Subjective reaction to vertical mechanical shocks of various waveforms

Abstract

Two laboratory studies have been conducted to assess the discomfort caused to seated male subjects by exposures to vertical whole-body mechanical shocks. The first experiment was designed to obtain judgements of the discomfort of single “up” or “down” damped sinusoidal impulses. Sixteen subjects used the method of magnitude estimation to indicate the discomfort produced by damped sine waves at nominal frequencies of 1, 4 and 16 Hz, with damping ratios of 0·125, 0·250 and 0·707 and vibration dose values ranging from 0·6 to 4·0 m s −1·75. Analysis was also conducted to determine whether the growth of discomfort with increasing magnitude was influenced by frequency, duration or direction of motion. The results suggested that a single frequency weighting and a single duration weighting is appropriate at all magnitudes. Analysis was also conducted to determine whether the discomfort depends on the frequency, duration and direction of shocks when they are presented at a constant vibration dose value using frequency weighting W b . There was no difference between the discomfort caused by “up” and “down” shocks of the same vibration dose value, but the W b frequency weighting slightly underestimated sensitivity to low-frequency shocks. The second experiment sought to compare two methods of evaluating stimuli containing between one and 16 repeated shocks. The vibration dose value [∫ o T a 4(t) dt] 1 4 and a measure of dose related to “energy”, [∫ o T a 2(t) dt] 1 2 , were employed to predict the reduction in shock magnitude required to counteract any increased discomfort with a greater number of shocks. Subjects employed the method of paired comparison. Judgements of the relative discomfort of the stimuli presented in pairs indicated that the vibration dose value provided the more accurate method of evaluating the discomfort of the shocks.