IXS compressional sound velocity as a function of density. Blue crossed circles (Fiquet et al., 2001); green dot centre circles (Antonangeli et al., 2004b); red open circles (Antonangeli et al., 2012). Lines are linear fits to the distinct experimental data sets. The star is the ultrasonic determination reported in Mao et al. (1998). PREM is shown as crosses.