Abstract: [Objective] The aim of this study was to investigate the effects of density and moisture content (MC) on ultrasound velocities along the longitudinal direction in wood.[Method] Epoch XT device was used to measure the longitudinal ultrasound velocities of six species wood (softwood:Chinese fir, Red pine, Mongolian scotch pine; Hardwood:Manchurian walnut, Asian white birch, Mongolian oak) with equilibrium moisture content (EMC) obtained at different relative humidity levels (20℃, 50%, 65%, 85% and 90%RH). The relationships between longitudinal ultrasound velocities and wood oven-dried density or MC were analyzed, respectively. The decrement of longitudinal ultrasound velocities per unit MC of six species wood were compared with each other as well.[Result] For given RH, the EMC varied among six species wood, and the degree of variance increased with the increasing RH. The oven-dried density of the three hardwood species were higher than that of the three softwood species, but this trend was not found in longitudinal ultrasound velocities. The relationship between wood oven-dried density and longitudinal ultrasound velocities was analyzed in two different manners:for each species and all six species. The results showed that longitudinal ultrasound velocities were not dependent on oven-dried density for each species wood, and the coefficients of determination R² were below 0.24. When six species wood were considered together, longitudinal ultrasound velocities tended to decrease with the increasing oven-dried density, but this tendency was not significant either, the coefficient of determination in this case was found to be only 0.42. Longitudinal ultrasound velocities decreased linearly as MC increased, the coefficient of determination R²was above 0.91. While less difference among softwoods was observed on decrement of longitudinal ultrasound velocities per unit MC, as compared to hardwoods. For softwoods, the decrement of longitudinal ultrasound velocities per unit MC was almost the same for Red pine and Mongolian scotch pine, which value was 42.6 m·m^-1%^-1 and 42.4 m·m^-1%^-1, respectively, and the value of Chinese fir was the lowest, 38.1 m·m^-1%^-1. For hardwoods, the value of Mongolian oak (55.4 m·m^-1%^-1) was 2.4 times higher than that of Asian white birch (23.1 m·m^-1%^-1). [Conclusion] No significant correlationship between longitudinal ultrasound velocities and wood oven-density was found in this study. It was hard to establish a direct dependence of density on longitudinal ultrasound velocities. The velocities of wave propagation in wood is probably much more affected by the microstructure of particular species, such as rays percentage, the length of tracheids, vessels and fibers. Below the fiber saturation point, as MC increased, longitudinal ultrasound velocities of six species wood decreased. It is attributed to the adsorbed water increasing the acoustic resistance of cell wall. Furthermore, microstructure, extractives, thyloses and rays percentage of particular species could be the major reasons for the differently influence of MC on six species wood longitudinal ultrasound velocities.

Key words: ultrasound wave, wood density, equilibrium moisture content, longitudinal ultrasound velocities