Frequency dependent mechanical properties of violin varnishes and their impact on vibro-mechanical tonewood properties

Abstract

Violin varnishes influence the vibrational properties of tonewood. However, the frequency dependence of the varnish influence and mechanical properties of typical varnishes has received little attention. The viscoelastic properties of various violin varnish materials over the audible frequency range were characterized by dynamic mechanical analysis. The properties of the studied varnishes showed comparable frequency dependencies. For all varnishes, E increased and tan(δ) decreased with increasing frequency. The results were in good agreement with an analytical mechanical model, which was used for additional numerical FEM calculations. The approach of numerically determining varnish-induced changes in the vibrational properties on basis of the individual wood and varnish properties was confirmed through comparison with experimental results obtained in an earlier study. The latter procedure was subsequently used to analyse varnish-induced changes in the eigenfrequencies of a violin soundboard. The results revealed that the frequency dependence of the varnish properties determined the specific influence of varnishes on the vibrational properties of tonewood, which should be taken into account when assessing the impact of varnishes.

Keywords

• Violin varnish
• Viscoelasticity
• Frequency dependence
• Dynamic mechanical analysis (DMA)
• Numerical modal analysis
• FEM