  Several theoretical approaches to disordered media predict that acoustic
waves should undergo a quartic increase in their attenuation coefficient with
increasing frequency in the sub-terahertz region. Such Rayleigh-type scattering
would be related to the anomalous low-temperature plateau in the thermal
conductivity and to the so-called boson peak, i.e. an excess of vibrational
modes above the Debye density of states at around 1 THz. Brillouin scattering
of light allows the measurement of sound absorption and velocity dispersion up
to about 0.1 THz while inelastic x-ray scattering is limited to frequencies
larger than about 1 THz. We take advantage of the advent of ultrafast optical
techniques to explore the acoustical properties of amorphous SiO2 layers in the
difficult but crucial frequency region within this gap. A quartic scaling law
with frequency is clearly revealed between 0.2 and 0.9 THz, which is further
shown to be independent of temperature. This strongly damped regime is
accompanied by a decrease in the sound velocity already starting from about 0.5
THz, in line with theories. Our study assists to clarify the anomalous
acoustical properties in glasses at frequencies entering the boson peak region.
