A little bit of magic

The effects of speed, part 3

In the final three blogs in this set of five, I will look at what setting a high maximum running speed specification does to the noise generated by a passing train. For these three blogs I have relied heavily on a manual produced by a consultant for the US Department of Transport Federal Railroad Administration (US FRA), which is a comprehensive bible for all train noise freaks. The document is called High-Speed Ground Transportation Noise and Vibration Impact Assessment and may be found here. The material for this current blog comes mainly from Chapter 2 of the manual.

The “good book” says, in section 2.2, that noise emanating from a conventional train, on the WCML say, comes from two sources, these being propulsion/machinery noise (from the motors used for propulsion and any fans used to cool them) and rolling noise (from the wheels and the rails); the latter noise source being the most dominant at full speed. There is also a third potential source of noise, aerodynamic noise (resulting from airflow moving past the train), but this is not significant at conventional speeds.

However, aerodynamic noise becomes the dominant noise source for high speed trains; the good book says at over 180 mph (290 kph). Once it becomes dominant, aerodynamic noise increases far more rapidly with speed than propulsion or rolling noise. The change from mechanical noise dominance to aerodynamic noise dominance is significant because it not only changes the nature of the noise (in technical terms the “energy spectrum” of the noise) but the location of the source of the noise (principally the height of the effective noise source above the rails).

For a conventional train the effective location of the noise source is about a half metre above the rails. For a high speed train the effective aerodynamic noise source is shifted higher above the rails and the spectrum is more towards the lower end of the frequency band (refer to section 2.2.3 of the US FRA manual). These quite different noise characteristics of conventional and high speed trains have a marked impact on the effectiveness and nature of noise barriers.

Firstly, 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. So such barriers will be less effective for HS2.

Secondly, the increased effective height of the noise source for a high speed train means that, in order to be effective, the barriers must extend higher. The US FRA manual suggests barrier heights of 15 feet (4.6 metres) or more above the top of the rail.

So what height noise barriers do you think HS2 Ltd is planning to use? The answer is in paragraph 8.10.13 on page 99 of Volume 1 of the Appraisal of Sustainability Main Report (available here). Here HS2 Ltd says:

Noise reduction would be equivalent to that achieved by use of 3m high noise barriers (or bunds) at all candidate areas for mitigation, or at viaducts, by 2m high barriers; noise-absorbent barriers have been assumed throughout.

So the standard trackside barrier for HS2 will be only two-thirds the height recommended by the US FRA manual. Why is HS2 Ltd doing this? Is HS2 Ltd not aware of the way that the effective source of noise rises up the train when the speed is taken above 300 kph?

Well yes, HS2 Ltd appears to be fully aware of this problem, but it does not see higher barriers as feasible. Instead, HS2 Ltd has come up with a “magic” way of reducing the effects of aerodynamic noise. This is mentioned in paragraph 6.3.3 on page 50 in Appendix 5.4 to the Appraisal of Sustainability Main Report (available here). This paragraph says:

To mitigate potential impacts in areas of high operating speeds, there is a need to control aerodynamic noise through advanced rolling stock design. Without first mitigating the source of aerodynamic noise, wayside noise barriers are not likely be as effective or feasible, due to the required increase in barrier height, to provide shielding to the entire train.

No explanation is given of the techniques that will be employed to “control aerodynamic noise through advanced rolling stock design” or how they will work. Also no indication is given of whether the ability to reduce aerodynamic noise at source is proven technology or whether it is just an aspiration. The indications are that the latter is the case, because in 6.3.5 on the same page the text talks about a “reduction in noise emissions at source based on the anticipated noise control improvements in the next generation of high speed rolling stock” (my underscoring).

Just so that it doesn’t go unnoticed, this little revelation in paragraph 6.3.5 means that not only are HS2 Ltd assuming that this advanced rolling stock will allow lower barriers to be used but that the noise power emanating from the train will be lower also. I will discuss this further piece of magic (or is it sleight of hand) in the next blog but one.

Speaking as someone who is going to have to live with the noise from HS2, I find this all very worrying. I took the opportunity of the seminar hosted by HS2 Ltd in March to submit a written question on this topic and I have been promised an answer. In the unlikely event that I get one, I will let you know what it is.

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