Electric field heating of electrons in thin metal films deposited on silicon substrates has been used to determine the phonon transmission across the metal-silicon interface. A simple rate equation model is used to fit the electron temperature as a function of applied field. From the fit the authors have established that the phonon transmission deviates from the usual acoustic mismatch model. The authors find the phonon escape to comprise two components: ballistic escape of phonons from the film and strong scattering close to the interface.