# Defect Detection in Solid Timber Panels Using Air-Coupled Ultrasonic Imaging Techniques
## 3 Results and Discussion  
Knots, cracks, and wormhole-like defects were analyzed for their effects on air-coupled ultrasonic signal propagation in timber panels. Figure 7 shows two typical defects. A 70 mm \(\times\) 70 mm sample area shown in Figure 7a contains a natural knot defect. A 70 mm \(\times\) 70 mm sample area shown in Figure 7b contains a natural crack in the middle of a knot. Compared to Figure 7a, defect detection on Figure 7b is more challenging since both crack and knot defects are fixed, making it difficult to distinguish. Both sinusoidal signal and Barker-coded signal were used as excitation signals. In Figure 8, sinusoidal signals were used as the excitation signals. The signals received by the receiver were filtered and then amplified as the response signal. Figure 9 shows the filtered signals at the receiving end using the excitation signals shown in Figure 5c. It presented a much higher SNR compared to the sine signals as excitation signals shown in Figure 8, making capturing the main lobe peak voltage easier. Figure 8: The filtered waveform of the received signal. Figure 6: The process of cross-correlation matched filtering at the receiver. Figure 7: Timber samples with knot defect (**a**) and combined knot and crack defect (**b**). The amplitude (C-scan) images of the sample shown in Figure 7 using two different excitation signals are presented in Figure 10, the received signal amplitudes are normalized and colored to represent the attenuation. Comparing Figure 10a to Figure 10b, it is demonstrated both methods are reliable inspection for knot detection. However, the size of the knot in Figure 10a is much less than the actual size of this defect due to the limited precision of the system and noise interference. The image size of Figure 10b matches the actual size. Furthermore, the result shown in Figure 10a demonstrates no detection ability when combined defects occur. The result shown in Figure 10b, on the contrary, demonstrates a much higher resolution to distinguish combined defects due to different colors at the edge of both defects. It is proved that applying the Barker code method can improve the measurement accuracy compared to taking a sinusoidal signal as an excitation signal. Thus in this paper, Barker code pulse compression technology was used for all the other scenarios. A 70 mm \(\times\) 70 mm sample area shown in Figure 7a contains a natural knot defect with a diameter of 22 mm. The amplitude (C-scan) image is presented in Figure 11a and the TOF (D-scan) image in Figure 11b, respectively. Comparing Figure 7a to Figure 11a, the imaging result (Figure 11a) was essentially consistent with the actual defect size (Figure 7a). However, Figure 11b shows that the TOF method did not perform as well as the amplitude method. It is probably because the attenuation differences were amplified by the amplifiers at the receiving end, while TOF is not sensitive to knot defects, making it much harder to distinguish from normal areas. It is thus demonstrated that the ACU method is a reliable inspection for knot detection. However, the diameter of the knot in Figure 11a is 24 mm, which is 2 mm larger than the actual size. The error was mainly because of the following reasons:  
Figure 10: C-scan imaging results of the sample with sinusoidal signal (**a**) and Barker coded signal (**b**) as excitation signals. Figure 9: Filtered pulse compression signal of 13-bit Barker code. * The main cause of errors was the limited specifications of the adopted typical commercially available piezo-ceramic transducers, which were selected to reduce cost. To improve detection accuracy, advanced transducers, such as those made of cellular polypropylene, could be used which are more powerful and have smaller scattering angles. * Timber properties. Because wood grows naturally, the features of wood such as grain orientation and annual rings, lead to anisotropy and heterogeneity even in areas without defects. * Insufficient localization accuracy of the stepping motor could also influence the results.