  The attenuation of longitudinal acoustic phonons up to frequencies nearing
250 GHz is measured in vitreous silica with a picosecond optical technique.
Taking advantage of interferences on the probe beam, difficulties encountered
in early pioneering experiments are alleviated. Sound damping at 250 GHz and
room temperature is consistent with relaxation dominated by anharmonic
interactions with the thermal bath, extending optical Brillouin scattering
data. Our result is at variance with claims of a recent deep-UV experiment
which reported a rapid damping increase beyond 100 GHz. A comprehensive picture
of the frequency dependence of sound attenuation in $v$-SiO$_2$ can be
proposed.
