## 3 Results
### Realization and Evaluation of the Second Phase  
The aim of the authors in the pre-test phase was to analyse the effects of different settings on the results of the modal analyses to find settings under which comparative measurements of the modal parameters of three shape-, dimension- and mass-correlated circular plates with different infill patterns produced by the FDM method could be carried out. For the experimental conditions, the following was required:  
* Realization of the experiments under the same conditions to minimize their influence on the values of the estimated modal parameters;
* Time efficiency in performing a larger series of measurements;
* Sufficient accuracy of the obtained measurement results. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Stage**} & \multirow{2}{*}{**MVRD**} & \multirow{2}{*}{**MVRD**} & \multirow{2}{*}{**MVRD**} & \multicolumn{6}{c}{**Estimated Damping Ratio (\%)**} & \multirow{2}{*}{**MVRD**} \\ \cline{3-3} \cline{8-12}  & & & & & & & & & & & & & & \\ \hline Iref & 1.460 & 1.432 & 1.242 & 0.841 & 0.706 & 0.611 & 0.761 & 0.630 & 0.724 & 0.557 & 0.587 & —Based on the analysis of the results and time requirements of the individual stages of the pre-test phase, the authors decided to implement the comparative EMAs under the conditions presented in Table 8. The specimen fixture, hardware, and software means of the measurement chain were the same as in the pre-test phase. To validate the results, the measurements were realized 10 times. Within each measurement, FRF plots (one example for each specimen can be seen in Figures 17a, 18a and 19a) were obtained, from which CMIF plots (Figures 17b, 18b and 19b) were extracted using SVD. Figure 17: Plots of the analysed specimen with honeycomb infill pattern: (**a**) FFRs obtained from one of the measurements realized; (**b**) CMIFs obtained from the 10 measurements realized. Figure 18: Plots of the analysed specimen with star infill pattern: (**a**) FFRs obtained from one of the measurements realized; (**b**) CMIFs obtained from the 10 measurements realized. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
\begin{tabular}{c} 1 to \\ 48 \\ \end{tabular} & 49 & 1600 & 400 & 10 & 10 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Settings used in the second (comparative) phase of tested specimens’ modal parameters. H = honeycomb, S = star, C = concentric, DOFs = degrees of freedom (excitation points), REF = reference degree of freedom (locations of response measurements), NoM = number of measurements, NoE = number of evaluations. of the selected measurement settings on the modal parameters of a circular plate with a honeycomb infill pattern realized in the 0-800 Hz frequency span. Since the specimen was printed from PLA filament, in order not to influence it, the measurement of the responses of the structure was carried out in a non-contact way using a vibrometer working on the laser Doppler principle. The analysis was divided into four stages:  
* The reference MISO analysis of the structure with 48 DOFs and one reference degree of freedom, realized with a frequency resolution of 2 Hz;
* The MISO analysis of the structure with 24 and 96 DOFs, respectively, and one reference degree of freedom, realized with a frequency resolution of 2 Hz;
* The MISO analysis of a structure with 48 DOFs and one reference degree of freedom realized with a frequency resolution of 1 Hz and 0.5 Hz, respectively;
* The MIMO analysis of a structure with 48 DOFs realized with a frequency resolution of 2 Hz, with emphasis on the choice of different numbers and locations of reference degrees of freedom. It was found that reducing or increasing the number of excitation points (i.e., DOFs) resulted in vertical shifts between the CMIF plots and significant differences in the heights of the individual CMIF peaks as well. These phenomena could have occurred due to the accuracy of the singular value decomposition, which depends on a sufficient number of DOFs to capture the response of all modes in the system.