Nonlinear subjective and biodynamic responses to continuous and transient whole-body vibration in the vertical direction

The effect of the magnitude of continuous and transient whole-body vibration in the vertical direction on both subjective and biodynamic responses of human subjects has been investigated experimentally. Additionally, the relation between the subjective responses and the dynamic responses has also been studied. Twelve subjects were exposed to sinusoidal continuous vibrations at five frequencies (3.15-8.0 Hz) and at three magnitudes (0.5-2.0 m s−2 rms). They were also exposed to transient vibrations that were modulated one-and-half cycle sinusoidal waveforms at the same frequencies as the continuous vibrations and at three magnitudes corresponding to the magnitudes used for the continuous vibrations. Discomfort was measured by the method of magnitude estimation with reference stimuli having frequency components in the middle of the frequency range used in this study. The driving-point dynamic responses (the ratio between the force and the motion, i.e., acceleration and velocity, at the driving point) were also measured and divided by the responses to the reference stimuli used in the measurement of discomfort so as to allow the comparison of the dynamic responses with the discomfort responses. Both the discomfort estimates and the normalised driving-point dynamic responses were influenced by the stimuli magnitudes, especially with the continuous vibration. At 3.15 and 4.0 Hz, the discomfort estimates and the normalised mechanical impedance and apparent mass increased significantly with increases in vibration magnitude from 0.5-2.0 m s−2 rms. Magnitude estimates for discomfort were correlated with the normalised mechanical impedance and apparent mass in the frequency range investigated. For the transient vibrations, the discomfort estimates and the driving-point dynamic responses were interpreted as responses in frequency bands around the fundamental frequency of the input motion. The results indicate similar nonlinearities in discomfort and driving-point dynamic responses associated with the principal body response within the range 3.15-8 Hz. The nonlinearity in discomfort at these frequencies may be partially caused by the nonlinear dynamic response of the body and is sufficient to require consideration in methods of predicting discomfort caused by vertical whole-body vibration.