Discomfort of Seated Persons Exposed to Low Frequency Lateral and Roll Motion

Passengers of land transport are exposed to horizontal and rotational oscillations at frequencies less than 1 Hz which may cause vibration discomfort and motion sickness. Previous knowledge of human responses to motion is insufficient for predicting the discomfort caused by low frequencies. The objective of this thesis is to improve understanding of subjective responses to lateral and roll oscillation (presented in isolation and in combination) at frequencies less than 1 Hz in order to establish a predictive model of comfort. The first of five experiments tested the predictions of a conceptual model of motion sickness. Illness ratings were obtained over a 30-minute exposure to 0.2 Hz fully roll-compensated lateral oscillation where the point of full roll-compensation was either at the seat surface (i.e. 'seat compensation') or at head height (i.e. 'head compensation'). Median illness ratings were greater during 'head compensation', showing some support for the motion sickness model, but differences were not statistically significant. Age, stature and body weight had no effect on illness ratings, but Asians were more than three-times as likely to experience 'mild nausea' than Europeans. It is concluded that differences in the position of full roll-compensation in transport vehicles are less important for motion sickness than inherent differences in passenger populations. The next four experiments used the method of magnitude estimation to determine the vibration discomfort caused by lateral oscillation, roll oscillation, and fully roll-compensated lateral oscillation with a variety of seating configurations. In the second experiment, lateral acceleration between 0.2 and 1.0 Hz caused less discomfort when sitting with a backrest than when sitting without a backrest on both a rigid seat and on a cushioned train seat; contrary to the predictions of current standards. In the third experiment, 0.25 to 0.4 Hz lateral acceleration in the plane of the seat caused similar discomfort regardless of whether the acceleration was due to lateral oscillation or roll oscillation through the gravitational vector, but above 0.4 Hz, discomfort from the roll was far greater. At frequencies less than 0.5 Hz, fully compensating the lateral acceleration with roll improved comfort compared to uncompensated lateral acceleration, but at greater frequencies, roll-compensation worsened comfort and caused discomfort similar to pure roll oscillation at 1 Hz. The fourth and fifth experiments examined differences in discomfort caused by the rigidity of the seat pan and the height of the backrest. In the fourth experiment, discomfort was greater on a soft foam seat than on a rigid seat during lateral oscillation below 0.63 Hz, during roll oscillation below 0.5 Hz and during fully roll-compensated lateral oscillation between 0.315 and 0.5 Hz. In the fifth experiment, discomfort was greater without a backrest than with a short backrest for lateral oscillation between 0.315 and 0.5 Hz. Contrary to current standards, discomfort was also greater without a backrest than with a high backrest for lateral oscillation below 1 Hz and for roll oscillation below 0.5 Hz. In addition, sitting with a backrest was beneficial for comfort with fully roll-compensated lateral oscillation between 0.4 and 0.63 Hz. The results of the five experiments were collated to provide recommendations for the improvement of current vibration standards. On the basis of experiment 1, a new multiplying factor for the prediction of vomiting incidence in an unadapted group of male Asian adults is offered. On the basis of the four discomfort experiments, modifications to current frequency weightings for lateral acceleration and roll acceleration are offered so as to extend the prediction to frequencies less than 0.5 Hz. Guidance for the prediction of discomfort with fully roll-compensated lateral oscillation is also provided. The thesis is concluded with recommendations for future research.