Towards Dynamic Copies of Guitars – Shape Optimization vs. Material Variability

EasyChair Preprint no. 13451

Abstract

Although many good luthiers exist and they build excellent instruments some instruments even stick out of these high quality instruments and some luthiers, like Antonio Torres or Antonio Stradivari for example, have received almost irrefutable iconic status. To many musicians their instruments are unrivaled and real myths have grown around them. We present a methodology that allows the replication of reference instruments based on their vibrational characteristics rather than solely on their geometry. It allows building instruments that sound alike rather than merely look the same as geometric copies do. This is achieved by compensating unavoidable differences in vibrational properties between reference and replica caused by the natural variability of the wood through specific geometric adaptions. The challenge of such an approach is to reliably and non-destructively identify material parameters of the reference in order to be able to predict the necessary geometry modifications before the replica is built. As summarized in this contribution we first focus on matching the vibrational response of individual elements of a guitar, namely the soundboard. Two soundboards made from spruce with initially equal geometry and Torres bracing were manufactured. One is defined as reference soundboard while the other represents the copy to which eventually geometric modifications are applied. The eigenfrequencies and eigenmodes, identified with experimental modal analysis, are used to compare the soundboards. Although the same tonewood from the same batch was used the relative difference of the first few eigenfrequencies is about 5% initially due to the natural variability of the tonewood. A detailed and experimentally validated numerical finite element model of the soundboards is developed to predict suitable geometry modifications that can reduce the difference of eigenfrequencies between the soundboards.

Keyphrases: dynamics of guitars, parametric model order reduction, shape optimization

@Booklet{EasyChair:13451,
  author = {Alexander Brauchler and Pierfrancesco Cillo and Pascal Ziegler and Peter Eberhard},
  title = {Towards Dynamic Copies of Guitars – Shape Optimization vs. Material Variability},
  howpublished = {EasyChair Preprint no. 13451},

  year = {EasyChair, 2024}}