Discomfort from sinusoidal oscillation in the pitch and fore-and-aft axes at frequencies between 0.2 and 1.6 Hz

Low frequency pitch and fore-and-aft oscillations arise in many modes of transport. Pitch oscillation rotates a seat through the gravity vector giving rise to a fore-and-aft acceleration in the plane of the seat: the measurement of fore-and-aft acceleration does not discriminate between the component of this acceleration arising from pitch and the component arising from horizontal acceleration in the fore-and-aft direction. The objectives of this study were to investigate whether fore-and-aft acceleration in the plane of the seat was an adequate predictor of vibration discomfort arising from low frequency oscillation in both the pitch and fore-and-aft axes, and to determine the effect of a backrest on discomfort during pitch and fore-and-aft oscillation at low frequencies. Twelve male subjects used the method of magnitude estimation to judge the discomfort produced by sinusoidal oscillations in the pitch and fore-and-aft axes at 10 frequencies between 0.2 and 1.6 Hz, while seated with and without a backrest. For both pitch and fore-and-aft oscillation, the rate of growth of discomfort with increasing vibration magnitude decreased with increasing frequency of oscillation, indicating that the frequency-dependence of discomfort is magnitude-dependent. At frequencies greater than about 0.4 Hz with a backrest, and at frequencies greater than about 0.8 Hz without a backrest, fore-and-aft acceleration in the plane of the seat arising from pitch oscillation caused greater discomfort than the same acceleration produced by fore-and-aft oscillation. A backrest increased discomfort with pitch oscillation at frequencies greater than about 0.63 Hz, but tended to decrease discomfort during fore-and-aft oscillation. The prediction of discomfort caused by low frequency pitch and fore-and-aft oscillation requires that both components are measured and assessed according to their separate effects, taking into account any beneficial and detrimental effects of a backrest.