The longitudinal sound velocity vL of pure SiO2 aerogels with densities in the range of 5 to 500 kg m−3, either made from TEOS (tetraethoxysilane), TMOS (tetramethoxysilane), or waterglass was measured. The variation of vL with density ρ for evacuated aerogels is more complex than described earlier: The simple scaling law vL∝ρα, with α=1.3, only holds for ρ>100 kg m−3; below this density α≊0.8 is valid. Data are also collected for sintered SiO2 aerogels. For specimens with densities around 100 kg m−3 or below, sound velocity measurements under variation of air pressure have also been performed. The dependence of the sound velocity on air pressure can be correctly described by a simple theory involving quasi-isothermal compression of the gas within the pores. From these findings a reliable method to determine the macroscopic aerogel density is derived.