  The lattice dynamics of LaFeAsO_{1-x}F_{x} (x=0, 0.1) and PrFeAsO_{1-y}
(y~0.1) are investigated using inelastic x-ray scattering and ab-initio
calculation. Measurements of powder samples provide an approximation to the
phonon DOS, while dispersion is measured from a single crystal of
PrFeAsO_{1-y}. A model that agrees reasonably well with all of the data at room
temperature is built from results of ab-initio calculations by softening the
strength of the Fe-As bond by 30%.
