The directivity of noise radiated by a railway wheel in situ

The most important noise source from railways is rolling noise, which is produced by vibration of the track and wheel, induced by their combined surface roughness. The component of noise radiated by the wheel is generally the greatest above 2 kHz and increases in overall importance as the train speed increases. The current models used to predict the sound radiation from the wheel consider this to be located in free space; in practice, however, the rail beneath the wheel, the ground in close proximity, the presence of the bogie frame around it and the train body above it can modify the wheel radiation. This paper investigates the influence of these factors on the sound power and the directivity of the wheel in both vertical and horizontal planes. The sound radiation of the wheel is analysed by using a combination of the Finite Element Method for vibration and the acoustic Boundary Element Method with Adaptive Order in three dimensions. The geometries of the rail, ground, bogie frame and the train body are introduced in the acoustic model to investigate their influence on the sound radiation from the wheel. It is shown that these nearby structures affect the sound power of the wheel only at low frequency, but they have a significant influence on the directivity of the sound in the whole frequency range. Of the various structures considered, the presence of the rail and the bogie frame has little effect on the wheel radiation, whereas the ground and car body have a greater effect. Finally, new approximate expressions are developed for the wheel directivity in the presence of the train body that can be used in engineering models.