Vertical vibration of seated subjects: effects of posture, vibration level, and frequency.

The transmissibility of 12 seated subjects to sinusoidal vertical vibration was measured at 12 frequencies in the range of 7 to 75 Hz. Transmissibility, the ratio of seat vibration to vertical head vibration measured on a bite-bar, was determined at each of six levels of vibration (0.2 to 4.0 m/s2 r.m.s.) in two body postures. The postures were defined by the body positions that maximised and minimised the sensation of vibration at the subjects' heads. Measurements of the fore-and-aft (ax), lateral (ay), and pitch motions of the head were measured on three subgroups. Posture was found to have a very large effect on the vibration of the head with an approximate 6:1 difference in the mean vertical head vibration of the subjects between the two postures in the range 35 to 50 Hz. In both postures the mean transmissibility of the subjects decreased as frequency increased from 7 to 75 Hz and there were statistically significant reductions in transmissibility with increasing levels of vibration. Some results from individuals are presented to illustrate the large differences between subjects. The reactions of the subjects to the vibration are outlined and some implications of the results are discussed.