The picture below shows noise radiation from a regional train picked up by a microphone array. The train passed the array at 200 km/h, direction right-to-left. Main noise sources are indicated by red color: wheels, in particular front wheels, and pantograph.
At train speeds below 250 km/h, mechanical defects on wheel/rail running surfaces cause most of the railway noise. Defects may be small scale irregularities, caused by tread brakes acting directly on the wheel running surface, as well as large scale (several centimetres) wheel flats. A wheel flat may arise when the friction coefficient between rail and wheel is very low, so the wheel does no longer roll on the rail, but only slides along. When the wheel slides, a flat grows. This is a problem in particular during autumn, since leaves falling from trees form a greasy paste on the rail running surface. Another cause for a wheel flat may be a blocked brake. Each time a flat hits the rail, a great amount of noise is emitted. A wheel flat increases noise as well as costs to protect the environment from the noise. Also, it wears down the track, increasing maintenance costs. Vast amounts are yearly spent on noise barriers along railway lines as well as on wheel and rail maintenance. Sound videos are effective tools to monitor noise from railway wheels and to detect defect wheels at an early stage.
At train speeds above 250 km/h, wind turbulence around the pantograph causes most noise from the train. At the same time, normal height track side noise barriers have no effect, since the pantograph is located on the train roof. Therefore, understanding noise generation is essential in order to construct a low noise pantograph. The sound video below of sound radiation at 800 Hz from a train at 200 km/h gives insight into the generation process. Noise generation around the pantograph seems to be a complex process, since the source location is obviously moving around between different parts of the pantograph.