## 4 Experimental Results  
Experimental research was carried out for several simulated lesions, ranging from a small change on the surface of the plate (simulated as the tile's bore), through a change in the structure (hole in the plate), up to two damages in the form of holes. The obtained results were compared with the case of no damage. Examples of signals recorded for particular types of damage are presented as follows: Figure 6a for the undamaged board; a case with minor damage (bore) in Figure 7a; plate significantly damaged (two holes) in Figure 8a. The comparison of such extreme cases makes it possible to assess the validity of the adopted assumptions regarding both the possibility of using the proposed method and its eventual usefulness in field conditions. Figure 5: Sample signal waveforms recorded during measurements. Figure 6: Registered signal for an undamaged plate (**a**) detail 1, (**b**) detail 2, (**c**), detail 3 (**d**) of the discrete wavelet transform (DWT), Daubechies 8 wavelet, N—number of time intervals. At the present stage of research, the quantitative assessment concerns the differences in the number of single pulses visible in the signal and the wavelet coefficients. It can be observed that for single, minor damage, there is one impulse, and for a larger number of damages, more impulses were registered. Hence, the recorded number of pulses or peaks in the details of the wavelet transform are quantitative indicators in the proposed method. Moreover, indicators independent of the mechanical wave excitation force in the analyzed element are sought; therefore, a direct comparison of, e.g., signal amplitudes is not a reliable indicator. Analyzing the above examples of waveforms, one can notice some regularities in the signals or details of the wavelet transform. While the absolute values, from the point of assumption of the method, are not the most important, the differences in the signal waveforms, and especially the details, allow indicating characteristic features related to the state of the sample. Thus, for the state without damage in the details of the wavelet transform, and even in the signal at the excitation with a small amplitude, no characteristic features were characterizing this state. The waveforms were similar to the noise (as in the example) or to the distribution corresponding to the typical, relaxing course of the AE impulse. On the other hand, in the case of a damaged board, it is important to determine the beginning of the signal analysis properly--there is a difference in the initial phase of the details for different types of damage, as well as during the signal. Figure 7: Registered signal for a plate with a recess (**a**), detail 1 (**b**), detail 2 (**c**), detail 3 (**d**) of DWT, Daubechies 8 wavelet, N—number of time intervals. These differences may be related to the presence of a different number of points of reflection of waves propagating in the metal, i.e., the presence of holes may cause additional multiple reflections of the wave; and the overlap of waves reflected on each other. This may result in changes in the amplitude and fall time of the signal. It has been observed that increasing the number and size of damage causes an increase in the amplitude and duration of the signal  
Evident disturbances of the transformed signal characterized by high peaks in the transformation window for particular DWT details can be considered as tangible information about the presence of damage in the structure. The signals obtained from excitation of the undamaged panel and with the defect in the form of a cavity were analyzed over 16,384 time-intervals, whereas in the case of a plate with two holes for 65,536 intervals. It was necessary to fulfill the condition of dyadic discrete wavelet transform and provide 2N data for decomposition. During the analysis of the obtained signal, the authors tested different ranges (4, 6, 8, 10) of Daubechies wavelet.