Arboriculture & Urban Forestry 36(3): May 2010 SINGLE PULSE SONIC AND ULTRASONIC TECHNIQUES The velocity of the propagation of sound waves is much faster in wood than in air (Bootle 2005). In a solid me- dium, the velocity of a sound depends on the type of wave and the elasticity and density of the material (Pollard 1968). Acoustic instruments usually measure the wave veloc- ity (ν) in wood. The wave velocity in m s-1 is given by: Equation ν = Ε ρ Where Ε is the modulus of elasticity and ρ is the density (Ouis 2003). Velocity is therefore dependent on factors such as species, moisture content, temperature, and the anatomical direction in which the sound is transmitted (Mishiro 1996). It is also difficult to translate the velocity of sound to physical properties, because wood is an anisotropic material (Nicolotti et al. 2003; Socco, et al. 2004; Bucur 2006a; Maurer et al. 2006; Schubert et al. 2009). Most instruments for ultrasonic analysis operate at fre- quencies between 50 kHz to 5 MHz (Bucur 2003). Instru- ments operating above 1 MHz can create images of the ob- jects they scan as resolution increases at these frequencies (Bucur 2003). To minimize attenuation of the ultrasound signal the frequency of the signal must be low, but this results in de- creased resolution and, in some instances, the wavelength of the signal may be large enough that regions of different wood densities may be obscured (Ouis 2003; Socco et al. 2004). The Fujikura-Arborsonic Decay Detector is based on a simple pulse-echo method measuring the transmission time of an ultra- sound pulse (Wade 1975). It delivers an ultrasound pulse of 77 kHz that passes through the stem. The signal speed is approximately 2000 m s-1 through undegraded cell walls. In timber, the usual mode of propagation of ultrasound is via the cell walls. When cells are degraded the ultrasound signal speed is slowed; the more decay the slower the signal. The Fujikura-Arborsonic Decay Detector operates using a transducer (which sends the signal) and a receiv- er (that receives the signal) on opposite sides of the tree. A 45 mm diameter bark plug is first removed to provide good contact to the wood. The known distance between the transducers in millimeters is divided by two to obtain the expected ultrasound propagation time reading in microseconds. The expected times are given in a table according to the diameter of the tree. It does not produce a tomographic image. The maximum recommended tree diameter for the Fujikura-Arborsonic Decay Detector is 1.4 m. Larger trees require quadrant testing, with transducers and receivers at 90-de- grees to one another, rather than at opposite sides of the tree. As decay deep in the core of the tree is not as important a concern in larger trees, testing in this way is appropriate (Smiley 1992). Single pulse ultrasonic devices were able to detect vari- ous types of defects and changes in wood quality in some trees (Nicolotti and Miglietta 1998; Sandoz 1999). Decayed wood did not significantly reduce the transmission time of the signal produced by the Fujikura-Arborsonic Decay Detector in Vic- torian blue gum, a result inconsistent with willow oak (Quer- cus phellos L.) and nuttall oak (Quercus nuttallii E.J. Palmer), where decay did reduce transmission times (Xu et al. 2000; Johnstone 2005). The Fujikura-Arborsonic Decay Detector per- BREAKING CORE SAMPLES 1 Fractometers measure decay by assessing the mechanical proper- ties of an extracted core of wood. The Fractometer I measures the force required to bend a core sample (radial bending frac- ture strength) and the radial angle prior to breaking (stiffness) (Bethge et al. 1996). Fibers and wood rays must be oriented parallel to the front of the Fractometer, which, the developers claim, simulates the fiber loading due to wind (Mattheck et al. 1995). Measurement is in “fractometer units” (FU), which can be converted into units of pressure (MPa) (Bethge et al. 1996). The theoretical basis for developing the Fractometer is clearly Hooke’s law, which states that a change in form is pro- portional to the deforming force or F = k ∆L, where F is the force pulling on an object, ∆L is the increase in length, and k is a proportionality constant (Giancoli 2005). The change in form on the Fractometer sample is measured by the angle set- ting and the deforming force by the pressure required to break the sample. The modulus of elasticity can be derived from Hooke’s law and is the constant “k” (Pollard and Harris 1968). The size of the sample cores used in the Fractometer (5 mm di- ameter) largely precludes the use of the device in trees that have high wood density. Species such as eucalypts usually require increment coring with a motorized corer (Downes et al. 1997). Motorized in- crement corers yield a 12 mm sample too large to be tested by the ©2010 International Society of Arboriculture 123 formed slightly better on spotted gum (Corymbia maculata) samples that were hollow, but was not successful at all in hol- low willow oak or nuttall oak (Xu et al. 2000; Johnstone 2005). As with the Silvatest that requires stripping pieces of bark 30 mm in diameter to provide good contact (Nicolotti and Miglietta 1998), another disadvantage as “noninvasive” test instruments is bark must be taken from the trunk, thus wounding the tree. Stress wave assessment is another method of assessing wood using sonics, rather than ultrasonics. A stress wave is a complex mixture of frequencies, various components of which travel through solid, liquid, and gas with differing velocities (Wade 1975). Stress wave assessment of wood has been successfully mod- eled in utility poles in laboratory settings (Bulleit and Falk 1985). The Metriguard Stress Wave Timer uses this approach (Mattheck and Bethge 1993). A hammer struck against pins inserted into the xylem sends a signal across the trunk. It detects changes in wood quality but may be less accurate than ultrasound because of the number of frequencies involved. The Metriguard requires species- specific reference tables. Inconsistencies in readings can occur because the hammer is not always struck with the same force (Ni- colotti and Miglietta 1998). Interpretation is complicated as the velocity of the sound may be slowed by bacterial wetwood, de- cay, and in some cases inaccurate measurement due to excessive wind speed (Mattheck and Bethge 1993; Yamaguchi et al. 2001). Perhaps the best use for single path stress wave time-of-flight test- ing is, as Wang and Allison (2008) suggest, as an initial screen- ing process that may justify more sophisticated investigations. A more advanced analysis of acoustic single path stress waves can be performed by reworking the data using Fourier transformations (Lawday and Hodges 2000). Short-time Fou- rier transforms of stress waves predict the extent of wood de- cay, rather than just the presence of decay. Acoustic techniques utilizing multiple path stress waves are classed as “tomog- raphy” techniques and are discussed in a following section.
May 2010
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