## 5 Results and Discussions  
The frequency response function (FRF) of the WTS test specimen with and without HCDP is plotted in a narrow frequency band and a one-third octave band. The narrow frequency band spectrum in this study is mainly used to extract the information about the number of resonance frequencies (47.7 Hz, 93.0 Hz, 126.6 Hz, 142.2 Hz, 162.5 Hz, 282.8 Hz, 335.9 Hz) that are occurring below 355 Hz, see Figure 7. In the present experimental study, one-third octave band is more suitable for comparing vibration attenuation, see Figure 8. In this contribution, only the one-third octave bands around the most prominent resonance peaks are of special interest. To make these frequency ranges easily recognizable, gray patches in the FRF are plotted. Furthermore, for each HCDP position on the WTS test specimen (see Figure 6) the difference in the vibration amplitude in comparison to the reference specimen around resonance peaks is plotted, see Figure 9. The gray patches are also used in this plot to identify the one-third octave frequency band around which the highest vibration amplitude is occurring. The horizontal red line in the difference plot (see Figure 9) represents 0 dB. The region above the red line shows an increase in the vibration amplitude and the region below the red line represent a reduction in the vibration amplitude. In Figures 7-9, FRF and the vibration attenuation obtained by attaching the MU configuration on the outer wall of the generator ring (see Figure 6e) is plotted. MU HCDP is obtained by joining the SU HCDP together. In this case, SU HCDP filled with stone  
Figure 6: Location of HCDP on the WTS test specimen (**a**) Test specimen without HCDP (Reference test specimen), (**b**) SU HCDP mounted between the stator ring and stator arm (**c**) MU HCDP assembled between the stator ring and stator arm, (**d**) SU HCDP on the outer wall of the stator ring (opposite to excitation point), (**e**) MU HCDP on the outer wall the stator ring (opposite excitation point), (**f**) MU HCDP on the inner wall the stator ring (opposite excitation point), (**g**) SU HCDP on the inner wall of the stator ring (excitation point side), (**h**) Configuration 1, (**i**) Configuration 2, (**j**) Configuration 3.  
powder is mounted together with a SU HCDP filled with sand to create MU configuration. Both SU HCDP are having a filling ratio of FR1. The resonance frequency of this system is plotted in the narrow band spectrum, see Figure 7. In the one-third octave band plot, it is visible that mounting the MU HCDP on the outer wall of the WTS ring reduced the vibration amplitude over the whole frequency range from 45 Hz to 355 Hz except for the central frequency of 125 Hz, where this configuration increases the vibration amplitude by approximately 1 dB, see Figure 8. As mentioned before, the present damping concept is only analyzed for the most prominent resonance peak, which is marked through gray patches. Therefore, the resonance peak occurring around 125 Hz is not crucial for this damping concept. Furthermore, the sum level (SL) value of the surface velocity (logarithmic measure of the effective surface velocity relative to a reference value) for the MU configuration on the outer wall of the generator ring (150.28 dB) in comparison to the SL value for the reference test specimen (158.28 dB) also confirms the tremendous damping achieved by this MU HCDP setup. This MU setup can reduce the vibration amplitude up to 12 dB (dark blue dotted line), see Figure 9. In contrast, the same MU HCDP configuration attached to the inner wall of the WTS ring (see Figure 6f) shows almost no efficiency in reducing the vibration amplitude (brown dashed line). This phenomenon can also be observed in the higher SL value (160.39 dB) in comparison to the reference specimen SL value (158.28 dB). The above mentioned results show that the position of HCDP on the WTS test specimen is very crucial for reducing the vibration amplitude.