## 3 Results
### Realization and Evaluation of the Pre-Test Phase
#### 3.1.3 Results Obtained in the III Stage of the Pre-Test Phase  
The number of analysis lines in an analysed signal defines the balance between the time and frequency resolution and influences the process of performing FFT. A larger number of analysis lines improves the resolution of the frequency domain, as the frequency spectrum is divided into smaller bins. On the other hand, a larger number also increases the time window, i.e., the duration of the signal being transformed, computation time, and memory usage.  
Analyses of the symmetric structure with repeated modes near the same frequency or closely spaced modes, where a high-resolution FFT is required to separate them, can be a reason for using a larger number of analysis lines. The MTC Hammer V21.0.0 software, using which the EMAs described in this article were carried out, allows the user to set the frequency span \(F_{s}\) and the number of analysis lines \(N\). The frequency resolution of and block length \(T\) are then calculated as follows:  
\[df= \frac{F_{s}}{N}, \tag{2}\] \[T= \frac{1}{F_{s}}. \tag{3}\]  
This stage of the pre-test phase is intended to analyse the modal parameters obtained for three different numbers of analysis lines, i.e., \(N\) = 400 (set in the I\({}^{\text{ref}}\) stage), \(N\) = 800 (set in the III\({}^{\text{A}}\) stage), and \(N\) = 1600 (set in the III\({}^{\text{B}}\) stage). As the same 0-800 Hz frequency span was set in each of the measurement stages, it led to changes in the frequency resolution from 2 Hz to 1 Hz and finally to 0.5 Hz. The mean courses of the CMIF plots obtained from the three measurements realized in the I\({}^{\text{ref}}\), III\({}^{\text{A}}\), and III\({}^{\text{B}}\) stages can be seen in Figure 12.  
Figure 11: Mode shapes of the clamped specimen with honeycomb infill pattern evaluated in the II\({}^{\text{B}}\) stage (model with 96 DOFs) of the pre-test phase.  
Figure 12: Comparison of mean singular curves of CMIF plots obtained from the three experimental modal analyses carried out in the I\({}^{\text{ref}}\), III\({}^{\text{A}}\), and III\({}^{\text{B}}\) stages of the pre-test phase.  
In contrast to the courses from the second stage of the pre-test phase, in this case, it is not possible to observe the mutual vertical shift of the singular lines. However, it is obvious that the larger the number of analysis lines, the higher, sharper, and narrower the CMIF peaks. Since the damping ratio is estimated based on the width of the resonance peaks, narrower peaks can lead to an underestimation of the damping ratios. Moreover, in each measurement realized, an exponential weighting of the response was used. The combination of a higher frequency resolution with exponential weighting of the responses leads in general to a minimization of the spectral leakage. Such a reduction in leakage contributes to the sharpness of the peaks as well, resulting in a lower damping ratio. Although there was no change observed in the natural frequencies, a significant decrease in the damping ratio was obtained, especially for the first five modes (see the quantitative comparison of the results in Section 3.2). No new or closely spaced modes were found in this measurement stage. The estimated mode shapes look the same as those from the I\({}^{\text{ref}}\) stage.