Moisture-related adjustment factor to obtain a reference ultrasonic velocity in structural lumber of plantation hardwood

Wave propagation techniques based on ultrasound for timber grading are influenced by the moisture content (MC) of wood. The ultrasound velocities at an equilibrium MC (V₁₂) and above saturation point (V_sat) were used to calculate moisture-related adjustment factors (k_H). Tests were conducted using plantation hardwoods (Corymbia citriodora, Eucalyptus grandis and Eucalyptus pellita) grown in Brazil. Below the fiber saturation point (FSP), k_H values were between 0.66% and 0.83%, which concurred with intervals proposed in existing literature for softwoods. However, the k_H inferred from the velocities obtained at MCs above the FSP increased (from 10% to 30%) with increasing density. The sound velocity under saturated conditions (V_sat) obtained by adjustment equations was 2–9% lower than the actual V_sat obtained from the tests. The obtained adjustment factors may be useful for the standardization of nondestructive timber grading based on ultrasound propagation.