## 3 Design of the Test Specim
The geometrical properties of HCDP are given in Figure 3. The thickness of the top face sheet, bottom face sheet, and wall thickness of honeycomb cells are equal. The WTS test specimen without HCDP is considered as the reference test specimen and the obtained results are compared to the WTS test specimen with HCDP attached. For assembling the HCDP at different positions of the WTS test specimen, six rectangular shape cuts are made in the HCDP, which are used to place small steel blocks of thickness \(0.9h_{\text{DP}}\). Afterwards, with the help of an aluminium thin sheet and screws, the HCDP is attached to the stator ring walls. This assembling technique of HDCP to WTS stator is useful for avoiding plastic deformation in the cores during the fastening process. The idea behind using screw joints is that the HCDP can be assembled and disassembled without any complexity. Additionally, the WTS test specimen parts, i.e., stator arm and stator ring along with HCDP can be reused for each  
To study the efficiency of the HCDP on the vibration response of the WTS test specimen, four different materials, namely: sand, stone powder, rubber powder, and tungsten powder are chosen. In addition, the influence of an empty HCDP on the vibration response of the WTS test specimen is studied. The idea behind this investigation was to show that the damping is mainly because of granular materials and not merely the design modification effect. The grain size of the chosen granular material particles is distributed non-uniformly. The distribution of particle size and their corresponding average particle size is analyzed by our project partner IFAM-Dresden. The selection of materials for this study was chosen based on their availability in the market and costs. Furthermore, the environmental friendly character of the materials and their recycling properties were also taken into consideration. Along with the influence of materials on vibration attenuation of WTS test specimen, the effect of filling ratios (FR) is also studied. For this purpose, two different filling ratios, namely: FR1 and FR2 are chosen, where FR2 = 0.5 FR1. It should be noted that no HCDP was filled with a 100% FR as it has been shown in the previous study that it is not effective for vibration attenuation [17]. In this contribution, the filling ratio is defined as the ratio between the amount of particles filled and the total volume of the cavity. The FR for each HCDP, filling material mass, and the total mass of the HCDP are stated in Table 2. The distribution of the granular materials in each honeycomb cell is uniform. The granular particles inside each cell are filled in such a way that they do not experience any compression and move freely inside the cavity. Hence, it is crucial not to fill the cavity with a 100% FR. Because this will restrict the movement of particles, which will then act as a rigid body and not as a particle damper. Therefore, this criterion is necessary to make particle damper more effective as this will increase the probability of collisions of the particles with each other, as well as with the HCDP walls. Furthermore, the influence of single-unit (SU) and multi-unit (MU) HCDP on vibration reduction are studied and compared with each other. MU particle dampers are obtained by assembling the SU particle dampers together.