The discomfort from exposure to low frequency rotational and translational vibration

Abstract

The discomfort arising from exposure to low frequency oscillation is a matter of some interest in many forms of transport. However there is a dearth of knowledge about the discomfort which arises from exposure to vibration at low frequencies. This thesis investigates the effects of frequency, magnitude and direction arising from the exposure of seated passengers to roll, pitch, fore-and-aft and lateral oscillation at frequencies between 0.2 and 1.6 Hz. The objectives of this research were to investigate whether: • the discomfort arising from low frequency rotational and translational oscillation could be satisfactorily predicted based on knowledge of the acceleration in the plane of the seat; • the presence of a backrest would exacerbate discomfort during both rotational and translational oscillation; • the adoption of different postures during vibration would affect the reported discomfort. Achieving these objectives required determination of the frequency-dependence and magnitude dependence of discomfort arising from exposure to rotational and translational oscillation at frequencies less than 1.6 Hz. The investigations used the psychophysical method of magnitude estimation to determine the rate of growth of discomfort with increasing vibration magnitude, and contours of equivalent discomfort. Four studies within-axes contributed new information about the effect of vibration frequency on the discomfort arising from exposure to rotational and translational oscillation. A series of smaller studies determined the relationship between the discomfort arising within different axes, and a study investigated the effect of posture. Each study employed 12 subjects. At frequencies greater than 0.4 Hz, exposure to motion in the rotational axes caused greater discomfort than the equivalent motion in the translational axes. At lower frequencies, the acceleration in the plane of the seat was a good predictor of the discomfort experienced irrespective of whether oscillation was in the rotational or translational axes, except when the discomfort arising from fore-and-aft and pitch oscillation on a seat with no backrest was compared. It was found that the presence of a backrest exacerbated discomfort during rotational oscillation at frequencies greater than 0.4 Hz but reduced discomfort during fore-and-aft oscillation. The presence of a backrest increased the incidence of discomfort arising at the head neck interface. The ability of subjects to voluntarily control their posture was found to be low, except at the lowest frequencies. The sensitivity of subjects to angular displacement increased at approximately 6 dB per octave at frequencies greater than 0.4 Hz, whereas during translational acceleration on a flat rigid seat the rate of increase in sensitivity was lower at approximately 3 dB per octave. When subjects were seated on a rigid seat with backrest and exposed to translation acceleration the discomfort arising from motion was approximately independent of frequency over the range 0.2 to 1.6 Hz. The observed effect of vibration magnitude on the frequency dependence was small, but variations in the rate of growth of discomfort with frequency indicate this magnitude-dependence would be in settings where a greater range of accelerations was present. important Existing standardised methods of prediction (e.g. BS 6841) were compared to the results with minimal modification (linear extension of the asymptotic frequency weightings). Where discomfort is dominated by oscillation in a single axis at a single input (e.g. fore-and-aft oscillation with no backrest) the prediction was considered sufficient for many practical purposes. However, where the presence of a backrest influenced discomfort the quality of prediction deteriorated markedly and the standardised method cannot be recommended.

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