At speeds above 250 km/h (planned speed up to 350 km/h) aerodynamic noise is dominant – very problematic to soundproof with conventional noise barriers (standard height 4-5 m).
“High-speed line noise is one of the few significant pollutants and negative impacts (after land take and landscape severance) that lines can have. It is such a significant and specific phenomenon that it has been addressed by acoustic specialists for years.
Noise at such high speeds behaves in a different way to what we are used to with standard or upgraded lines. The passage of a train is acoustically more akin to a gunshot than a line source, the frequency spectrum of the noise and its sources change. At speeds of about 280 km/h, the aerodynamics of the car body begins to contribute significantly to the resulting noise, but above all the aerodynamic noise of the pantograph, which is very difficult to influence, given its main function, to supply the traction unit with an electric current by contact of a few cm2. At a speed of 300 km/h, the pantograph already contributes as much to the noise production as the wheel/rail source. The noise propagating from the wheels (from the ground) can be reduced quite successfully by technical means. Noise ‘radiated’ from the collector at height cannot be captured by noise barriers in most cases and therefore this effect must be anticipated and not rely entirely on the effectiveness of noise barriers. Compared to conventional rail, there is a difference in the much better quality of the carriageway and the vehicle body, with its distinctly aerodynamic shape, which has a positive effect on reducing the level of noise emitted. Shorter trains running at 2 – 3 times the speed of conventional rail mean a substantial reduction in exposure time to the track environment.
The maximum permissible noise levels are set in accordance with Government Regulation 88/2004 Coll., amending Government Regulation No. 502/2000 S., on the protection of health against the adverse effects of noise and vibration. The calculation of indicative predicted noise levels from the operation on high-speed lines is based on the predicted traffic performance on the line. However, detailed acoustic calculations cannot be carried out at this stage of the documentation.
The preliminary effort was to ensure a minimum distance of 250 m from inhabited settlements and zones as an acceptable compromise, which was further modified by other requirements according to the severity of local conflicts of interest and subsequent choice of priorities. This will, of course, in future necessitate the need to address the problem by technical means according to local conditions, i.e. noise barriers, bunds, screening structures, cuttings, and tunnels.”
Source: https://link.springer.com/content/pdf/10.1007/s11431-016-0649-6.pdf (350 km/h – 107.2 dBA, 305 Hz @ 7.5m (pg. 568, 569))
Crosswinds significantly increase noise: “The sound pressure level grew approximately as the 6th power of the pantograph speed.” https://link.springer.com/article/10.1007/s11431-019-1500-2
Pg1: “..sound power of the aerodynamic noise grows according to the 6th–8th power of the running speed..Noise that exceeds standards has become the main limiting factor of train speeds and restricts the sustainable development of high-speed railways.”: https://link.springer.com/content/pdf/10.1007/s11431-016-0649-6.pdf
It appears that for over 300kph, pantograph, insulator, contact strip, etc. are the limiting factors for both passengers and the environment. It’s fairly broad spectrum sound, and impossible to alleviate without extensive and very costly noise barriers which can add 1/3 to the cost of the line.
Noise assessment, monetization: https://journals.open.tudelft.nl/ejtir/article/view/3347/3504