The application of photoacoustic spectroscopy (PAS) to a-Si:H films prepared by ion beam deposition (IBD) is described. The PAS spectra are found to be characterized by a subgap absorption tail superimposed on the Urbach edge. The systematic changes in these features with ion beam hydrogen composition are reported. Films made with a low concentration of hydrogen show a strong excess absorption tail whereas this feature is significantly reduced in films fabricated using a higher concentration of hydrogen. The PAS data have been used to estimate the density of defect states responsible for the subgap tail. These results are shown to be consistent with the conclusions deduced from photoconductivity and esr measurements and point to two hydrogenation regimes with different gap state structures in the IBD material.