The mathematical theory of geometric diffraction with a circular elastic‐wave source and receiver in a fluid half‐space shows that the phase at maximum received amplitude does not correspond to the plane‐wave value, i.e., ωl−kz. The excess phase 〈φ〉 plays the rôle of phase shift in an electrical network. Therefore, the value of −d〈φ〉/dω can be used to estimate the delay, due to geometrical diffraction in arrival of the bulk of a pulse. Under usual pulse echo measurement conditions, the velocity measured will seldom be 0.01% smaller thanv0, the true plane‐wave phase velocity. It is assumed here that measurements are made to parts of the pulse where steady‐state geometrical diffraction is occurring. It is further assumed that transient diffraction effects exist for only a small fraction of the pulse. The possibility of diffraction contributing to changes in velocity caused by an external parameter (strain, magnetic field, etc.) is discussed.