## 4 Experimental Verification
### Experimental Results  
For the purpose of removing the noise, the received signals are filtered by a Chebyshev band-pass filter [39] with a bandwidth of 140 kHz to 260 kHz. Figure 17 shows the experimental signal in the time domain and frequency domain. Figures 18-22 are the experimental results. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimen** & \begin{tabular}{c} **Actual** \\ **Depth (mm)** \\ \end{tabular} & \begin{tabular}{c} **Reconstructed** \\ **Depth (mm)** \\ \end{tabular} & \begin{tabular}{c} **Estimation Error** \\ **of Depth** \\ \end{tabular} & \begin{tabular}{c} **Actual Incidence** \\ **Angle (\({}^{\circ}\))** \\ \end{tabular} & \
\begin{tabular}{c} **Reconstructed** \\ **Angle (\({}^{\circ}\))** \\ \end{tabular} &
\begin{tabular}{c} **Estimation Error** \\ **of Angle** \\ \end{tabular} \\ \hline Case 10 & 1 & 0.8705 & 12.95\% & 90 & 90.00 & 0.00\% \\ Case 11 & 1 & 0.8640 & 13.60\% & 135 & 134.84 & 0.12\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The depth and angle evaluations of Case 10 and Case 11. Figure 16: Schematic of the experimental setup. Figure 17: Band-pass filtering of an experimental signal. (**a**) Original signal; (**b**) spectrum of the original signal; (**c**) filtered signal; (**d**) spectrum of the filtered signal. Figure 18: Experimental results of Case 1 to Case 3. (**a**) Wavenumber distribution of Case 1; (**b**) depth distribution of Case 1; (**c**) wavenumber distribution of Case 2; (**b**) depth distribution of Case 2; (**d**) wavenumber distribution of Case 3; (**e**) depth distribution of Case 3. The white circle represents the actual location of the damage. The actual defect depth of Case 1 and Case 3 is 1 mm, while the actual defect depth of Case 2 is 0.5 mm. Figure 19: Experimental results of Case 5 and Case 7. **(a)** Wavenumber distribution of Case 5; **(b)** depth distribution of Case 5; **(c)** wavenumber distribution of Case 7; **(d)** depth distribution of Case 7. The white circle represents the actual location of the damage. The actual depth of the defect is 1 mm. Figure 20: Experimental results of Case 8 and Case 9. **(a)** Wavenumber distribution of Case 8; **(b)** depth distribution of Case 8; **(c)** wavenumber distribution of Case 9; **(d)** depth distribution of Case 9. The white circle represents the actual location of the damage. The actual depth of the defect is 1 mm. The experimental results have good agreement with the simulation results. Figure 18 shows that the depth estimation of the deep hole is more accurate than that of the shallow one. In addition, the depth estimation of the big hole is more accurate than that of the small one. Figure 19 reveals that 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. Figure 20 shows that the proposed approach is capable of identifying the adjacent defects with different sizes. Meanwhile, it indicates that the impact that the big hole exerts on the small one is connected to their positions relative to the excitation source when they are in the same line. Figure 21 shows that the width of the rectangular groove is larger than its actual value. Figure 22 shows the direction identification for an inclined rectangular groove. Tables 7 and 8 indicate that the evaluation results still need to be improved due to the existence of multi-reflection between the adjacent defects and the spatial window effect, which will be analyzed in the discussion section. Figure 21: Experimental results of Case 10 and Case 11. (**a**) Wavenumber distribution of Case 10; (**b**) depth distribution of Case 10; (**c**) wavenumber distribution of Case 11; (**d**) depth distribution of Case 11. The white rectangle represents the actual location of the damage. The actual depth of the defect is 1 mm. Figure 22: The direction identification of an inclined rectangular groove based on the experimental result. (**a**) 90\({}^{\circ}\); (**b**) 135\({}^{\circ}\).