Effects of vertical mechanical shocks and body posture on discomfort

Abstract

The discomfort caused by vertical vibration depends on the magnitude and frequency of vibration, but little is known about how discomfort depends on the magnitude and frequency of mechanical shocks or on body posture. The main objectives of this thesis were to advance understanding: (i) of how the discomfort caused by a vertical mechanical shock depends on the nominal frequency, magnitude, and direction of the shock and seating dynamics, and (ii) of the effects of body posture on vibration comfort when sitting and standing.

Three of the four experiments presented in this thesis investigate the discomfort caused by mechanical shocks in an upright sitting posture. The first experiment compared the frequency-dependence of discomfort caused by shocks and sinusoidal vibration in the range 0.5 to 16 Hz at vibration magnitudes less than ±9.4 ms-2. A different frequency-dependence was found for shocks and for vibration, with shocks being less uncomfortable than vibration at frequencies greater than 4 Hz. The difference is explained by shocks containing energy at frequencies other than their fundamental frequency. The rates of growth of discomfort depended on frequency, indicating an effect of magnitude on the frequency-dependence of discomfort caused by shocks and vibration. A second experiment investigated the effect of shock direction (i.e., up or down) on discomfort in the range 2 to 5 Hz with peak accelerations from 7 to 11 ms-2. Upward displacements at frequencies from 2 to 4 Hz were more uncomfortable than downward displacements when the peak acceleration approached or exceeded 1 g. This was explained by the human body leaving, and subsequently impacting with, the seat. A third experiment found that a three degree-of-freedom model is able to predict SEAT values of blocks of polyurethane foam when people are exposed to shocks in the range 1 to 16 Hz. Predicted and measured SEAT values were consistent with subjective responses at most frequencies and magnitudes investigated.

A fourth experiment investigated how the discomfort caused by vertical vibration depends on the frequency and magnitude of vertical vibration (0.5 to 16 Hz at 0.3 to 3.2 ms-2 r.m.s.) in four postures. The frequency-dependence of discomfort was equivalent to the standardised frequency weighting Wb when sitting upright, sitting leaning forward, and standing with straight legs. When standing, bending the legs increased discomfort in the range 2 to 4 Hz but reduced discomfort at frequencies greater than 5 Hz, consistent with the effects of bending the legs on biodynamic responses.

There are four main findings from the research reported in this thesis: (i) The same methods can be used to predict the discomfort caused by shocks and vibration but the optimum frequency weighting for evaluating shocks depends on the shock magnitude; (ii) Shocks with fundamental frequencies in the range 4 to 16 Hz cause less discomfort than vibration of the same frequency and magnitude; (iii)The SEAT value is a useful predictor of seat comfort and a three degree-of-freedom model can be used to predict SEAT values of occupied foam cushions during exposures to vertical shocks in the range 1 to 16 Hz with peak accelerations less than 1g; (iv) The frequency-dependence of discomfort caused by vertical vibration is similar in normal standing and when sitting upright or sitting leaning forward, but differs when standing with bent legs.

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