Acoustic measurements on trees and logs: a review and analysis

Acoustic technologies have been well established as material evaluation tools in the past several decades, and their use has become widely accepted in the forest products industry for online quality control and products grading. Recent research developments on acoustic sensing technology offer further opportunities to evaluate standing trees and logs for general wood quality and intrinsic wood properties. Although the concept of using acoustic velocity as an effective measure of stiffness applies to both standing trees and felled logs, the method typically used to measure acoustic velocity in trees is different from that used in logs. Consequently, there is a significant difference in measured velocity values between trees and logs. Other factors affecting tree-log velocity relationships include tree diameter, stand age, operating temperature, and wood moisture content. This paper presents the fundamentals of acoustic wave propagation in trees and logs and discusses two different mechanisms of acoustic velocity measurement, time-of-flight for standing trees and resonance for logs. Experimental data from previous studies are reviewed and analyzed to examine the strength of the tree-log velocity relationships and discuss the factors that influence tree velocity deviation.