## 3 Results and Discussion
### Vibration Levels in Stacked Units and the Effect of FM, and Transmissibility  
The recorded vibration levels displayed in the PSD plots are shown in Figure 6 for various amounts of FM space and layer positions in all three orientations. In all SAVER positions, the results showed that the highest vibration levels occurred in the vertical direction, followed by lateral and then longitudinal. In the vertical direction, the most intense vibration levels in higher layers were found in the frequency range of 8-50 Hz; below 8 Hz, the vibration intensity was almost the same in higher layers as on the floor. Additionally, around 40-50 Hz, cross-frequency could be observed, above which the intensity decreased rapidly below the floor intensity. The reason for the same vibration intensity between 1 and 8 Hz could be that the applied sample stacked unit, by its mass, moved together with the vehicle in this frequency range. The results also showed that in the longitudinal direction, this cross-frequency could also be found above, in which the intensity also decreased below the floor's intensity. However, in a lateral orientation, the vibration intensity in the higher layers was already higher than that in the lower region (below 8 Hz). This was also true for the longitudinal direction if any FM space (10% or 20%) was applied.  
Figure 6: PSD plots of various layers in a stacked unit and on the floor at FM 0%, 10%, and 20%, in the (**a–c**) longitudinal, (**d–f**) lateral, and (**g–i**) vertical axes.  
Figure 7 presents the transmissibility plots for vertical orientation in the layers of the stacked unit with various amounts of FM space in the frequency range of 1-200 Hz. The peaks of the transmissibility ratio in the third layer were 11.9 (21 Hz), 7.2 (15 Hz), and 11.8 (16 Hz) at FM 0%, 10%, and 20%, respectively. At FM 10%, the highest peak was observed in the first layer. This phenomenon can be attributed to some shock (impact) impulse that may have occurred due to potholes or severe road roughness during this measurement. Nevertheless, it is clearly visible that the amplification at FM 0% and in a higher layer was significant in the frequency range of 8-50 Hz. However, if any FM space was applied, this frequency range was wider, as shown by FM 10% and 20%. It also has to be noted here that the middle layer was partly suppressed by the package above it, and this is reflected in the recorded and calculated levels. This phenomenon has already been observed in previous research in laboratory vibration simulations for unitized load shipments [16]. Table 4 reports the overall G\({}_{\text{rms}}\) values of the calculated vibration data for the various FMs and layers in the range of 1-200 Hz. The highest overall G\({}_{\text{rms}}\) values were observed in the third layer in each case in the vertical direction, followed by the lateral and longitudinal directions. In the analyzed frequency range of 1-200 Hz, the overall G\({}_{\text{rms}}\) values increased as layers were added. In the vertical direction, the overall G\({}_{\text{rms}}\) value was 1.22-1.48, 1.29-20.3, and 1.08-2.28 times higher than those on the floor, in the longitudinal direction, and in the lateral direction, respectively.