ACOUSTIC WAVE PROPAGATION IN STANDING TREES —PART II. EFFECTS OF TREE DIAMETER AND JUVENILE WOOD
Keywords:Acoustic waves, COMSOL Multiphysics software, dilatational wave, plane wave, wave front, wave velocity, trees.
The objective of this study was to investigate the effects of tree diameter and juvenile wood on acoustic wave propagation in standing trees. Two-layer tree models with various diameters and proportions of juvenile wood were constructed to examine the effects of these two factors on propagation patterns and velocity of acoustic waves. The simulation results and analysis indicated that acoustic wave propagation in trees is dependent on both tree diameter and propagation distance. In the context of time-of-flight (TOF) acoustic measurement on standing trees with a test span of 1.2 m, when tree diameter is 10 cm or less, or slenderness is twelve or greater, wave propagates as quasi-plane waves in tree trunk, and the tree velocity determined using the TOF method is then comparable to the log velocity measured using the acoustic resonance method. When tree diameter is 40 cm or larger, or slenderness is three or less, wave propagates as dilatational waves in the tree trunk; thus, the three-dimensional wave equation should be considered for wood property prediction. When tree diameter falls between 10 and 40 cm or slenderness falls between 3 and 12, wave propagation is in a transitional phase. Mathematical models were developed to convert the tree velocity in the transition mode to the resonance velocity. It was found that juvenile wood content resulted in a 113-m/s reduction in acoustic velocity. In addition, our analysis indicated that wave propagation in standing trees is controlled by the wood properties of entire cross section, not just the outerwood. Therefore, the wave velocity measured on standing trees reflects the global properties of the wood between the two measuring points.
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