  We have performed an approximate ab initio calculation of vibrational
properties of hydrogenated amorphous silicon (a-Si:H) using a molecular
dynamics method. A 216 atom model for pure amorphous silicon (a-Si) has been
employed as a starting point for our a-Si:H models with voids that were made by
removing a cluster of silicon atoms out of the bulk and terminating the
resulting dangling bonds with hydrogens.
  Our calculation shows that the presence of voids leads to localized low
energy (30-50 cm^{-1}) states in the vibrational spectrum of the system. The
nature and localization properties of these states are analyzed by various
visualization techniques.
