We calculated the acoustic velocity contrasts between perovskite and postperovskite in MgSiO3, FeSiO3 and Al2O3 compounds, using the planewave pseudopotential method for a pressure range of the Earth's lower mantle. Both FeSiO3 and Al2O3 pv and ppv have about 20% smaller shear moduli than Mg-phases at deep mantle pressures, though the bulk moduli are comparable, consistent with previous studies. For the relative variations of shear to compressional velocities, bulk to shear velocities and density to shear velocity at constant pressure due to variations in Fe and Al content up to 20 mol%, we find RS/P of 1.5~2.2, RΦ/S of 0.1~0.4 and Rρ/S of -1.0~-0.2. Both Fe and Al influence elasticity of ppv more than that of pv. Therefore velocity contrasts across the postperovskite phase change would decrease with increasing Fe and Al content. We find that positive velocity variations are hardly produced in pv with the MORB composition.