## 3 Results
### Realization and Evaluation of the Second Phase
It should be mentioned that only slight changes occurred concerning the natural frequencies (MVRD < 1%) and damping ratios (MVRD = 2-3%) estimated in the reference stage. However, the reduced number of DOFs caused the inaccuracy in the estimation of the mode shapes. On the other hand, the larger number of DOFs led to an increase in the mode shape accuracy and measurement time requirements as well. A larger number of analysis lines, set through the use of FFT, contributes to increasing the frequency resolution. In this case, it was not possible to observe the mutual vertical shift of the singular lines; however, higher, sharper, and narrower CMIF peaks occurred. Moreover, the exponential weighting of the responses combined with the higher frequency resolution led to a minimization of the spectral leakage, contributing to larger sharpness of the peaks as well. Although there was no significant change in the natural frequencies obtained (MVRD < 0.4%), a radical decrease in the damping ratios, especially for the first five modes (MVRD = 16-20%), was observed. By comparison of CMIF plots obtained from the MIMO analysis, it was found that each combination of reference degrees of freedom used contributes to the magnitudes of singular values differently. In two stages, six repeated modes of the structure were estimated. On the other hand, there were some stages in which not all of the modes were found. This finding confirmed that the choice of the appropriate number as well as the position of the reference degrees of freedom plays an important role in the estimation of the modal parameters. When comparing the modal parameters, it was found that the achieved relative differences in the natural frequency values (MVRD = 0.21-0.43%) and damping ratios (MVRD = 2.27-5.95%) were approximately at the same level as those for the other measurements. The advantage over the previous phases was the acquisition of the modal parameters of the repeated modes and the higher accuracy of the obtained mode shapes. The disadvantage was the increase in the time duration of the experiments, as it was doubled and tripled, respectively. In the second (comparison) phase of the measurements, three structures (i.e., circular plates) with the same shape and dimensions manufactured by the FDM method were analysed. The structures with 15% honeycomb, star, and concentric infill pattern densities differed very slightly in mass (see Table 2). To compare the modal parameters of these structures, it was necessary to perform measurements under the same boundary conditions and with the same measurement parameters set. Although, due to the realization of the MIMO analysis in the pre-test phase, the obtained singular curves contained richer mode shape information, leading to a more accurate and detailed reconstruction of the mode shapes, based on the comparison of the obtained results and the time requirements of the analyses, the authors decided to perform 10 MISO analyses of each structure. The results of the measurements represented by the repeating (almost identical) CMIF plots validated that there was no error on the part of the experimenters in the individual measurements. A comparison of the mean singular curves of the CMIF plots obtained from the 10 measurements, carried out for each specimen, can be seen in Figure 20. From this comparison, as well as from Table 9, it is obvious that the natural frequencies of the analysed circulate plate with a concentric infill pattern are significantly lower than those of the other two structures. This confirms the fact that the specimens with honeycomb or star infill patterns have higher stiffness than those with concentric infill patterns. The relatively small values of the damping ratios obtained for each specimen indicate their prolonged vibrations and strong response to excitation at or near their natural frequencies. The standard deviations evaluated for each analysed modal parameter reach very small values, indicating the validity of the results presented here.