Sound tuning in asymmetrically braced guitars

Abstract

Guitar bracings generally follow a symmetrical design in contrast to the asymmetrical assembly of the sound post and bass bar in violins. However, one of Conde's progressive constructions to combine the classical Spanish guitar with the flamenco guitar is based on an asymmetrical bracing. Mobility measurements on a well-preserved exemplar reveal four strong resonances between air modes A0 and A1, whereas other guitars reveal only one or two resonances in the same region. These are equivalent to the intensively discussed signature modes in violins, which are believed to prominently determine sound quality. The paper develops a model to explain the existence of the four resonances and how these can be tuned in terms of frequency and intensity. The tuning parameters for three of these resonances comprise (i) a measure of asymmetry, (ii) a measure of how wide the bridge overhangs relevant braces, (iii) a measure of coupling between the top and back plates, and finally (iv) the tuning between the top and back plates. Data from modeling are referenced against a lumped experimental setup and measured data from the guitar. Asymmetrical bracing constructions leverage mutual mode coupling and increase radiation efficiency, but they are more difficult to tune than symmetrical constructions.