  Making use of the classical second moment sum rule, it is possible to convert
a series of constant-Q x-ray Brillouin scattering scans (Q momentum transfer)
into a series of constant frequency scans over the measured $Q$ range. The
method is applied to literature results for the phonon dispersion in liquid
vitreous silica and in glassy polybutadiene. It turns out that the constant
frequency scans are again well fitted by the damped harmonic oscillator
function, but now in terms of a Q-independent phonon damping depending
exclusively on the frequency. At low frequency, the sound velocity and the
damping of both evaluations agree, but at higher frequencies one gets
significant differences. The results in silica suggest a new interpretation of
x-ray Brillouin data in terms of a strong mixing of longitudinal and transverse
phonons toward higher frequencies. The results in polybutadiene enlighten the
crossover from Brillouin to Umklapp scattering.
