Résumé
Wood is a very hygroscopic material. When submitted to different relative humidity (RH), which can typically be the case when an instrument is played in different places, the internal moisture content of wood will be changed. This is well known to result in dimensional swelling or shrinkage, and will also modify the wood mechanical or acoustical properties. The equilibrium moisture content (EMC) dependence of vibrational properties (specific modulus of elasticity E’/γ which is related to resonance frequencies, and damping coefficient tanδ) has been well-studied for spruce wood along the grain [Obataya et al. 1998] and for various wood species in the three main directions of wood [Suzuki et al. 1980]. However, equilibrium state is only reached after a relatively long time (weeks), whereas the humidity changes encountered when an instrument is played in different places involve short time scales (hours) and are typically out-of-equilibrium. Previous research have shown the existence of transitional effects, notably on damping [Sasaki et al. 1988; Hunt and Gril 1996]. However these studies considered a single direction of anisotropy each, and wide steps of changes in relative humidity. The present work aims at observing transitional moisture effects on vibrational properties of violin-making woods (spruce and maple) over different steps and different histories of RH changes, in longitudinal and radial directions of wood.