Simultaneous non-destructive identification of multiple elastic and damping properties of spruce tonewood to improve grading

The tonewoods used in musical instruments are finely selected and are subject to current and upcoming availability issues. Wood grading assesses the quality of the tonewood and depends on both subjective and objective criteria. Traditionally, grading criteria based on mechanical properties consider mainly the longitudinal direction. This paper investigates the elastic and damping properties of spruce wood used in the making of acoustical guitars and violins. In this context, the mechanical grading properties will be studied in multiple directions. A non-destructive vibrational characterisation method is used to simultaneously identify at least six mechanical properties of wood and leads to the determination of the elastic and damping parameters of wood along the different material directions. The main objective is to quantify the variability of the mechanical properties in different material directions as a function of the grading attributed by the wood seller. The results show that material anisotropy decreases as the grade decreases. This is attributed to strong increase of the specific elastic modulus in the radial direction and the specific shear modulus in longitudinal and radial plane. Moreover, grading seems to be assigned as a function of specific elasticity and loss factor in the longitudinal direction, which is tied in with the preferences of instrument makers. Following the assumption that high specific modulus is desirable, the classification based on instrument makers choices can be improved for new grading criteria that consider directions than longitudinal. Moreover, the substitution of wood species and the proposal of new materials exhibiting equivalent elastic and damping properties can be proposed.