A precise relation between the acoustic index and the amount of doped Ge in silica is established using both energy and wave dispersive spectroscopies on preform pieces and stimulated Brillouin scattering in optical fibers. It is shown that the relation is in very good agreement with a simple model involving only GeO(2) and SiO(2) bulk parameters. The change in the acoustic index is found to be 3.97x10(-3)/wt. %GeO(2) at 1550 nm.