Predicting the discomfort caused by tri-axial whole-body vibration

Standardised methods of evaluating whole-body vibration assume that the discomfort caused by multi-axis vibration can be predicted from a power summation of the discomfort caused by the vibration in each axis. The ‘power’ is either two, for the root-sums-of-squares (r.s.s.) method, or four, for the root-sumsof-quads (r.s.q.) method. This study investigated whether the prediction of the discomfort caused by 7-s periods of tri-axial vibration depends on the frequency of random vibration (in three one-octave bands over the frequency range 1.4 to 11.3Hz) with a frequency-weighted vibration dose value of 1.0 ms-1.75 in each direction.Eighteen seated subjects provided magnitude estimates of their discomfort when exposed to single-axis vibration and when exposed to tri-axial combinations of the single-axis motions. The linear sum of magnitude estimates of discomfort caused by single-axis vibration, and the r.s.s. of the magnitude estimates of discomfort caused by single-axis vibration, tended to overestimate the discomfort caused by tri-axial vibration. The greatest magnitude estimate of discomfort caused by any component of the tri-axial motion, and the r.s.q. of magnitude estimates of discomfort caused by single-axis vibration, tended to underestimate the discomfort caused by tri-axial vibration. Over all tri-axial vibration stimuli investigated, there were statistically significant differences in the optimum power for summing magnitude estimates to predict the discomfort caused by tri-axial vibration. There was a highly significant correlation between the optimum power for summing magnitude estimates of discomfort and previously investigated rates of growth of discomfort when increasing the magnitude of the translational components within the tri-axial vibrations. It is concluded that the optimum power for summing vibration magnitudes differs for different combinations of tri-axial vibration because the relation between the vibration magnitude and vibration discomfort (i.e., the rate of growth of vibration discomfort) varies with both the frequency and the direction of whole-body vibration