Résumé
Silica aerogels with different microstructures were synthesized by one-step and two-step process. Acoustic waves in these materials were studied by Brillouin scattering. As expected, propagative phonons are observed for high-density samples. However, for some two-step samples an asymmetric profile of the Brillouin line is found which was not observed in previous investigations. This behaviour is attributed to some relaxation process coupled to the phonons. After an oxidation heat treatment these relaxations disappear. For low-density samples, the crossover to fracton modes is investigated. Values of the sound velocities,
ν
0, are compared for different series of samples and the data are correlated to structural information.