In August 2005 extensive acoustic testing was carried out on behalf a client throughout their forest estate. This testing comprised of two parts. The first involved the sampling of upcoming harvest areas to attain an average acoustic velocity of the standing resource using the Director ST300. The second part was a destructive trial to establish a relationship between acoustic velocity results of the standing trees to those found in cut logs. This article aims to just review the use of the Director ST300 and to help impart an understanding of the science behind the tool.
Introduction to Standing Tree Acoustic Testing
Stiffness and fibre properties of wood are fundamentally important for producers of lumber and pulp and paper. Acoustic speed or the speed at which sound travels through wood, is well proven to be an accurate measure of wood stiffness [see Pellerin and Ross, 2002]. With recent technological advances it is now possible to measure stiffness in standing trees.
Traditionally the segregation of logs by internal quality characteristics has relied on knowledge of tree age, species, geographic location and climate. The development of acoustic testing now allows the forest owner to broadly segregate logs based on their inherent stiffness quality allowing more discerning supply to end markets [NZFI, 2002].
At least two log markets benefit from logs segregated on stiffness, these being the structural lumber market and veneer logs for LVL production.
In 2005 the New Zealand timber structures standard NZS3603 was amended requiring verification testing on visually and machine stress graded structural lumber. A “Verified Timber” quality assurance programme and brand was established, supported by the Timber Industry Federation, where random static bending tests are now carried out on lumber designated for certain applications. A consequence of this legislative change is that for certain lumber applications the wood fibre must be of a certain level of stiffness [Verified Timber Ltd, 2005]. Acoustic testing standing timber allows sawlog purchasers to source their logs from stands of known average stiffness improving their chance of meeting the new timber standards.
There are instances where grade recovery and financial return have been shown to trace back to initial log selection and more specifically green log acoustic speed [Carter, Wang, et al 2005]. Individual log stiffness (via acoustic) testing and grading provides the best certainty to the log buyer in terms of correlation with lumber or veneer stiffness however using average stiffness of logs per stand provides a low-cost method to capture the benefits of segregation [Carter et al, 2004].
Stiffness of wood can most accurately be measured by subjecting a sample to a static bending machine test. A force is applied and the resistance to deflection is recorded from which the static modulus of elasticity (MOE) is calculated. The equipment required is not practical for operational use in the forest, and even more so, not for use on standing trees.
What has developed in the past few years is the technology to measure dynamic MOE, which is strongly correlated with Static MOE. Through measuring the velocity of an acoustic wave through wood and the density of the material in question, this measure can be determined, as shown in the formula below [Carter, Wang et al, 2005].
stiffness = (V^2)*D
Where V = velocity of sound wave through the wood (km/s), and D = wood density (kg/m 3)
The density of wood is relatively constant so the velocity of a sound wave through wood, as measured by acoustic tools such as the Director ST300, can be used as a direct indicator of dynamic MOE [Carter, Wang et al, 2005].
So What’s the ST300 Tool ?
Tools for measuring acoustic velocity in logs have been around for a few years and have been shown to produce extremely accurate results. Fibre-gens Director HM200 handheld field assessment tool tested against laboratory equipment showed r2 values of 0.9763 [Ross & Wang, 2005]. The Director HM200 is now widely used through Interpine and others options through New Zealand and overseas.
The Director ST300 tool is new to the market and uses several technologies, including ultrasound, lasers and impact induced stress waves to assess wood stiffness in standing trees. There have been various trials carried out on thousands of trees of different species and ages across North America, Australia and New Zealand to establish how acoustic speeds in standing trees compare to those obtained in the resulting logs. Velocities recorded in standing trees are typically higher than those recorded from logs but the correlation between the two is strong.
For example : A trial of 50 radiata pine trees in Eyrewell Forest, Canterbury, New Zealand found a strong correlation (r 2 = 0.67) between acoustic velocities in standing trees and the manufactured logs from the same trees. An Interpine trial on 80 trees, established similar results with r 2 of 0.65 when velocities from pruned and second unrpuned log were combined.
Correlations between acoustic values from standing trees to cut logs increase as diameter and age decrease. A trial in young 10 year old trees showed an r 2 of 0.90 between standing tree and log velocities [Carter, Wang et al, 2005].
So what does this mean ? Well their is now a well developed understanding of the Director HM200 velocity results compared to structural product out-turn of logs. We can then use this regression to apply a target ST300 value for the standing resource. For example a target 3km/sec HM200 result for the first and second logs, will likely result in a expected ST300 result at the tree base of 3.9km.sec. Further trials and validation of these results is required, although inital results are positive.
Literature Cited – Quick Refs
CARTER, P. WANG, X. ROSS, R. BRIGGS, D. “NDE of logs and standing trees using acoustic tools – Technical application and results”, 2005.
CARTER, P. BRIGGS, D. ROSS, R. WANG, X. “Acoustic Testing to Enhance Western Forest Values and Meet Customer Wood Quality Needs”, 2004.
New Zealand Forest Industries [periodical],”Acoustic tools for wood quality applications”. P29. June, 2002.
PELLERIN AND ROSS, “Non-destructive Evaluation of Wood” Forest Products Society, USA, 2002
ROSS, R. WANG, X. “A review of the Use of Acoustic Speed to Assess Standing Timber Quality” USA 2005.
VERIFIED TIMBER Ltd, “Verified Timber – Information Sheet” www.verifiedtimber.co.nz, NZ, 2005
Footnote : ST300 and HM200 are tools developed and marketed by Fibre Gen. This article is just a review completed by Interpine Forestry Ltd and may not represent the views of Fibre Gen. For more information on the tools themselves review their website found here :