Nilsson C., Jones C.

In tramway tracks and some new kinds of mainline railway track, the rail is embedded with elastomeric material in a concrete slab. Embedded rails possess noise radiation characteristics different from those of non-embedded rails. Largely, this is due to a change from line-dipole type radiation to line-monopole behaviour. There are also effects of the waves propagating along the rail both in the rail itself and in the embedding material. Several of these waves exhibit strong attenuation with distance and shorter wavelengths than those found in non-embedded rails. The sound radiation behaviour of conventional tracks can be modelled adequately with adjustments to two-dimensional modelling schemes but to study the radiation behaviour in detail for embedded rail a three-dimensional approach is necessary. Normal three-dimensional numerical models are very costly and difficult to apply to this problem involving infinite wave propagation. Here, a new method, based on a combined waveguide finite element and boundary element models in the wavenumber domain is presented. This method finds decaying waves in an infinite rail using a waveguide finite element model. These waves are subsequently projected onto a number of two-dimensional boundary element models, each with a different wavenumber imposed in the longitudinal direction. The radiated power is calculated in the wavenumber domain, whereas the full three-dimensional response can be recovered via a reverse Fourier transform. A study of the radiation characteristics of some embedded rail cases is presented showing important differences in behaviour compared to unembedded rails. In particular the smaller bending stiffness of tram rails compared to main-line rail gives the potential of a acoustic short circuiting effect at some frequencies that might be used as a noise reduction mechanism.