Measuring the low-frequency response of an acoustic guitar

Abstract

The low-frequency response of an acoustic guitar is strongly influenced by the combined behavior of the air cavity and the top plate. The sound hole–air cavity resonance (often referred to as the Helmholtz resonance) interacts with the first elastic mode of the top plate creating a coupled oscillator with two resonance frequencies that are shifted away from the frequencies of the two original, uncoupled oscillators. This effect has been modeled using finite elements for the top plate and boundary elements for the air cavity with rigid sides and back and no strings. The natural frequencies of both the individual and combined oscillators will be compared to measurements. The modes and natural frequencies of the coupled and uncoupled top plate were obtained using experimental modal analysis. An array of electret microphones was used above the sound hole to compare the phase of acoustic pressure with that of the top plate vibration. The uncoupled acoustic mode was then measured after placing sandbags on the top plate to restrict its motion. The measurements confirm the mode shapes, natural frequencies, and damping of the model.