Until now, the micromechanical wing crack model of Ashby & Sammis 1990 describing the brittle field of rocks has only been used for failure predictions. However, it gives relevant information regarding the crack opening process, so this model is extended to compute elastic wave speeds according to the cracked solid theory of Kachanov. Laboratory experiments at various confining pressures on Westerly granite were realised to fit its parameters and test this extension. A decent correlation between modelled and measured velocities shows that this model might be used for seismic velocities and crack-induced anisotropy computation. Furthermore, the validity of the model is discussed through an energy budget and comparisons with in-situ measurements of velocity changes.