6:46pm Friday 3rd September 2010
RESIDENTS who live along the length of HS2’s proposed 9.6 km twin-bore tunnel from the M25 to Old Amersham should not think this tunnel will shield them from noise. Rather they need to be aware that they will suffer from serious noise annoyance because of the tunnel vents.
In the tunnel trains will be travelling at 320 km/h – a little slower than their maximum operational speed. Vents which connect the tunnel to the surface are needed to allow the pressure, which will build up in front of the trains, to escape.
If there were no vents the air pressure in front of the trains would slow the trains down.
Even with the vents, compressed air builds up in front of the trains because they will fit fairly snugly in the tunnel.
People become aware of a train travelling on the surface because there is a build-up of noise as it approaches.
This build-up reaches a crescendo and then declines as the train passes.
For regions around a tunnel vent however, there will be no warning of a train approaching. Instead there will be a rapid change from normal ambient noise levels to a very loud unexpected BANG! or BOOM! as the compressed air shoots up the vent immediately before the train passes. This will occur nearly every two minutes as there will be 14 trains per hour each way.
There will be a similar situation at tunnel portals as trains unexpectedly exit from the tunnel ends.
Marilyn Fletcher, Chiltern Manor Park, Great Missenden
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