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Backrests influence the comfort of seated people. With 21 subjects sitting with three backrest heights (no backrest, short backrest, high backrest) discomfort caused by lateral, roll, and fully roll-compensated lateral oscillation was investigated at frequencies between 0.25 and 1.0 Hz. With lateral oscillation, the short backrest reduced discomfort at frequencies less than 0.63 Hz and the high backrest reduced discomfort at frequencies less than 1.0 Hz. With roll oscillation, the high backrest reduced discomfort at frequencies less than 0.63 Hz, but increased discomfort at 1.0 Hz. With fully roll-compensated lateral oscillation, the short backrest reduced discomfort at 0.4 Hz and the high backrest reduced discomfort at 0.5 and 0.63 Hz. As predicted by current standards, a backrest can increase discomfort caused by high frequencies of vibration. However, a backrest can reduce discomfort caused by low frequencies, with the benefit depending on the frequency and direction of oscillation and backrest height. [ABSTRACT FROM AUTHOR]

The detection of a vibration may be reduced by the presence of another vibration: a phenomenon known as ‘masking’. This study investigated how the detection of one frequency of vibration is influenced by vibration at another frequency. With nine subjects, thresholds for detecting fore-and-aft backrest vibration were determined (for 4, 8, 16, and 31.5-Hz sinusoidal vibration) in the presence of a masker vibration (4-Hz random vibration, 1/3-octave bandwidth at six intensities). The masker vibration increased thresholds for perceiving vibration at each frequency by an amount that reduced with increasing difference between the frequency of the sinusoidal vibration and the frequency of the masker vibration. The 4-Hz random vibration almost completely masked 4-Hz sinusoidal vibration, partially masked 8- and 16-Hz vibration, and only slightly masked 31.5-Hz vibration. The findings might be explained by the involvement of different sensory systems and different body locations in the detection of different frequencies of vibration. [ABSTRACT FROM AUTHOR]

The discomfort caused by lateral oscillation, roll oscillation, and fully roll-compensated lateral oscillation has been investigated at frequencies between 0.25 and 1.0 Hz when sitting on a rigid seat and when sitting on a compliant cushion, both without a backrest. Judgements of vibration discomfort and the transmission of lateral and roll oscillation through the seat cushion were obtained with 20 subjects. Relative to the rigid seat, the cushion increased lateral acceleration and roll oscillation at the lower frequencies and also increased discomfort during lateral oscillation (at frequencies less than 0.63 Hz), roll oscillation (at frequencies less than 0.4 Hz), and fully roll-compensated lateral oscillation (at frequencies between 0.315 and 0.5 Hz). The root-sums-of-squares of the frequency-weighted lateral and roll acceleration at the seat surface predicted the greater vibration discomfort when sitting on the cushion. The frequency-dependence of the predicted discomfort may be improved by adjusting the frequency weighting for roll acceleration at frequencies between 0.25 and 1.0 Hz. [ABSTRACT FROM AUTHOR]

Abstract: Lateral acceleration causes discomfort but how the discomfort depends on the frequency of acceleration or characteristics of seating is poorly understood. Using magnitude estimation, twelve male subjects rated the discomfort caused by lateral oscillation at eight frequencies (0.2–1.0 Hz) across four seating conditions (a rigid seat and a train seat, both with and without backrests). Discomfort increased with increasing frequency of lateral acceleration in a similar manner for all four seating conditions. However, at all frequencies and with both seats there was less discomfort when sitting with backrest support than without. Least discomfort occurred on the train seat with backrest and greatest discomfort on the rigid seat without backrest. Current standards predict an additive effect of backrest on vibration discomfort, but the findings show that low frequency lateral acceleration can cause less discomfort when sitting with a backrest than when sitting on the same seat without a backrest. [Copyright &y& Elsevier]

Abstract: Current methods for evaluating seat vibration to predict vibration discomfort assume the same frequency weightings and axis multiplying factors can be used at the seat surface and the backrest irrespective of the backrest inclination. This experimental study investigated the discomfort arising from whole-body vertical vibration when sitting on a rigid seat with no backrest and with a backrest inclined at 0° (upright), 30°, 60°, and 90° (recumbent). Within each of these five postures, 12 subjects judged the discomfort caused by vertical sinusoidal whole-body vibration (at frequencies from 1 to 20 Hz at magnitudes from 0.2 to 2.0 m s−2 r.m.s.) relative to the discomfort produced by a reference vibration (8 Hz at 0.4 m s−2 r.m.s.). With 8-Hz vertical vibration, the subjects also judged vibration discomfort with each backrest condition relative to the vibration discomfort with no backrest. The locations in the body where discomfort was experienced were determined for each frequency at two vibration magnitudes. Equivalent comfort contours were determined for the five conditions of the backrest and show how discomfort depends on the frequency of vibration, the presence of the backrest, and the backrest inclination. At frequencies greater than about 8 Hz, the backrest increased vibration discomfort, especially when inclined to 30°, 60°, or 90°, and there was greater discomfort at the head or neck. At frequencies around 5 and 6.3 Hz there was less vibration discomfort when sitting with an inclined backrest. [Copyright &y& Elsevier]