Acoustic studies for the non-destructive testing of wood.
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Acoustic technologies are an established means of measuring wood stiffness. Acoustic tools are useful because they are non-destructive, relatively cheap, and quick and easy to use. These tools have been used in tree breeding programs, silvicultural applications, and log grading systems.
Acoustic tools work by measuring the speed of sound in wood, which is related to two key wood quality parameters: stiffness and density. However, several factors make this measurement difficult. Wood structure is complex, being both anisotropic (directionally variable) and inhomogeneous (spatially variable). Additionally, wood is highly attenuating to acoustic waves, which means it is also dispersive.
In this thesis, several aspects of acoustic testing of wood are explored. A review is provided of existing findings in the field, and an overview of the requisite wave mechanics is provided. Following this, a series of original studies on the topic are described.
Several models are proposed for the effect of the wood-loading on a spike-shaped acoustic transducer. It is demonstrated that these models, known as radiation impedance models, determine the low-frequency performance of the transducer.
A methodology is proposed for determining the frequency-dependent transferfunction of an acoustic wave propagating in wood. Several transmission-line models are proposed for the observed behaviour of green wood. Of the proposed models, the Zener model with spherical spreading is shown to match the measured data most closely.
A new time of flight (ToF) measurement system, Wireless Treetap, is presented. The hardware, software, and testing of this system is described. A field test of the system is described. The findings of this test demonstrate that the system is feasible, however some problems regarding its robustness remain unresolved.
A new method is proposed for measuring the acoustic velocity variation along the length of a harvested tree stem. An array of transducers is used to measure a series of waveforms in two Pinus radiata stems. It is shown that two of the proposed processing methods: time of flight and reflected-wave cancellation, are insufficient for determining the velocity variation. However, it is shown that the position of the resonant nodes gives an indication of the variation in acoustic velocities between each half of the stem.