## 4 Validation and Discussions  
The vertical acceleration values obtained from this simulation work were validated by the testing data gathered from real-life car and driver system operating at the condition identical to this simulation set up. A standard hatchback car was maintained to achieve a speed of 30-35 m/h from static state, whereas a human male driver of 78 kg mass occupied the driver seat. The road terrain had no visible sign of roughness, and acceleration vs. time data was recorded for 60 s. The vibration measurement equipment--NI 9234 USB module, CompactDAQ chassis, and transducer Dytran 3055--were utilized to record data at different points of driver body. Testing set up and sample raw test data for thigh of human male driver are shown in Figures 8 and 9, respectively. \begin{table}
\begin{tabular}{c c} \hline
**Human Segment** & **Average Frequency (Hz)** \\ \hline Head & 2.06 \\ Chest & 1.20 \\ Waist & 1.21 \\ Upper arm & 1.17 \\ Lower arm & 3.39 \\ Thigh & 4.70 \\ Leg & 0.93 \\ \hline \end{tabular}
\end{table} 
Table 1: Average frequencies of human segments. Figure 8: Test set up: (**a**) standard laptop with the signal processing tool “m+p Analyzer”, (**b**) NI 9234 module for measurement, and (**c**) Dytran 3055 sensor mounted on human driver thigh. Figure 7: Frequencies at the points of interest. The testing data for vertical acceleration were collected in SOT (In-built file type from \"m+p Analyzer\") format for a time span of 60 s, and to match to the time duration of simulation, the test data were curtailed and filtered to initial 10 s in XLS (Excel. Spreadsheet) format. For comparison purpose, test data and the respective simulation results were merged into a single plot area, and the contrasts are displayed in Figure 10.  
For human head, human waist, and human upper arm, the upper and lower limits of the testing data were well below the respective limits of simulation results. For human chest, human lower arm, and human thigh, the upper limits of accelerations from simulation were above the respective upper limits of test data, whereas magnitudes of the lower limits of accelerations from testing data exceeded the respective limits from simulation results. For human leg, both the limits from simulation results were lower that the respective testing data. The testing data were based on both the human body and the car seat, where numerous parameters were associated while measuring the acceleration output,  
Figure 10: Vertical accelerations of human segments from simulation (blue) and testing (red). Figure 9: Raw vibration test data for human driver thigh: (**a**) acceleration vs. time, (**b**) power spectrum density vs. frequency. and each of the parameters was capable of manipulating the output characteristics on its own. In contrast, at the present stage of this analysis work, the simplified human model was utilized and no car seat was associated with the human model in finite element simulation. Thus, the mismatches between the testing data and simulation results were inevitable. To validate the frequencies obtained from this simulation work, past references in the relevant field were consulted. While establishing the biodynamic model of human body [36], the frequency ranges in the for-aft and vertical directions were taken as 0.5-12 Hz and 0.5-20 Hz, respectively. Vibro-acoustic analysis on the human body [45] presented a set of frequency data for different human organs and parts, as shown in Table 2.  
From the past investigations in the field of human biomechanics, it is inevitably clear that the maximum permissible level of frequency inside the human portions are generally restricted to a limit of 20 Hz. The international standards on biodynamics and ergonomics ISO/TC 159/SC 1, ISO/TC 159/SC 3, and ISO/TC 159/SC 4 also outline the guidelines to design the human interfacing structures not exceeding this limit. International standard vibration transmission study [46] on 12 human objects reported frequency level as 4 Hz, whereas the peak values were between 8 Hz and 9 Hz.