## 3 Results and Discussion
### Time-Frequency Analysis of Velocity Measurements
For that reason, the FFT was computed and plotted to determine the maximum amplitudes of velocity in the frequency domain (mechanical resonances) as detailed in Figure 8. The frequency response functions (FRF) are compared in Figure 8a,c for 10 Hz to 100 Hz and in Figure 8b,d for 100 Hz to 1000 Hz. These signals were smoothed applying a 30-point FFT filter. For the window between 10 and 100 Hz (Figure 8a,c), the two highest resonance peaks were evidenced between 10 and 60 Hz. Figure 6: Time-domain results of velocity between 100 and 1000 Hz in \(e_{1}\) and \(e_{2}\). P1 and P2 measurements for each sample. (**a**) Group 1 (UR1, SMR1, and RP1). (**b**) Group 2 (UR2, SMR2, and RP2). Furthermore, the same ripening stage signals exhibited a similar behavior mainly for the measurement taken in P2 where amplitudes and resonance levels are quite close. However, signals taken in P1 showed some differences between SMR1 and SMR2 where SMR1 appeared to resonate at a lower resonance than SMR2. The evidenced structure in the signals showed that the experiments were well controlled, since the same peaks were evidenced for both directions. In addition, it is noted that resonance peaks keep close between the signals of each ripening stage. The resonance frequencies relations advise that the ratio stiffness/mass is lower in the unripe stage than the ripe one. Moreover, it indicates that signals move from unripe to ripe forwardly. Moreover, in the range from 100 to 1000 Hz, the highest resonance peaks were evidenced between 500 and 1000 Hz for the FFT of velocity (Figure 8b,d). In addition, both  
Figure 8: Fast Fourier transform (FFT) of the velocity measurements. At the \(e_{1}\) direction: (**a**) 10–100 Hz and (**b**) 100–1000 Hz. At the \(e_{2}\) direction: (**c**) 10–100 Hz and (**d**) 10–1000 Hz. Figure 7: Mean RMS of velocity measurements in directions \(e_{1}\) and \(e_{2}\). (**a**) 10 to 100 Hz. (**b**) 100 to 1000 Hz. measurements are correlated, since they contemplate the fruits dynamics after 500 Hz. All ripening stages echoed their characteristic resonance peaks. For this case, the signals begin with a high amplitude at 100 Hz and then decrease until higher amplitude resonances appear after 550 Hz. Nevertheless, this range can observe that the velocity resonance peaks shift differently when the ripening stage changes. For instance, the peaks of the ripened fruits appear first, which are followed by the unripe and semi-ripe resonating at higher resonances. This permits us to observe that the stiffness changes at these frequencies mainly influence the mechanical resonances. The URs present the highest stiffness, which are followed by the SMRs and the RPs. We can conclude that the similar behavior for the signals obtained for the same ripening stage fruits indicates that the resonances resemble the intrinsic properties of these. Star marks were labeled at resonance peaks of the signals. The values of frequency are listed in Table 3. It is important to mention that the peaks identification procedures were not performed using any methodology, since we consider that these are observable from a simple view while understanding that there are different methodologies that allow a procedure more replicable for the resonant peak extraction.