Apparent mass and head vibration transmission responses of seated body to three translational axis vibration

Abstract

The apparent mass and seat-to-head vibration transmissibility response functions of the seated human body were investigated under whole-body vibration exposures to fore-aft (x), lateral (y), and vertical (z) applied individually and simultaneously. The experiments were performed with 9 adult male subjects to measure the biodynamic responses to single and uncorrelated three-axis vibration with and without hands and back supports under different magnitudes of random vibration in the 0.5–20Hz frequency range. The apparent mass and the head vibration transmission responses were derived using two different frequency response function estimators based upon the cross- and auto-spectral densities of the response and excitation signals, denoted as H1 and Hv estimators, respectively. The two methods resulted in identical single-axis responses but considerably different responses under multi-axis vibration. The responses derived from the Hv estimator revealed significant coupling effects of three-axis vibration, which could be directly related to contributions of cross-axis responses observed under single-axis vibration, particularly those attributed to sagittal plane motion of the upper body. Such coupling effect, however, was not evident in the three-axis responses derived using the commonly used H1 estimator. The results also revealed significant effects of hands and back support conditions on the coupling effects of multiple axis vibration and the measured responses. The results suggest that biodynamic responses of the seated body exposed to simultaneous three-axis vibration, commonly encountered in work vehicles, differ considerably from the widely reported responses to individual axis vibration. A better understanding of the seated human body responses to uncorrelated three-axis whole-body vibration could be developed using the power-spectral-density based Hv estimator. Relevance to industryThe seated body biodynamic responses to multi-axis whole-body vibration and knowledge of coupling in the responses are essential for developing more efficient analytical models of the seated human body for applications in vehicular seating design and dynamics, and for deriving improved frequency-weighting for exposure assessment.