# Vibration Energy Harvesting from Raindrops Impacts: Experimental Tests and Interpretative Models
## 3 Experimental Results
### Statistical Analysis  
Figure 5 displays a statistical analysis of the experimental results through box plots representing the four quartiles and the median of the three characteristic parameters of the harvester response, obtained from 10 tests for each configuration. It is straightforward to observe that the dominant frequency had a low dispersion and there was high compatibility between the frequency computed from indoor and outdoor test results. Damping ratios and peak-to-peak voltages showed a higher dispersion. The outdoor tests showed higher damping ratios than the indoor tests, with both the empty and full spoon. Nonetheless, the distributions of indoor and outdoor tests in the same conditions (empty or full spoon) did overlap. Conversely, the peak-to-peak values of indoor and outdoor tests significantly differed and the indoor values were larger. The distribution of the indoor and outdoor values of the peak-to-peak voltage did not overlap. However, it is interesting to note that, in both the indoor and outdoor tests, the observed mean peak-to-peak voltage obtained with the full spoon was larger (by about four times) than the mean peak-to-peak voltage obtained with the empty spon. This result confirms the validity of the harvester with the spoon shown in previous studies [21] and suggests that the water layer is effective even if the operating conditions are slightly different. The difference between indoor and outdoor tests can be explained considering that in indoor tests the diameter of the drop was more repeatable and that the impact point of the drop on the harvester was carefully set in the middle of the spoon. The impact point cannot be controlled in outdoor tests. In order to investigate the effect of different impact positions, the response to impacts on various locations of the harvester device was measured using the indoor set-up. Figure 6 shows the four impact locations selected:  
* position 1, on the edge of the spoon farthest from the piezoelectric harvester;
* position 2, on the center of the spoon;
* position 3, on the edge of the spoon closest to the piezo;
* position 4, outside of the spoon, on the piezo itself (roughly 10 mm from the glued joint between the harvester and the spoon). Figure 7 compares the effect of the four different impact positions on the characteristic parameters of the response of the harvester with the full spoon. Again, 10 tests were considered in each configuration. The dominant frequency presented a low dispersion and was marginally affected by the different impact locations. Indeed, the dominant frequency corresponded to the frequency of the first mode of vibration of the device (harvester, spoon,  
Figure 5: Box plots representing the four quartiles and the median (in red) of the frequency, damping ratio, and peak-to-peak voltage registered in indoor (with artificial rain) and outdoor (with real rain) tests. (**a**) empty spoon, (**b**) full spoon. and water pool), which is an intrinsic characteristic of the system. Damping ratio and peak-to-peak voltage showed particularly high dispersion when the impact occurred on the edges of the spoon. In fact, when the impact is set to take place close to the edges, small perturbations can cause a displacement of the falling drop. As a consequence, some impacts may occur slightly inward with respect to the edge of the spoon (hence involving the water pool) and others may take place on the spoon sidewalls (resulting in a 'dry' impact, with a worse energy transfer). In general, impacts in position 3 generated lower voltages compared to impacts in position 1, which can be traced back to the lower arm of the impact force. It is particularly interesting to note that, when the impact occurred outside of the spoon, the generated voltage was particularly low, comparable to the lowest values obtained in outdoor tests; additionally, the same was true for some of the tests with the impact on one of the spoon edges.