Acoustic Wave Propagation in Standing Trees – Part 1. Numerical Simulation

Abstract

The use of acoustic waves for assessing wood properties in standing trees has been investigated extensively in recent years. Most studies were experimental in nature and limited to direct measurement of wave velocities in trees using a time-of-flight (TOF) method. How acoustic waves propagate in a tree trunk and how tree diameter, species, stand age, and juvenile wood affect wave propagation behavior in standing trees are not well understood. In this study, we examined propagation patterns of acoustic waves in a virtual tree trunk through numerical simulation using COMSOL Multiphysics® software (COMSOL, Inc., Burlington, MA). The simulation was based on the elastic theory of a solid medium with the assumption of an orthotropic material for a standing tree. Extensive acoustic measurements were conducted on green larch log samples to validate the simulation results. Our results showed that the wave front maps of the tree model from numerical simulations were consistent with those obtained through TOF measurements of the log samples, indicating that the simulation results were accurate and reliable. Wave propagation patterns of the tree model revealed that the side surface-generated acoustic wave expanded as a dilatational wave within a 0-to 1.2m transit distance, as the wave moved up along the tree model, the shape of the wave front gradually flattened and the wave eventually transformed into a quasi-plane wave from a 2.4-m transit distance.