Effect of thermal modifications on the anisotropic acoustic properties of spruce and their relevance for stringed instruments

Thermal modification alters wood properties and helps improving dimensional stability against humidity changes, making it a promising treatment for wood used in stringed instrument construction. Its effects on the anisotropic mechanical and acoustical properties, however, remain incompletely understood. In this study, spruce samples were analyzed to quantify changes in physical, mechanical, and acoustical properties following thermal modification at 160 °C. Density, orthotropic viscoelastic constants, and damping were measured using non-destructive techniques, while microstructural effects were examined via x-ray microtomography. Finite element analysis of a guitar soundboard assessed impacts on eigenfrequencies, mode shapes, and acoustic radiation. Results indicate that thermal treatment causes a slight reduction in density, a modest increase in longitudinal and radial stiffness, and a significant decrease in damping, leading to enhanced radiation ratio and acoustic conversion efficiency. Microstructural observations suggest that removal of resins and volatile extractives may underlie these changes. Finite element analysis shows that eigenmodes shape remain largely unchanged, with only minor shifts in eigenfrequencies. The combination of improved radiation efficiency and reduced damping could influence sound radiation and string-to-soundboard coupling.