Influence of the bridge on the vibrations of the top plate of a classical guitar

Abstract

In this work the effect of two different bridge configurations on the vibrations of the top plate of a classical guitar is presented. Experimental harmonic analysis and visualization techniques, in addition to detailed damped simulations using the finite element method, were used to obtain mobility functions and operating deflection shapes of a top plate. The mobility functions were obtained in the following sequence. First the mobility functions were obtained on the top plate without a bridge attached. Then a bridge was glued to the top plate and new measurements were made. Finally the already attached bridge was cross-cut in situ without detaching it from the top plate, and the measurements were repeated. Those specific designs were chosen on the grounds of simplicity of construction both experimentally and in FEM simulations (no particular preference for those designs is implied). The assembly and the specific design of the bridge have shown to have considerable influence on the response of the top plate regarding the mode shapes above 300Hz. Depending on the geometry of the bridge, its deflections can either be comparable to the maximum deflections of the top plate or can have amplitudes so small that the bridge effectively creates a nodal zone on the plate vibrations. This suggests that the shape, the stiffness, and the mass properties of the bridge may play a role in the sound quality of the instrument.