The effect of the position of the axis of rotation on the discomfort caused by whole-body roll and pitch vibrations of seated persons

Methods of predicting the discomfort caused by rotational vibration of subjects seated away from the axis of rotation from a knowledge of the discomfort caused by single-axis vibration were investigated. The method of category production was used, in which ten seated subjects adjusted the level of sinusoidal vibration until it could be described as “uncomfortable” on a given semantic scale. Judgments were made for four frequencies (2,4, 8 and 16 Hz) for vibrations in each of five single-axis motions (roll, pitch, fore-and-aft, lateral, vertical) and for roll and pitch vibrations with subjects sitting various distances and directions away from the axis of rotation. For similar vibration conditions there was no difference in levels described as “uncomfortable” between vibration in the fore-and-aft and lateral axes. Rotational vibration in “pure” roll produced greater discomfort than the same level and frequency of rotational vibration in “pure” pitch. Sensitivity to vibration acceleration decreased with increasing frequency in the fore-and-aft, lateral, roll and pitch axes. Sensitivity to vertical acceleration showed only a small decrease with increasing frequency. The efficiency of the prediction methods depended upon the vibration axis and frequency and the direction from the axis of rotation. The level of vibration producing discomfort caused by rotational vibration of subjects seated away from the axis of rotation (i.e., with combined single-axis motions) was adequately predicted by the most severe single-axis component of the motion and by the root-mean-square of the equivalent levels of the combined-axis motions. The most severe component alone method provided the best prediction procedure for many vibration conditions.