## 3 Fabrication and Experimental Verification of Interconnected Metamaterials  
We describe the stopband analysis for attenuating flexural vibration and the frequency response analysis performed to verify the vibration reduction performance of the stopband in the previous section. In Section 3, we experimentally verify the vibration-reduction performance of the multi-celled interconnected metamaterial. For vibration testing, a multi-celled structure was fabricated by using a 3D printer. Figure 8 describes the manufacturing process for the multi-celled interconnected metamaterial. Figure 7: Displacement field of a multi-celled interconnected metamaterial along the z-direction at 2000 Hz (out of stopband) and 1188 Hz (center frequency of stopband); frequency response of (**a**) bare plate, plates with (**c**) 5 \(\times\) 5 and (**e**) 5 \(\times\) 10 metamaterials at 1188 Hz; frequency response of (**b**) bare plate, plates with (**d**) 5 \(\times\) 10 and (**f**) 10 \(\times\) 10 metamaterials at 2000 Hz. Figure 8a shows the resonator part used in the finite element analysis model. As mentioned in Section 2.1, the resonator part included a combination of solid and shell elements, considering the computational cost. As shown in Figure 8b, to 3D print the multi-celled metamaterial, the metamaterial was modified by using the numerical analysis model. In this study, Sindoh 3D 2X was used to fabricate the multi-celled metamaterial. The geometrical size of the model to be manufactured must be smaller than 228 mm \(\times\) 200 mm \(\times\) 300 mm, which is the size of the 5 \(\times\) 5 multi-celled structure. Considering constraints in manufacturing, the multi-celled structure of the 5 \(\times\) 5 module was fabricated twice to make a 5 \(\times\) 10 structure. Figure 8c shows the multi-celled structure fabricated by the 3D printer. The printer setting used in this manufacturing are shown in Table 2. Because the resonator part of the finite element model of the interconnected metamaterial is composed of an isotropic material, it must be manufactured with an 'infill' set to 100%. As the interconnecting element is modeled as a 1T shell, the height of each layer was set to 0.2 mm for stable lamination. The printing velocity was set to 40 mm/s. Increasing the printing velocity and layer height can reduce the working time, but it also reduces the quality of the product. If the infill density is lowered, the manufacturing speed increases, but the dynamic characteristics of the structure are significantly changed because empty spaces occur inside the resulting structure. Therefore, for 3D-printer-based manufacturing, the setting parameters should be carefully determined considering the shape and size of the structure. Using the printer settings in Table 3, one module includes 154 layers, and manufacturing requires 53 h 10 m. Figure 8d shows the schematics used for assembling  
Figure 8: Fabrication process of the interconnected metamaterial, using (**a**) a numerical analysis model; (**b**) modified model for additive manufacturing; (**c**) assembly scheme of the 5 \(\times\) 5 multi-celled metamaterial and host structure; (**d**) assembly schematics of metamaterial. multi-celled metamaterials. As shown in Figure 8c,d, The multi-celled metamaterial of Figure 8c is attached to the aluminum plate of 1T thickness, using an adhesive layer. Figure 9 shows the experimental setup for impact testing to verify the vibration-reduction performance of multi-celled interconnected metamaterials. The multi-celled metamaterial fabricated by a 3D printer was attached to the center of a 1T thick aluminum plate with dimensions of 200 mm \(\times\) 800 mm. The vibration responses of the bare plate and the plate with a 5 \(\times\) 10 multi-celled structure were compared. As shown in Figure 9, the excitation force was imposed at the end of the plate, and the vibration response was measured at the center of the other side.