Subjective and biodynamic responses of seated subjects exposed to whole-body vertical vibration at low frequency

Abstract

As the magnitude of vertical whole-body vibration increases, the resonance frequency in the apparent mass of the human body reduces and there are changes in the frequency-dependence of acceleration equivalent comfort contours. It is unclear to what extent these two ‘nonlinearities’ are related. This thesis seeks to advance understanding of the combined influence of the magnitude and the frequency of whole-body vertical vibration on the subjective and biodynamic responses of the seated human. Specifically, the research was designed to identify whether the nonlinearity in the subjective responses reflects the nonlinearity in the biodynamic responses, and whether comfort would be better predicted from the force applied to the body.
The first experiment was designed to investigate how the biodynamic and subjective responses of seated subjects (20 males and 20 females) depend on the frequency, magnitude, and waveform of vertical vibration when they were exposed to sinusoidal and random vibration. The vertical apparent mass and fore-and-aft cross-axis apparent mass obtained with random and sinusoidal vibration were both nonlinear but similar at the same overall magnitude (0.1, 0.2, 0.4, 0.8, and 1.6 ms-2 r.m.s.). With both increasing acceleration and increasing force, the rate of growth of discomfort depended on the frequency of vibration. Both acceleration and force equivalent comfort contours (the magnitude of vibration expressed as a function of frequency which produces similar degrees of discomfort) varied with the magnitude of vibration. The equivalent comfort contours were less dependent on the magnitude of vibration when expressed by force than by acceleration. There were statistically significant positive correlations between the biodynamic responses and subjective responses at all frequencies in the range 1 to 16 Hz.
The second experiment investigated subjective and biodynamic responses to a series of upward and downward vertical mechanical shocks at 13 fundamental frequencies (1 to 16 Hz) and 18 magnitudes (unweighted peak accelerations in the range 0.12 to 8.0 ms-2). The optimum stiffness and optimum damping of two lumped parameter models fitted to the measured acceleration time history decreased as the shock magnitudes increased. With both models, and with both downward and upward shocks, the median principal resonance frequency of the apparent mass of the body decreased from 6.3 to 4 Hz as the magnitude of the shocks increased from 0.05 ms-1.75 to 2.0 ms-1.75 VDV. There was no consistent difference in the rate of growth of discomfort between acceleration and force, or between upward and downward shocks, or between lower magnitude and higher magnitude shocks.
The final experiment compared subjective responses of the human body with a rigid seat and a soft seat. With increasing magnitude of vibration (both acceleration and force), the rate of growth of discomfort was dependent on the frequency of vibration, but did not differ between the rigid seat and the soft seat. There were no significant differences in either the force or acceleration equivalent comfort contours on the rigid seat compared with those on the soft seat. The frequency-dependence of the force equivalent comfort contours showed less nonlinearity than the acceleration equivalent comfort contours with both the rigid and soft seat conditions.
In conclusion, this study indicates some similarities in the nonlinearity of subjective responses and biodynamic responses of the seated body exposed to vertical vibration. Although force equivalent comfort contours are also nonlinear, they showed less dependence on the magnitude of the excitation than acceleration equivalent comfort contours.

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ORCID for Mike Griffin: orcid.org/0000-0003-0743-9502

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