## 5 Discussion  
In this paper, the size of a spatial window is 16 mm, while the diameter of a circular flat bottom hole is no more than 10 mm. Hence, this spatial window will not only contain the propagating waves located at the damage, but also contain those located at the undamaged area. According to the dispersion curves, it should be noted that the wavenumber of the A0 mode will decrease with the increase in the frequency thickness product. More specifically, the wavenumber located at the damage is larger than that located at the undamaged area. Moreover, the deeper the circular flat bottom hole is, the more prominent the difference is. Therefore, this window effect cannot be ignored. (1) The depth estimation of the deep hole is more accurate than that of the shallow one, as revealed by comparing the results of Case 1 and Case 2. The frequency-thickness product of the shallow hole is much larger than that of the deep hole, which means that the wavenumber of the former is smaller than that of the latter. Additionally, there is no significant difference between the shallow hole and the undamaged area in terms of the wavenumber. According to Equation (11), it is easy to understand that the shallow hole will be easily affected by the window effect relative to the deep hole. (2) The depth estimation for the big hole is more accurate than that of the small one. This conclusion was drawn by comparing the results of Case 1 and Case 3. One reasonable explanation for this is that the local wavenumber is affected by the difference in size between the defect and the spatial window. The small diameter of a circular flat bottom hole means that the spatial window will contain more propagating waves located at the undamaged area where the wavenumbers are small. According to Equation (11), it is easy to understand that the local wavenumber will decrease as the lower wavenumbers are present in the spatial window. The smaller the diameter of a circular flat bottom hole is, the smaller the local wavenumber and the reconstructed depth will be. (3) The results of Case 4 and Case 5 indicate that the better test result appears under the big gap between defects when two circular flat bottom holes are symmetrical to the excitation source. Apparently, multi-reflection between two defects is low when they are far apart from each other. Furthermore, they are affected at the same level as they are symmetrical to the excitation source. (4) The circular flat bottom hole far away from the excitation source seems smaller than the near one when the excitation source and two identical defects are in the same line. Moreover, the closer the two defects are, the more obvious the effect is. This conclusion was obtained by comparing the results of Case 6 and Case 7. \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen** & **Actual Depth (mm)** & **Reconstructed Depth (mm)** & **Estimation Error of Depth** \\ \hline Case 1 & 1 & 0.9432 & 5.68\% \\ Case 2 & 0.5 & 0.4490 & 10.20\% \\ Case 3 & 1 & 0.8673 & 13.27\% \\ No. 1 & 1 & 0.9308 & 6.92\% \\ Case 5 & No. 2 & 1 & 0.9401 & 5.99\% \\ No. 1 & 1 & 0.9152 & 8.48\% \\ Case 7 & No. 2 & 1 & 0.9277 & 7.23\% \\ No. 1 & 1 & 0.9339 & 6.61\% \\ No. 2 & 1 & 0.8636 & 13.64\% \\ No. 1 & 1 & 0.8866 & 11.34\% \\ Case 9 & No. 2 & 1 & 0.9246 & 7.54\% \\ \hline \end{tabular}
\end{table} 
Table 7: The depth evaluations of Case 1 to Case 3, Case 5, Case 7, Case 8, and Case 9. \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Specimen** & **Actual Depth (mm)** & **Reconstructed** & **Estimation Error** & **Actual Incidence** & **Reconstructed** & **Estimation Error** \\  & **Depth (mm)** & **Depth (mm)** & **of Depth** & **Angle (\({}^{\circ}\))** & **Angle (\({}^{\circ}\))** & **of Angle** \\ \hline Case 10 & 1 & 0.8510 & 14.90\% & 90 & 92.37 & 2.63\% \\ Case 11 & 1 & 0.8345 & 16.55\% & 135 & 143.46 & 6.27\% \\ \hline \end{tabular}
\end{table} 
Table 8: The depth and angle evaluations of Case 10 and Case 11.