## 3 Concept of Tests
### Description of the Experiment  
For the purpose of conducting the experiment, a measurement stand was built, the diagram and view of which is shown in Figure 3.  
The test sample (plate) was excited using a modified hammer tinsmith. The modification consisted of attaching a piezoelectric plate and a polymer tip to its flexible boom. This treatment allowed recording the signals generated by the piezoelectric while hitting the sample with the hammer and minimizing the possible recontacts of the excitation with the sample. The precise knowledge of the moment of impact and the identification of the presence of possible, unintentional repetitions allows the starting point of the observation to indicate the signal coming from the transducer. The signals generated by the transducer were amplified and filtered in the preamplifier. The preamplifier has a built-in bandpass filter with a frequency range of \(20\div 1000\) kHz. Then the signal was amplified in the amplifier and recorded with a resolution of 16 bits and a sampling rate of 1 MS/s in a binary form using a measuring card in the memory of the computer.  
Before each test, the calibration of the recording circuit with the use of the Hsu-Nielsen method was performed. The measurements were carried out in three series of five excitations for different distances from the transducer. This number of attempts was considered sufficient to indicate the characteristics of the signal-dependent on the distance and type of damage. These tests were carried out first for the undamaged panel and then repeated for each type of damage: a small cavity, a large cavity, a hole and two holes.  
The mutual location of the defects, the transducer, and the impact points in the case with two holes are shown in Figure 4. The remaining defects were modeled in the upper half-plane (above a line drawn through the points of excitation).  
Figure 3: Scheme (**a**) and view (**b**) of measurement stand.  
Before the analysis, the recorded signals were denoised using the wavelet transform with a discrimination threshold equal to 10 mV to prevent the impact of equipment noise on the calculation results.  
Based on catalog data and preliminary research, it was estimated that the noise of the measuring equipment was at the level of 6 mV. Hence, it was assumed, with some margin, that signals with amplitudes less than 10 mV should be rejected. At the same time, in order not to lose the information contained in the signal, it was decided to use the most effective noise reduction mechanism.  
As shown in work [21], the use of the wavelet transform is an effective method of noise removal and guarantees minimal information loss. The assumption of the algorithm is to minimize the mean square error. The procedure algorithm includes three main steps:  
1. signal decomposition;
2. filtration of detail transformation;
3. reconstructing the signal based on the filtered details.  
The appropriate selection of the wavelet used for filtration requires comparing the noise reduction results obtained for different wavelets.  
Exemplary signals coming from both excitation and measuring transducer are shown in Figure 5.  
Figure 4: Diagram (**a**) and top view of the excited plate (**b**) for a case with two defects—holes; orange circle—location of the transmitter, letters A–E excitation points, circles—locations of damages (holes).