# Defect Detection in Solid Timber Panels Using Air-Coupled Ultrasonic Imaging Techniques
## 2 Materials and Methods  
Chinese fir panels were used as test specimens to examine the measurement technology in the through-transmission mode. Samples were 150 mm \(\times\) 150 mm solid timber panels. The density of timber panels used was 450 kg/m\({}^{3}\). The wood equilibrium moisture content was approximately 10% at 50% relative air humidity and the test temperature was 25 \({}^{\circ}\)C. The sawn lumber was the plains-sawn lumber. To simplify the defection detection process,the sawn lumber was pre-sorted and free from shape defects (twist, bow, cup, etc. ), rots, reaction (compression) wood, and slope of grain. Panels with a thickness from 10 mm to 40 mm with natural and artificial defects were used. Experiments indicated that the ACU method could not be used for defect detection in Chinese fir panels thicker than 40 mm because of the dramatically increased signal attenuation, making it impossible to distinguish between noise interference and useful signals indistinguishable. Table 1 shows the testing parameters applied at the testbed. Chinese fir was selected because it is one of the most widely used building materials in China and many other Asian countries [24]. However, defects, such as knots, wormholes, and cracks, are of great concern in engineering applications. The reference samples selected for this study had natural defects, such as knots and cracks. Wormholes and additional cracks were simulated using drilling. These defects were considered to explore the potential of ACU for the detection of different types of defects. Moreover, measurements on areas without defects were performed for comparison. The adopted commercially available piezo-ceramic ACU transducers (NU200E, produced by Jinci Technology Ltd., Shenzhen, China) had a center resonance frequency of 200 kHz. The experimental setup is shown in Figure 1. Firstly, a discrete sinusoidal signal (or an excitation signal modulated with a 13-bit Barker code) with a frequency of 200 kHz was produced by an output channel of the main controller. It was then processed by a 1 st order passive RC high pass filter to remove DC signals. After passing a voltage following the circuit, the signal was amplified by a two-stage power amplifier circuit composed of two operational amplifiers (OPA455 for the first stage and OPA544 for the second stage) and related capacitors and resistors. At last, it was boosted to the voltage of 400 Vpp to excite the transmitter. The 200 kHz frequency was a trade-off between the resolution and the penetration depth. Experiments on the same testbed showed this frequency achieved optimal results. The ultrasonic signal propagated into the sample. On its path, it passed through several boundaries (e.g., sample surface, air, and defects) with different acoustic impedances, where the wave was partly reflected with significant energy losses. The distance between the transducer and the receiver was set to 15 cm according to the experiments, while the wood panel was placed in the middle of it. After being transmitted through the sample, the signal was first received by the receiver. Figure 2 shows a received waveform. The first part of the signal received is the electromagnetic wave interference caused by the high voltage excitation and filtered in the signal processing process. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of excitation cycles & 8 \\ Pulse frequency & 200 kHz \\ Scanning rate & 20 mm/s \\ Scanning step & 2 mm \\ Pre-amp gain & 70 dB \\ Barker code & 13 bit \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Testing parameters. Figure 1: Experiment setup (a) and transducer layout (b) of ACU measurement system. The signals received by the receiver were very weak, so a high-precision amplifier circuit composed of an instrumentation amplifier AD8421 was set to amplify the received signals. The AD8421 is a low-power, extremely low noise, low bias current, and high-speed instrumentation amplifier that is ideally suited for data acquisition applications.