Effects of inter-subject variability and vibration magnitude on vibration transmission to head during exposure to whole-body vertical vibration

In this paper, the effect of inter-subject and intra-subject variabilities on transmission of vibration through seated human subjects is discussed using experimental results. The experimental study targeted three representative postures (backrest, erect, and forward lean on table) while performing sedentary activities and under three magnitudes (0.4, 0.8, 1.2 m/s rms) of vertical vibration. The frequency range considered is 1 Hz–20 Hz as representative of those likely prevailing in wide range of vehicles. The data sets are investigated in terms of STH (seat-to-head) and BTH (back support-to-head) transmissibilities and phase differences, and respective coherences under the magnitudes and postures undertaken. In addition to determining the effect of different frequencies, subjective readings were collected at vertical backrest support postures at representative frequencies using the Borg CR 10 comfort scale. The responses show significant variations in transmissibility and phase among all of the subjects. In all postures, the mean STH transmissibility increases with increasing vibration magnitude at body resonance frequency, which lies approximately between 4.5 Hz and 6 Hz. Resonance in STH transmissibility of erect and forward lean on table posture visibly tends to shift to a lower frequency with increasing vibration magnitude. The subjective reading obtained, in terms of discomfort level, match with experimental data sets and provides evidence that human body resonance frequency or discomfort zone is around 5 Hz. Therefore, it might be concluded that the inclusion of vibration magnitude, posture, and inter-subject variabilities in the prediction of seat biodynamic response is essential. The development of biodynamic models and design of seat should include the variation of STH and BTH transmissibility and phase in different possible postures under different vibration magnitude.