The work reported in this thesis addresses the problem of structure-borne sound transmission from impacts on lightweight stairs. The primary aim was to provide a laboratory method for characterisation of lightweight stairs as structure-borne sound sources, which will give input data for prediction of the sound transmission in heavyweight building situations. By treating the stair system, combined with impact source(s), as an active component, available methods for active sources could be adapted. The component powers of a timber staircase attached to a solid wall in a staircase test facility have been determined in-situ by use of a reciprocal method. It was shown that the force perpendicular to the wall surface is dominant, moments can be neglected. The force induced power can be predicted from contact free velocity and mobility or by the blocked force as stairs constitute high mobility sources in heavyweight buildings. A practical characterisation is proposed that is based on the reception plate method. It is demonstrated that real walls and floors can be used as reception plates along with a power calibration that circumvents problems in estimating the plate mass, mean squared velocity and total loss factor for non-isolated reception plates. The sound transmission is predicted using EN 12354 and it is confirmed that the prediction gives values within acceptable engineering accuracy. A deterministic model that accounts for the modal coupling of structure and room is used to predict the sound transmission at low frequencies. For the case considered, a major difficulty was found in the modelling of the wall vibration field, mainly due to the boundary conditions that do not correspond to idealised conditions, such as pinned or free edges.