Survey and evaluation of classical guitar soundboard design methods with finite element analysis

Designing the soundboards of guitars based on an acoustical and structural approach would ideally allow for the realization of instruments with reproducible acoustical properties and structural stability. This task is challenging because wood, the most common material used for this purpose, is a natural material with variable properties and building instruments using strict geometrical tolerances alone does not ensure reproducible results. Several approaches have been developed so far, some based on tradition and, more recently, on measurement of material properties and computer optimization. In this article, some approaches used to design classical guitar soundboards are reviewed and evaluated. An original builder-friendly method, based on simple definitions of mass and stiffness, is also considered. Finite element analysis is used to evaluate their robustness against variability in wood density and orthotropic stiffness by using the experimentally measured properties of 29 spruce specimens. The results are assessed by comparing the coefficient of variation of acoustically relevant parameters (eigenmodes, eigenfrequencies, mass, and monopole mobility) as well as structurally significant ones (mechanical stiffness of the soundboard and bridge rotation angle). Additionally, the correlation between sound radiation coefficient and monopole mobility is examined. Finally, the practical applicability of these methods is evaluated and discussed.