The sound of silence

日期:2017-08-07 01:32:25 作者:缑屁韪 阅读:

By Mick Hamer in Delft THUNDERING train noise can be slashed in half using small knee-high noise barriers along the track, coupled with an acoustic hood that shrouds the train’s wheels, say researchers at TNO, the Netherlands applied research organisation in Delft. The researchers tested their ideas by blasting a stationary one-third scale model train in a laboratory with a loudspeaker producing sound at 110 decibels. It is based on the principle of “reciprocity”, says Marcel Janssens, who is one of the research team at TNO’s Institute of Applied Physics. A barrier that is effective at stopping noise from a loudspeaker beside the track from reaching a train will be just as effective at preventing train noise from reaching a trackside listener. It is easier to use this technique to take measurements than to excite the noise in the model train, Janssens told New Scientist. The main source of noise from a train, says Janssens, is the contact between the steel rail and the train wheels. Particular culprits are freight wagons and older passenger coaches with clasp brakes—iron pads that clamp the sides of the wheels to stop the train. The wheels have to withstand prolonged braking. “For trains going over the Alps, you have to brake for tens of kilometres,” says Janssens. Although the wheels are circular when they emerge from the workshop, repeated braking makes them almost polygonal, with a slight flattening every 6 to 8 centimetres of the circumference. The rails transfer the sound to the sleepers and the wheels pass the sound on to the rest of the vehicle. Each of these sources has its own characteristic frequency spectrum (see Graph). “If you want to do something to cut noise you have to tackle all four sources,” says Janssens. Although some European railways have cut noise with large barriers at the edge of the line, these are intrusive and expensive. TNO, in association with the Netherlands railway, wants to find ways of cutting noise that would be compatible with existing trains and tracks. So TNO turned to some old data from noise reduction experiments carried out by British Rail, Britain’s former state-owned railway operator. The TNO researchers found that a 60-centimetre high barrier close to the track, coupled with a housing that partially encloses the wheels and bogies, would reduce train noise by between 6 and 8 decibels. The barrier would need to be about 50 centimetres from the edge of the track and made of a sound-absorbing material, while the acoustic hood could be made of metal or plastic. The findings are likely to be particularly interesting to European railways, where noise is a common source of complaint. “We would seriously look at anything that would mitigate noise especially to people living close to the track,” says a spokeswoman for Railtrack,