Duration of whole-body vibration exposure its effect on comfort

Most human response to vibration standards imply that low vibration levels are acceptable for longer periods than higher levels. In such standards it is usually assumed that the relationship between exposure duration and vibration level is of a similar form for a wide range of different types of motion. The experiment described in this paper was conducted to determine whether the relative discomfort produced by 4 Hz and 16 Hz sinusoidal whole-body vertical (az) vibration was dependent on the duration of the vibration exposure. Each of eight seated subjects was exposed to two 36-minute vibration sessions. Both sessions consisted of ten-second periods of 4 Hz and 16 Hz vibration alternating continuously. In one session the 4 Hz motion was set at the "standard" level of 0·75 m/s2 r.m.s. while the level of the 46 Hz "test" motion could be adjusted by the subjects. In the other session the 16 Hz motion was the standard at 0·75 m/sl r.m.s. and the level of the 4 Hz motion could be adjusted. The subjects were required to control the intensity of the test motion to compensate for periodic changes in its intensity made by the experimenter and so to maintain it at a level which produced similar discomfort to that caused by the standard motion. It was found that the relationship between the average levels of the two motions when adjusted to produce similar discomfort was independent of the vibration duration. The findings are discussed in relation to other laboratory research and the need for a better understanding of the effects of the duration of a vibration on its acceptability.