  We experimentally study the scattering of guided matter waves on an
amplitude-modulated optical lattice. We observe different types of
frequency-dependent dips in the asymptotic output density distribution. Their
positions are compared quantitatively with numerical simulations. A
semiclassical model that combines \emph{local} Floquet-Bloch bands analysis and
Landau-Zener transitions provides a simple picture of the observed phenomena in
terms of elementary \emph{Floquet photon} absorption-emission processes and
envelope-induced reflections. Finally, we propose and demonstrate the use of
this technique with a bichromatic modulation to design a tunable sub-recoil
velocity filter. Such a filter can be transposed to all species since it does
not rely on a specific internal level configuration of the atoms.
