We report on low-temperature (77 K) far infrared attenuated total reflection (ATR) spectroscopic measurements on a short period GaAs/AlAs superlattice, and discuss the optics of the modes observed. We assess the significance of the ATR technique for determining superlattice phonon parameters. We model the results using a linear chain model that incorporates the effects of interface roughness; it appears that this model is correctly modelling the phonon response.