  The dynamic structure factor of lithium-diborate glass has been measured at
several values of the momentum transfer $Q$ using high resolution inelastic
x-ray scattering. Much attention has been devoted to the low $Q$-range, below
the observed Ioffe-Regel crossover \qco{}$\simeq$ 2.1 nm$^{-1}$. We find that
below \qco{}, the linewidth of longitudinal acoustic waves increases with a
high power of either $Q$, or of the frequency $\Omega$, up to the crossover
frequency \OMco{} $\simeq$ 9 meV that nearly coincides with the center of the
boson peak. This new finding strongly supports the view that resonance and
hybridization of acoustic waves with a distribution of rather local low
frequency modes forming the boson peak is responsible for the end of acoustic
branches in strong glasses. Further, we present high resolution Brillouin
light-scattering data obtained at much lower frequencies on the same sample.
These clearly rule out a simple $\Omega^2$-dependence of the acoustic damping
over the entire frequency range.
