Individual variability and its effect on subjective and biodynamic response to whole-body vibration

Abstract

The experiment described in this paper was designed to evaluate the differences in discomfort and body transmissibility between and within different population groups and to attempt to discover physical causes of the differences. The responses of one hundred-and-twelve seated subjects to vertical (z-axis) sinusoidal vibration at 4 Hz and 16 Hz were investigated. The subjects were in three groups (56 men, 28 women and 28 children) and were of varied demography. In the first part of the experiment they were required to make seven judgments indicating which of seven levels (in the range 0·41 to 2·46 m/s 2 rms) of 16 Hz vibration caused more discomfort than a fixed level (1·0 m/s 2 rms) of 4 Hz vibration. In the second part of the experiment the transmission of vertical vibration from seat to head was measured at the same two frequencies. The large differences found between the responses of individual subjects are presented as frequency distributions. The subjective responses from each of the three populations were well approximated by log-normal distributions. Overall, the data indicate that, for the median subject, similar levels of the 4 Hz and 16 Hz vibration produced similar degrees of discomfort. However, within each population about 10% considered equivalence between the two motions occurred at a 16 Hz level more than double the median equivalent level and 10% considered equivalence occurred at less than about half the median equivalent level. For each subject group and at both frequencies the distributions of transmissibilities were approximately normal. Within groups there were significant correlations between transmissibility and subject size and between transmissibility and subjective response. The significance of the findings to future research and the application of human response to vibration data is discussed.