Plate vibrations and resonances of classical and folk guitars

Abstract

It has been shown recently [G. Caldersmith, J. Acoust. Soc. Am. 63, 1566 (1978)] that the two lowest resonances of a guitar act as a resonance doublet whose frequencies are spread apart by acoustic coupling between the top plate and the air in the sound hole. We have investigated the influence on these coupled modes, as well as on the higher modes of vibration, of different patterns of bracing used by guitar makers. In a fan-braced classical guitar, for example, the phase angle between the sound pressure near the top plate and that in the sound hole changes by nearly 180° as one passes through the lowest resonance [I. M. Firth, J. Acoust. Soc. Am. 61, 588 (1977)]. The phase change observed in cross-braced folk guitars tends to be smaller, however. Adding transverse bracing raises the frequency of all the resonances and changes the coupling between “air” and “wood” modes of vibration.