The use of acoustic technology has been recognized as an accurate, precise, and efficient means by which wood quality can be quantifiably measured (www.metriguard.com/fiber.htm ; Rippy 1998; Baldwin 1995). However, the process of measuring wood quality at the log level with acoustic tools has until recently been limited by a lack of ruggedness and mobility among the measurement tools themselves. Within the last several years, the portability of such tools has increased such that handheld measurement units are capable of quantifying key mechanical wood properties quickly, simply, and accurately (Carter and Lausberg FEIA 2002; Clark, Hartmann, Lausberg, and Walker 2002; NZFI June 2002; Young NZJF 2002).
Amid the NZ forest industry two handheld acoustic log measurement models have been developed for operational use. These models include the CHH Fibre-Gen Hitman tool and the Fletcher Challenge Forests (FCF) Sound Wave Acoustic Technology (SWAT) tool. The latter was developed primarily for FCF usage, while the Hitman tools were developed for both CHH and the open marketplace. Because of the availability of the Hitman tool, in addition to Interpine Forestry’s (IPF) close working relationship with CHH’s manufacturing wing both trialing and operationally implementing Hitman, the tool has become IPF’s choice model for testing structural log qualities via acoustic means.
Essentially, the Hitman tool measures the velocity of a sound signal reverberating through a log hit with a standard hammer. This procedure thus requires a single operator only, and produces an accurate result approximately within 1-2 seconds (NZFI 2002). Measurements can be provided in either feet or kilometers per second (Director HM 200 User Manual). To estimate log stiffness, which has clearly been demonstrated as strongly correlative with actual MOE (Rippy 1998; Verhey et al. 2002), sonic velocity (V) is squared and multiplied by the density (D) of the log (CHHFG pers. comm. 18/09/2003).
stiffness = (V^2)*D
A strong correlation exists between MOE and Hitman log sonic velocity, where previous trial data would suggest that r 2 values are approximately 0.98 for veneer and 0.97 for sawn timber (CHHFG proprietary data). Reversing the equation, the relationship might be interpreted more easily by understanding that the speed of sound in a structural material such as wood is dependant upon MOE and density (Green, Winandy, and Kretschmann 1999). Additionally, other wood properties such as grain direction, fiber length, and microfiril angle are all associated with stiffness and/or MOE as well as sonic speed (Green, Winandy, and Kretschmann 1999 1; www.metriguard.com/fiber.htm 2; www.metriguard.com/GRAIN01.htm 3; Carter and Lausberg FEIA 4; Clark, Hartmann, Lausberg, and Walker 2002 5). These references suggest stiffness decreases when grain direction (slope of grain from longitudinal direction) increases 1, and/or fiber length decreases 4,5, and/or microfibril angle (especially in the S2 wood fiber layer) increases 2,3. Thus, Hitman technology is capable of ascertaining not only a measure of stiffness, but also direct grain/fiber/mircofibril property assessments as well.
The use of acoustics for mechanically grading lumber is certainly not new technology. As a result, a large amount of wood property information has been determined under the acoustic umbrella. Furthermore, due to the strong correlation between Hitman sonic data with data calculated from current acoustic stress rating systems (i.e. Metriguard), the utilization of this type of value recovery technique (e.g. stress wave technology) has much to build on. In turn, the information gleaned from operational Hitman data may be utilized, and interpreted with similar intentions as Metriguard data, for value recovery purposes early on in the fiber supply chain.
Literature Cited – Quick Refs
Baldwin , R., Plywood and Veneer-Based Products, Miller Freeman Books, 1995
Carter, P. and Lausberg, M., Application of Hitman Ò Acoustic Technology – The Carter Holt Harvey Experience, FEIA, 2002
CHHFG personal communication, Peter Carter and Ted Downs, 18/09/2003
Clark, T.A., Hartmann, J., Lausberg, M., Walker, J.Fibre Characterization of Pulp Logs Using Acoustics, 2002
Director HM200 User manual, CHH Fibre-Gen
Green, D., Winandy, J., Kretschmann, D., Wood Handbook–Chapter 4–Mechanical Properties of Wood, 1999
New Zealand Forest Industries, page 29, June 2002
Rippy, R., Stress Wave Analysis of logs for Veneer Stiffness, M.S. Thesis, University of Idaho, 1998
Verhey et al. , Ultrasonic Veneer Grade Yields for Red Maple, Final Report for Northern Initiative’s VP Peter Cambier, 2002
www.metriguard.com/fiber.htm, Website, 2003
www.metriguard.com/GRAIN01.htm, Website Link, 2003
Young, G., Radiata Pine Wood Quality Assessments in the 21 st Century, NZ Journal of Forestry, Volume 47 no. 3, Nov 2002