Symmetry and asymmetry in guitar bracings and related sound tuning parameters
The low-frequency range strongly determines sound quality in classical Spanish guitars. In the range above the fundamental air mode A0 at roughly 100 Hz, and below 400 Hz, most guitars reveal one or two main plate/body resonances, only few guitars have up to four resonances. While guitar models come along with similar geometrical dimensions, the existence of multiple resonances traces back to construction: the symmetry/asymmetry of the bracing, the coupling of the top and the back plate, and the geometrical relation of bridge mass to the braces’ attack points. The sparse and very asymmetrical bracing in a guitar from Faustino Conde encourages the development of analytical models. A simple ‘top-plate-only’ model yields analytical solutions for the mentioned low frequency range. Another more complete ‘top-and-back-plate’ model cannot be solved by analysis but yields numerical solutions. Both models agree with findings on the guitar and with an experimental setup. The coupling between resonances grows with asymmetry, yielding modulated, lively sounds in asymmetrically braced guitars. All guitars could have multiple resonances independent from aspects of symmetry, but the parametrical space of related factors is wide and sound tuning is more delicate in asymmetrically braced guitars than it is in symmetrically braced guitars.