the train goes faster, that high speed trains are going to be really noisy beasts.

However just in case that you think that I have a real downer on high speed trains, I wish to point out one positive advantage of going very fast. High speed trains may be loud, but mercifully they don’t hang around very long.

This is well illustrated in graphs in two figures in the US FRA manual that I introduced in my previous blog (17 May); if you’ve not been paying attention, this document is called High-Speed Ground Transportation Noise and Vibration Impact Assessment (and may be found here). The two graphs are figures 2-3 and 2-4 on pages 2-5 and 2-6 and are reproduced below.

Pass by TGV at 180 mph (Source: USFRA)

Pass by Eurostar at 90 mph (Source: USFRA)

The two figures compare the sound levels experienced by an observer 82 feet (25 metres) from the track when passed by a TGV train at 180 mph (290 kph) and by a Eurostar train at 90 mph (145 kph). As well as travelling slower the Eurostar train is nearly twice as long as the TGV and this adds to the duration of the pass, but the difference in speed also contributes.

The point of the two graphs is to illustrate, that by one of the common measures, both train passes have the same noise nuisance value. This measure is Sound Exposure Level (SEL) and I will leave any detailed consideration of that until another day. However, the lesson that I suggest that we draw is that both passing speed and train length contribute to the noise nuisance level and HS2 will be bad on both counts.

There is one other feature that is illustrated by the two graphs. Take a look at the left-hand side of the graphs, which represent how the noise level increases for an observer as each train approaches. The TGV ramps up to its maximum noise in about half the time that the Eurostar and that maximum is higher anyway. In the parlance, the TGV has “a fast onset rate”.

The US Air Force has looked at the effect of fast onset rates on perceived noise nuisance levels. The USAF work, which was naturally on aircraft (The Effect of Onset Rate on Aircraft Noise Annoyance, Plotkin et al, US Air Force Systems Command May 1992), indicates that sounds with fast onset rates are more annoying than sounds with less rapid variation. I can find no reference to this effect in Appendix 5.4; has HS2 Ltd given any consideration to the possible increase in noise annoyance levels that this factor may cause?

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