The frequency response of sound pressure and top plate mobility is studied around the two first resonances of the guitar. These resonances are shown to result from a coupling between the fundamental top plate mode and the Helmholtz resonance of the cavity. A simple model is proposed for low- frequency guitar function. The model predicts frequency responce of sound pressure and top plate mobility which are in close quantitative agreement with experimental responses. The absolute sound pressure level and mobility level are predicted to within a few decibels, and the equivalent piston area of the top plate is determined. All parameters of this model can directly be derived from measurements of the frequencies of the two first resonances and of the cavity Helmholtz resonance. The Helmholtz resonance is found as the antiresonance in the spectrum of top plate mobility.