## 4 Results and Discussions
The data were then normalized to the absolute value of the probe voltage in the air using the following equation:  
\[\mathbf{V}_{\text{Rx}}=(\mathbf{V}_{\text{Rx-defect}}-\mathbf{V}_{\text{Rx- air}})/|\mathbf{V}_{\text{Rx-air}}| \tag{3}\]  
Figure 14: Experimental results: specimen No. 2, differential voltage magnitude on frequency dependences (**left**) and phase dependences on differential voltage magnitude (**right**) for individual flaw depths, probe B.  
Figure 15: Experimental results: specimen No.2, differential voltage magnitude on frequency dependences (**left**) and phase dependences on differential voltage magnitude (**right**) for individual flaw depths, probe C.  
where, \(\mathbf{V_{Rx}}\) [-] is the normalized voltage, \(\mathbf{V_{Rx-defect}}\) [V] is the voltage of the coil over the defect or defect-free material, and \(\mathbf{V_{Rx-air}}\) [V] is the voltage of the coil in the air. After normalization, the magnitude and phase of the normalized induced voltage were calculated. These values were next used for waveform construction with the purpose of comparing simulated and measured data. Therefore, the set of results was further statistically analyzed. First, the individual results of the measurement were subtracted from the corresponding results of the simulations, according to the equation:  
\[\Delta\mathbf{V}=\mathbf{V_{Rx-simulation}}-\mathbf{V_{Rx-measurement}} \tag{4}\]  
where \(\Delta\mathbf{V}\) [-] is the voltage difference, \(\mathbf{V_{Rx-simulation}}\) [-] is the normalized voltage of the simulated coil, and \(\mathbf{V_{Rx-measurement}}\) [-] is the normalized voltage of the measured coil. The difference \(\Delta\mathbf{V}\) between the two values was plotted and evaluated. The difference was plotted by three graphs for each probe. Figure 16 clearly shows that the largest differences between the resulting values obtained from the numerical simulations and experiments are observable for probes B and C. The degree of this uncertainty, on the other hand, is compensated by the better resolution of these probes, especially when detecting deeper defects (especially with the depth of 7 mm and 9 mm). The measurement results of the A probe and the B probe are more accurate than the C probe. The strongest difference was found around the frequency interval's edges, particularly at lower frequencies. The same oscillation that occurred in the measurement results of probe A was responsible for the oscillation. Furthermore, the addition of the aluminum shielding had a different effect on detection than the simulation predicted. Despite having the widest difference between the simulated and measured data, probe C presented sufficient detection ability. Table 3 presents the previous information through the prism of selected statistical parameters. This analysis was performed to better evaluate the results. For each statistical file, the median and standard deviation were calculated. The statistical set represented the results of individual coil geometry change differences. The consistency of the differences is described by these results. The results show that despite the initial oscillation of the waveform probe A performs the best results. The average difference in probe B is slightly higher. The results that are still relatively sufficient were obtained by probe C: the standard deviation is 12-14 times higher when compared to probe A.  
Figure 16: Difference between simulation and experimental results for individual flaws, specimen No.1: probe A (**left**), probe B (**middle**), and probe C (**right**). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{2}{c}{**Parameter**} & **Probe A** & **Probe B** & **Probe C** \\ \hline \multirow{2}{*}{Depth} & Median & \(-0.0049\) & \(-0.0059\) & \(-0.028\) \\ \cline{2-5}  & StD & \(0.0079\) & \(0.0084\) & \(0.1023\) \\ \hline \multirow{2}{*}{Length} & Median & \(-0.0043\) & \(-0.0067\) & \(-0.0467\) \\ \cline{2-5}  & StD & \(0.0083\) & \(0.0254\) & \(0.1251\) \\ \hline \hline \end{tabular}
\end{table}