Spruce wood specimens were hygrothermally treated at different temperatures (T, 95-140 °C) and relative humidity during heating (RHh, 0-92%). Their loss in dry mass (ML), specific dynamic Young's modulus (E′/ρ), mechanical loss tangent (tan δ), and CIELAB colour parameters were measured at 25 °C and 60% relative humidity (RH) before and after the hygrothermal treatment. The changes in physical properties by the hygrothermal treatment were formulated as functions of ML. Those ML dependencies were combined with the ML-time curve at 20 °C previously predicted by using a time-T-RHh superposition, and the changes in physical properties during long-term ageing were predicted. The predicted vibrational properties were stable at 0-80% RH, whereas a significant decrease in E′/ρ and increase in tan δ were predicted at 92% RH, suggesting serious degradation in acoustic quality in humid conditions. The predicted changes in colour reasonably agreed with those during natural ageing. The combination of hygrothermal treatment and time-T-RHh superposition is a useful tool to predict the effects of ageing in ambient conditions, particularly when the target property simply depends on the chemical changes in wood constituents.

Title: Effects of seasoning on the vibrational properties of wood for the soundboards of string instruments.

Spruce wood specimens were hygrothermally treated at different temperatures (T, 95-140 °C) and relative humidity during heating (RHh, 0-92%) to measure the loss in dry mass (ML) due to the treatment. The ML was approximated by a power function of the heating duration t, and the plots of ML versus t were successfully superposed as a single master curve by shifting the plots along the t axis using the shift factors a T and a H , representing the effects of T and RHh, respectively. The apparent activation energies calculated from the linear correlation between ln(a T ) and the reciprocal of absolute temperature were 109-116 kJ/mol, almost independent of RHh. Plots of a H versus RHh showed sigmoid shapes, suggesting that the a H value depended on the moisture content of the wood. The shift factors allowed the prediction of the ML of wood by hygrothermal treatment at arbitrary T and RHh. The predicted ML values showed good agreement with experimental ML values available in the literature.

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