Low-frequency resonances in acoustic guitar bodies significantly affect the perceptual impression of the sound and are, therefore, essential factors in guitar design. In the lowest frequency range, the body behaves as a coupled system comprising (i) Helmholtz-type resonance of the sound hole, (ii) monopole vibration of the top plate, and (iii) monopole vibration of the back plate. This behavior is conventionally modeled using a simplified three-degrees-of-freedom (3-DOF) structural-acoustic system, where all three components are represented as masses coupled via the air spring inside the body. In this study, the 3-DOF model was applied to several acoustic guitars. The modeling enabled each instrument to be described using a small set of simplified parameters and eigenvectors associated with the three components. Parameter estimation was performed via curve fitting based on the gradient descent method. Furthermore, the relationship between the low-frequency characteristics and the simplified parameters of the 3-DOF model was examined.
Kohei Mitaka (2025) studied this question.