  One of the key requirements for good thermoelectric materials is a low
lattice thermal conductivity. Here we present a combined neutron scattering and
theoretical investigation of the lattice dynamics in the type I clathrate
system Ba-Ge-Ni, which fulfills this requirement. We observe a strong
hybridization between phonons of the Ba guest atoms and acoustic phonons of the
Ge-Ni host structure over a wide region of the Brillouin zone which is in
contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host
cages. It occurs without a strong decrease of the acoustic phonon lifetime
which contradicts the usual assumption of strong anharmonic phonon--phonon
scattering processes. Within the framework of ab-intio density functional
theory calculations we interpret these hybridizations as a series of an
ti-crossings which act as a low pass filter, preventing the propagation of
acoustic phonons. To highlight the effect of such a phononic low pass filter on
the thermal transport, we compute the contribution of acoustic phonons to the
thermal conductivity of Ba$_8$Ge$_{40}$Ni$_{6}$ and compare it to those of pure
Ge and a Ge$_{46}$ empty-cage model system.
