Determination of Acoustic Properties of Railway Ballast

As a porous material, railway ballast has absorptive properties that are important for the noise produced by the railway system. To understand its acoustic behaviour, measurements are presented on 1:5 scale ballast. Following direct measurements of the flow resistivity and porosity, transfer function methods are used in a vertical impedance tube to determine the surface impedance and absorption coefficient of different depths of ballast. The complex wavenumber is determined through transfer function measurements within the medium in the vertical tube. The Johnson-Champoux-Allard model for porous materials is fitted to these measurements, allowing determination of the remaining parameters, thermal and viscous characteristic lengths and tortuosity. The parameters are finally adopted to predict the diffuse field absorption coefficient of the scale ballast and full-size ballast. These are compared with measurements, showing acceptable agreement.