Sensitivity of acoustic tools to variation in equilibrium moisture content of small clear samples of loblolly pine (Pinus taeda)

Abstract

There are several types of acoustic tools commercially available for wood characterization, but they are generally classified into resonance and time-of-flight (ToF) tools. This classification is based upon the mode of velocity estimation for wood. In this study, we explored how the equilibrium moisture content of small clear wood samples (2.5 cm × 2.5 cm × 41 cm) affect the predictive capabilities of two types of acoustic tools namely a microsecond timer (ToF) and a resonance log grader (resonance). The results indicated the acoustic velocity is sensitive to equilibrium moisture content of loblolly pine, and sensitivity to EMC is similar for both type of tools. The acoustic velocity decreased by 32.9 and 28.8 m/s for ToF and the resonance acoustic tools respectively for a unit increase in EMC below fiber saturation point (FSP); 5.4 and 6.1 m/s respectively for a unit increase in EMC above FSP although the slope was statistically equivalent to zero. Also, the static MOE of the green samples was overestimated by 16% by both resonance and ToF tools with oven-dried density, while it was 72% when estimated with density at test. The insignificant slope coupled with better accuracy in MOE supports the hypothesis that the cell wall controls the acoustic velocity while the water in the lumen of the cell wall is insignificant. These results bring into question the standard use of green density to estimate acoustic MOE of live trees and oven dry density is instead recommended.