| Popis: |
It has previously been assumed that the vibration discomfort of standing people can be estimated using the same procedures developed from for seated people. In this thesis, the discomfort of standing people exposed to vibration was investigated to improve understanding of the mechanisms responsible for discomfort and construct a model that may be used to predict the discomfort of standing railway passengers. The first of five experiments using the method of magnitude estimation and 6-s periods of vibration investigated how the discomfort of standing subjects exposed to fore-and-aft, lateral, and vertical sinusoidal vibration depends on the frequency of vibration. From the judgements of 12 subjects at each of the 16 preferred one-third octave centre frequencies from 0.5 to 16 Hz, frequency weightings were constructed for each direction. For vertical vibration, the weighting was similar to that recommended in standards, but the weightings for fore-and-aft and lateral vibration differed from that previously assumed. Horizontal vibration caused loss of balance at frequencies less than about 3 Hz, and it caused discomfort in the legs at higher frequencies. Vertical vibration caused discomfort in the upper body. To adjust the frequency weightings according to differences in sensitivity between directions, the second experiment with 12 subjects compared the discomfort caused by 4-Hz sinusoidal vibration in the fore-and-aft, lateral, the vertical directions. It was found that sensitivity was greater for fore-and-aft vibration than lateral vibration at frequencies less than 4 Hz and weightings were determined to assist the evaluation vibration in all three directions. The third experiment investigated the extent to which postural supports used by standing train passengers (vertical bar, shoulder support, and back support) affect discomfort caused by fore-and-aft and lateral vibration in the range 0.5 to 16 Hz. Supports that created a new path for the transmission of vibration to the upper-body increased discomfort over the range 4 to 16 Hz. The fourth experiment investigated how the root-mean-square method, the basic evaluation method in current standards but known to underestimate the discomfort caused by motions containing occasional peaks, could be modified for the evaluation of non-sinusoidal vibration. Using 1-Hz and 8-Hz random vibrations with a range of crest factors it was found that the discomfort of standing subjects was better predicted with an exponent around 3, rather than an exponent of 2 implicit in r.m.s. averaging. The final experiment determined a method for predicting the discomfort of tri-axial vibration. The cube root of the sum of the cubes of the discomfort caused by the single-axis components gave good estimates of the total discomfort for both 1-Hz and 4-Hz tri-axial vibration. Since it was found in the first experiment that the discomfort was generally proportional to the acceleration at the power 0.7. these results suggest that the root-sum-of-squares of the accelerations gives good estimates of the total discomfort for tri-axial vibration. The results of all experiments were combined in an empirical model for predicting the discomfort of standing people exposed to 6-s periods of vibration. It is concluded that there are two distinctly different mechanisms responsible for vibration discomfort when standing: postural instability and body vibration. Postural instability is dominant with horizontal vibration at frequencies less than about 3 Hz, whereas body vibration is dominant with vertical vibration and with horizontal vibration at frequencies greater than about 3 Hz. The discomfort of standing people is similar to the discomfort of seated people for vertical vibration, but fundamentally different with horizontal vibration due to postural instability at low frequencies and vibration attenuation in the legs at higher frequencies |
