  A 1D mechanical model for nanogranular films, based on a structural
interface, is here presented. The analytical dispersion relation for the
frequency and lifetimes of the acoustics breathing modes is obtained in terms
of the interface layer thickness and porosity. The model is successfully
benchmarked both against 3D Finite Element Method simulations and experimental
photoacoustic data on a paradigmatic system available from the literature. A
simpler 1D model, based on an homogenized interface, is also presented and its
limitations and pitfalls discussed at the light of the more sophisticated
pillar model. The pillar model captures the relevant physics responsible for
acoustic dissipation at a disordered interface. Furthermore, the present
findings furnish to the experimentalist an easy-to-adopt, benchmarked
analytical tool to extract the interface layer physical parameters upon fitting
of the acoustic data. The model is scale invariant and may be deployed, other
than the case of granular materials, where a patched interface is involved.
