Abstract
We study propagation and attenuation of sound waves in silica with two complementary experimental techniques, Brillouin light scattering and picosecond acoustics. Analysis of new accurate Brillouin data obtained in a large temperature range (4-1300 K), supplemented with literature data on a broad frequency range, shows that internal friction results from two main mechanisms: the interaction of sound with relaxing defects and with modes of the thermal bath (anharmonicity). Brillouin spectroscopy at high pressure (0-6 GPa) reveals the existence of a structural modification around 2~GPa in silica at room temperature. High frequency sound is observed for the first time around 250~GHz in silica with a new picosecond acoustic technique. These data demonstrates that the sound attenuation is still dominated by the anharmonicity at these mesoscopic wavelengths. The relation with a strong scattering regime of the attenuation at THz frequencies is discussed.