Are high frequency acoustic modes in glasses dominated by strong scattering or by lifetime broadening?

The attenuation of acoustic phonons in glasses increases with their frequen cy Omega owing either to internal friction or to scattering by static and d ynamical disorder. Friction leads to a finite lifetime, i.e. to homogeneous broadening. Conversely, scattering does not prevent excitations from remai ning long-lived, but plane waves become inhomogeneously broadened. In this paper we show that, at very high frequencies Omega similar to 1THz, acousti c modes enter a regime dominated by strong scattering in which they cease t o propagate as plane waves. This is consistent with all available informati on, whether macroscopic, simulation, or spectroscopic.