## 4 Experimental Results  
The new custom real-time acquisition system was tested in a series of low-energy impact loading experiments so as to not damage the laminate. The aim of those experiments  
Figure 6: Experimental set-up with the eight piezoceramic transducers attached with a bi-adhesive tape to a 1.4 mm aluminum plate, shown in the bottom of the figure, and the real-time electronic acquisition system, shown in the top. In the center of the plate, the low-energy hammer impactor is visible. The figure also shows the colored play dough covering the edges of the aluminum plate; this method was adopted to attenuate the signals received from multipath. Reproduced from [24]. was to verify the actual capabilities of the SHM system (i.e., the localization accuracy) processing signals acquired from different numbers of sensors. The fundamental symmetric (S\({}_{0}\)) and anti-symmetric (A\({}_{0}\)) group velocity dispersion curves for an aluminum plate with thickness of 1.4 mm are shown in Figure 7. These curves were obtained with the LAMB MATLAB toolbox GMM calculator. From Figure 7 we can evaluate the propagation velocity of the S\({}_{0}\) mode. This value was used to solve Equation (1). For our experiments we considered the propagation velocity v = 5150 m/s, corresponding to a frequency of fS0 = 650 kHz, which falls into a frequency range where the dispersive behavior of S\({}_{0}\) mode is negligible but still retains a small wavelength v/fS0 = 8 mm. We performed three impacts: impact P1 with position x\({}_{\text{i}}\) = 190 mm, y\({}_{\text{i}}\) = 150 mm, performed inside the ROI, delimited by sensors; impact P2 with position xi = 193 mm, y\({}_{\text{i}}\) = 32 mm, performed outside the ROI; and impact P3 with position xi = 200 mm, y\({}_{\text{i}}\) = 40 mm, performed outside the ROI. In Figures 8-13 we report the results for impacts inside and outside the ROI. In detail, Figure 8 shows the performance of the acquisition system detecting impact point P1 and processing the signal acquired by only four sensors: Rx1, Rx3, Rx5, and Rx7, whereas Figure 9 shows the result obtained by detecting impact point P1 and processing the signal acquired by all eight sensors (Rx1-Rx8). The estimated position of the impact point obtained by processing signals acquired by only four sensors (P1\({}^{\prime}\)-1 point) is also reported in Figure 8, whereas Figure 6 shows the estimated position of the impact point obtained by processing signals acquired by all sensors (P1\({}^{\prime}\)-2 point). To evaluate the trade-off between sensor density of an SHM system and error in impact localization, we processed the signal acquired by six sensors Rx1, Rx2, Rx3, Rx5, Rx6, and Rx7, and the estimated position of the impact point (P1\({}^{\prime}\)-3 point) is reported in Figure 10.  
Figure 7: Viewgraph showing the calculated S\({}_{0}\) and A\({}_{0}\) group velocity dispersion curves for an aluminum plate with thickness of 1.4 mm. Figure 8: Position of impact P1 (red circle) inside the ROI with coordinates \(\mathrm{x_{i}}\) = 190 mm, \(\mathrm{y_{i}}\) = 150 mm. The detected impact position P1\({}^{\prime}\)-1 (red star) with coordinates \(\mathrm{x_{p}}\) = 196 mm, \(\mathrm{y_{p}}\) = 134 mm was obtained by processing the ultrasonic signals acquired by only four sensors (Rx1, Rx3, Rx5, and Rx7). Reproduced from [24]. Figure 9: Position of impact P1 (red circle) inside the ROI with coordinates \(\mathrm{x_{i}}\) = 190 mm, \(\mathrm{y_{i}}\) = 150 mm. The detected impact position P1\({}^{\prime}\)-2 (red star) with coordinates \(\mathrm{x_{p}}\) = 192 mm, \(\mathrm{y_{p}}\) = 141 mm was obtained by processing the ultrasonic signals acquired by all sensors (Rx1–Rx8). Reproduced from [24]. Results shown in Figures 8-10 are summarized in Figure 11.  
In general, the installation of sensors is based on several constraints imposed by the target structure, while the impact event position is more unpredictable. It is, therefore, worthwhile analyzing impacts outside of the ROI.