## 4 Experimental Results
The full size of the area of the aluminum plate (500 mm \(\times\) 500 mm) was divided into a uniform grid of points spaced 1 mm apart  
Figure 11: Zoom of results shown in Figures 8–10. The illustration reports the real impact point P1 (red circle), the detected impact points P1’-1, P1’-2, and P1’-3 (red stars) together with the points 1% above the minimum of the error function (blue dots). Reproduced from [24]. Figure 10: Position of impact P1 (red circle) inside the ROI with coordinates x\({}_{i}\) = 190 mm, y\({}_{i}\) = 150 mm. The detected impact position P1’-3 (red star) with coordinates x\({}_{p}\) = 196 mm, y\({}_{p}\) = 142 mm was obtained by processing the ultrasonic signals acquired by six sensors (Rx1, Rx2, Rx3, Rx5, Rx6, and Rx7). Figures 12 and 13 demonstrate the ability of the electronic system to reveal, also, impacts outside the ROI with a good accuracy: in both cases, the error in the evaluation of \(x\)-axis and \(y\)-axis impact point coordinates was less than 10 mm. Another observation concerns the placement of sensors. In several real applications, it is not possible to install the sensors on the vertices of regular geometric shapes (squares, circles, hexagons, etc.) and thus positions are decided according to these, and other, constraints. From Figures 8-13 we can observe the position of the eight sensors that are symmetrical along the \(x\)-axis (Rx1, Rx3, Rx4, Rx5, Rx7, and Rx8) but non-symmetric along the \(y\)-axis (Rx2 and Rx6): these results demonstrate that the calculated impact point is well-identified even in cases of non-symmetric positions of sensors. The resulting estimates of the three impacts P1, P2, and P3 are summarized in Tables 2-4, respectively. The errors of the impact localization are reported in as a percentage with respect to the dimension of the monitored area, i.e., 500 mm. sensor reference coordinates. The main benefit of the AFE designed for this application is the possibility to select the best signal processing chain to avoid input saturation, with large energy impacts generating 1 to 10 Vpp amplitudes and retaining high SNR, in excess of 72 dB, in the 270-800 kHz bandwidth. We also point out that a certain redundancy in the number of sensors is always useful when considering the possible failure of single channels. Finally, thanks to the programmable configuration of up to 16 channels, this criteria for the selection of optimal sensor density will be investigated also on CFRP plates with the same sensor layout to account for the variability of velocity in composite laminates in different directions. Conceptualization, E.M.M. and L.C. ; methodology, A.B. ; software, E.M.M. ; validation, E.M.M. and A.B. ; formal analysis, E.M.M. ; investigation, A.B. and L.C. ; resources, L.C. ; data curation, A.B. ; writing--original draft preparation, L.C. ; writing--review and editing, L.C. ; visualization, E.M.M. ; supervision, L.C. ; project administration, L.C. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable. The authors wish to acknowledge the support of [PERSON], senior technician at the Ultrasound and NDT laboratory of the University of Florence, in conducting this work. The authors declare no conflict of interest.