Frequency Response Evaluation of Guitar Bodies with Different Bracing Systems

Mircea Mihălcică,Sorin Vlase,Mariana D Stanciu

doi:10.3390/sym12050795

Abstract

Wood is a natural composite, having a porous structure, with a complex elastic symmetry specific to orthotropic solid, influenced by three mutually perpendicular planes of elastic symmetry. The classical guitar is obtained from different wooden species, each of them having their own elastic properties and, as a whole, forming a lignocellulosic composite structure. Generally, some constructive parts of the classical guitar body are based on symmetry, starting from the structural features of wooden plates, which are symmetrically cut, and some patterns of the stiffening bars. The other elements, such as the strings system, are not symmetric. This study aims to evaluate the frequency responses of the guitar body as a symmetrical mechanical system from constructive points of view. Because theoretical results (analytic and numeric) regarding the symmetrical systems cannot be applied to quasi-symmetric systems, the dynamic response was analyzed from experiments performed on four types of classical guitar body (without neck), different from each other by the pattern of stiffening bars placed inside of the top plate. The experiments were performed using a Brüel&Kjær mini-shaker to excite the structure, and the signal was captured with accelerometers. The symmetric behavior of coupled plates from the guitar body was noticed in the case of an applied dynamic force of 110 Hz and 440 Hz, but in the case of 146 Hz, 588 Hz, 720 Hz, quasi skew symmetrical modes were recorded.

Highlights

The frequency response of the lignocelluloses plates with different structural and material characteristics offers useful information about the way in which the frequency of the excitation force overlaps with the plate’s normal frequency
Because theoretical results regarding the symmetrical systems cannot be applied to quasi-symmetric systems, the dynamic response was analyzed from experiments performed on four types of classical guitar body, different from each other by the pattern of stiffening bars placed inside of the top plate
The symmetric behavior of coupled plates from the guitar body was noticed in the case of an applied dynamic force of 110 Hz and 440 Hz, but in the case of 146 Hz, 588 Hz, 720 Hz, quasi skew symmetrical modes were recorded

Summary

Introduction

The frequency response of the lignocelluloses plates with different structural and material characteristics offers useful information about the way in which the frequency of the excitation force overlaps with the plate’s normal frequency. One of the most important characteristics of the stringed instruments is the possibility of resonating at excitation frequency of the strings. On this line, the amplification box made up of the ligno-cellulose plates takes over this function, being the reason of the necessity of analyzing the frequency response of the plates with FEM. The musical instruments with strings, such as the guitar, represent a complex mechanical structure that has seen numerous improvements over time, especially in case of bracing patterns of the soundboard, which has a significant effect on its mode shapes [1]. Several studies have analyzed how different bracing patterns change the modal properties of the top plate using Chladni’s method, through numerical or Symmetry 2020, 12, 795; doi:10.3390/sym12050795 www.mdpi.com/journal/symmetry

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