Using numerical simulation, we can explore the influence of some design parameters to the vibration suppression performance of the TMPD. We first investigate the enclosure geometry. The volume and width of the enclosure as well as all the other parameters are kept constant (i.e., the same as those listed in Tables 1 and 2), and we only change the ratio of the enclosure height to length, denoted by $\gamma $. This affects how the particles impact the enclosure ceiling. Again, we search for the response peaks for the first and second modes of the integrated system under each $\gamma $ ratio. Figures 14(a) and 14(b) show the results under different excitation levels 0.20 N, 0.24 N, and 0.30 N, respectively. As can be seen, for small ratio $\gamma $, the amplitudes of both the first and second resonant response peaks are relatively high for all the excitation levels applied. The reason is that, when the clearance height is small, many particles only have limited motion and also impact frequently with the enclosure floor/ceiling, so they behave more like added-on mass to the system. As the ratio increases, the energy dissipation effect of the TMPD increases and reaches certain optimal value. If the ratio further increases, the energy dissipation effect saturates since particles will not be able to reach the enclosure ceiling. The frequencies of the peak responses of the system under different ratios $\gamma $ indeed change, i.e., 5.66 Hz, 5.69 Hz, and 5.75 Hz under ratio $\gamma =1.0$, 2.0, and 3.0, respectively, for the first mode when the excitation amplitude is 0.24 N. The ratios of the remaining energy to the total work done by the external force, for the first and second modes, are listed in Table 4. It can be observed that the energy ratios are consistent with the results shown in Fig. 14.