Grading Green Timber Using Compression Waves

Grant Emms

New Zealand Acoustics, 18(2), pp.14- 18 . (2005).

Being a natural product, timber has inherently variable properties. For the purposes of structural use, one of the important properties is the longitudinal stiffness of the timber. The longitudinal stiffness of timber varies greatly from tree to tree and within a tree. Currently, mechanical graders are used to stiffness grade the final, dried, dressed product to remove unsuitable timber and to classify timber for specific purposes. Reject rates can easily be as much as 30%. It would be useful from an efficiency point of view to grade green timber and remove reject timber before cost is incurred from further processing. Unfortunately, the roughRsawn, sapRfilled green timber cannot be passed through a normal mechanical grader. One solution is to use an acoustics approach, where compression waves are generated in the timber and the propagation of the waves measured. From simple calculations of the speed of compression wave propagation in the timber, and measurements (or prior knowledge) of the density, the stiffness of the timber can be determined. This paper examines the principal of the acoustic measurements and the currently applied technology. The relationships between the acoustic measurements of green timber stiffness and mechanicallyRdetermined dried, dressed timber stiffnesses are shown.