Résumé
The attenuation of acoustic phonons in glasses increases with their frequency Ω growing either to internal friction or to scattering by static and dynamical disorder. Friction leads to a finite lifetime, i.e. to
homogeneous
broadening. Conversely, scattering does not prevent excitations from remaining long-lived, but piane waves become
inhomogeneously
broadened. In this paper we show that, at very high frequencies Ω ∼ 1THz, acoustic modes enter a regime dominated by strong scattering in which they cease to propagate as piane waves. This is consistent with all available information, whether macroscopic, simulation, or spectroscopic.