TRANSMISSION OF YAW SEAT VIBRATION TO THE HEAD

The transmission of yaw-axis vibration to the heads of seated subjects has been investigated at frequencies below 5 Hz. The variability between and within subjects and the effects of backrest contact, visual environment and the position of the centre of rotation have been investigated. The subjects sat on a rigid flat seat and were exposed to random motion at a magnitude of 1·0 rad/s2r.m.s. (root-mean-square) for 2 min. Head motion was measured in six axes using a light-weight bite-bar held between the teeth. Twelve male subjects participated in a study of the effect of backrest contact and visual conditions and one male subject participated in a repeatability study. A “back-on” posture (subjectʹs back in contact with the seat backrest) increased the frequency of maximum transmissibility from 2 to 3 Hz compared with a “back-off” posture. There was little change in transmissibility with the subjects sitting with their eyes open compared to their eyes closed. With increasing separations between a subject and the centre of rotation (at six distances from 0 to 500 mm with the subject facing outwards) there were large increases in lateral acceleration at the head.