Thermal treatment, moisture content, and vibration direction’s effect on dynamic properties of spruce wood (Picea abies) for musical instruments

The article examines the main and interactive effects of thermal modification (TM), relative humidity (RH), and direction of vibration on the density, modulus of elasticity (MOE) and shear modulus of spruce wood. Samples were thermally modified at 180, 200, and 230 °C and then equilibrated at 20%, 44%, 76%, and 88% RH. The MOE in the longitudinal direction and the shear moduli GLR and GLT were calculated from the natural frequencies of the first three lateral vibration modes of free-free specimen. EL-R and GLR were determined from the vibration frequencies when the specimens vibrated laterally in the longitudinal-radial plane, while EL-T and GLT were determined from vibration frequencies when the specimens vibrated in longitudinal-tangential plane. Density, MOE, and shear moduli decreased at the highest TM level by averages of 16%, 9.8% and 9.7%, respectively. Acoustic coefficients such as the sound velocity (c), the sound radiation coefficient (R) and the characteristic acoustic impedance (ACE) were determined. On average, c and R increased as a function of RH from 1% to 3% and from 10% to 15%, respectively, while ACE decreased from 7% to 13%. The test material was classified for making soundboards for musical instruments.