Combined Experimental and Numerical Investigation of Vibro-Mechanical Properties of Varnished Wood for Stringed Instruments

Abstract

The obvious purpose of varnishing a stringed instrument is the protection against wear and relative humidity changes as well as enhancing its aesthetic appearance. Besides, it is known that varnishing changes the acoustic properties of the wood [1]. Unlike studies on the chemical compositions of old varnishes, in searching for the “secret” of well-known historical instruments [2], the vibro-mechanical properties of varnished wood have been less studied. Nevertheless, the vibrational and mechanical properties of wood are influenced after varnishing [3]. Vibrational experiments show that varnishing changes the specific stiffness and internal damping of wood. Therefore, varnishing also influences acoustic properties that are commonly used to evaluate the sound quality of an instrument such as speed of sound, characteristic impedance, sound radiation coefficient and emission ratio.

For the investigation of these temperature and humidity dependent properties, we propose a combined experimental and numerical analysis approach. X-ray microtomography images reveal density differences at the interface of wood and varnish, depending on its composition and application procedure. In combination with neutron imaging, this approach is used to evaluate the extent of varnish penetration and that of other particles (e.g. pumice powder) into the porous wood structure. Further, we study the vibrational and mechanical properties at the material to instrument scale. Additionally, numerical analysis improves the material property determination and helps evaluating the combination of different wood species and varnishes. Our aim is that these results will contribute to the scientific understanding of the variations in mechanics, vibrations and sorptivity of varnished wood and facilitate the design of higher quality varnish compositions and treatment processes.