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Seat effective amplitude transmissibilities (SEAT values) quantify the influence of seats on discomfort caused by vibration. This study investigated SEAT values with vertical mechanical shocks at low, medium, and high magnitudes (0.5, 1.0, and 2.0 ms-1.75) at frequencies between 1.0 and 16 Hz. With 18 subjects sitting on a rigid seat and two foam cushions, SEAT values were measured objectively (from the transmitted vibration), measured subjectively (by the responses of subjects), and predicted (using a model of the seat-passenger system). Except with high magnitude low frequency shocks, predicted SEAT values were similar to measured SEAT values. With shocks causing subjects to rise off a cushion and subsequently impact the cushion (low frequencies around 1 g), subjective SEAT values were better estimated using predicted SEAT values than objectively measured SEAT values, because accelerometers on cushions exaggerated the impact of the falling subject. However, accelerometers on rigid seats will underestimate such impacts. [ABSTRACT FROM AUTHOR]

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]

The extent to which a seat can provide useful attenuation of vehicle vibration depends on three factors: the characteristics of the vehicle motion, the vibration transmissibility of the seat, and the sensitivity of the body to vibration. The ‘seat effective amplitude transmissibility’ (i.e., SEAT value) reflects how these three factors vary with the frequency and the direction of vibration so as to predict the vibration isolation efficiency of a seat. The SEAT value is mostly used to select seat cushions or seat suspensions based on the transmission of vertical vibration to the principal supporting surface of a seat. This study investigated the accuracy of SEAT values in predicting how seats with backrests influence the discomfort caused by multiple-input vibration. Twelve male subjects participated in a four-part experiment to determine equivalent comfort contours, the relative discomfort, the location of discomfort, and seat transmissibility with three foam seats and a rigid reference seat at 14 frequencies of vibration in the range 1–20 Hz at magnitudes of vibration from 0.2 to 1.6 ms −2 r.m.s. The ‘measured seat dynamic discomfort’ (MSDD) was calculated for each foam seat from the ratio of the vibration acceleration required to cause similar discomfort with the foam seat and with the rigid reference seat. Using the frequency weightings in current standards, the SEAT values of each seat were calculated from the ratio of overall ride values with the foam seat to the overall ride values with the rigid reference seat, and compared to the corresponding MSDD at each frequency. The SEAT values provided good predictions of how the foam seats increased vibration discomfort at frequencies around the 4-Hz resonance but reduced vibration discomfort at frequencies greater than about 6.3 Hz, with discrepancies explained by a known limitation of the frequency weightings. [ABSTRACT FROM AUTHOR]

Abstract: While walking on an instrumented treadmill, 20 subjects were perturbed by lateral sinusoidal oscillations representative of those encountered in transport: frequencies in the range 0.5–2 Hz and accelerations in the range 0.1–2.0 ms−2 r.m.s., corresponding to velocities in the range 0.032–0.16 ms−1 r.m.s. Postural stability was assessed from the self-reported probability of losing balance (i.e., perceived risk of falling) and the movements of the centre of pressure beneath the feet. With the same acceleration at all frequencies, the velocities and displacements of the oscillatory perturbations were greater with the lower frequency oscillations, and these caused greater postural instability. With the same velocity at all frequencies, postural instability was almost independent of the frequency of oscillation. Movements of the centre of pressure show that subjects attempted to compensate for the perturbations by increasing their step width and increasing their step rate. [Copyright &y& Elsevier]

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]

Abstract: Few studies have compared the discomfort caused by vibration in different directions, and few have investigated the vibration discomfort of standing people. This study was designed to compare the discomfort experienced by standing people exposed to sinusoidal vibration in the fore-and-aft, lateral, and vertical directions. Using the method of magnitude estimation, 12 subjects estimated the discomfort caused by 4-Hz sinusoidal vibration at 10 different magnitudes. At 4 Hz, subjects were less sensitive to lateral vibration than to fore-and-aft vibration (Ky /Kx  = 0.71), and more sensitive to vertical vibration than to horizontal vibration (Kz /Kx  = 1.95; Kz /Ky  = 2.77). Previous findings showing how the discomfort of standing people depends on the frequency of fore-and-aft, lateral, and vertical vibration were used to define frequency weightings that reflect relative sensitivity to vibration in each direction. The frequency weightings differ from those appropriate for seated people, and differ from the weightings for standing people in current standards that were mostly derived from understanding of the discomfort of seated people. [Copyright &y& Elsevier]

Abstract: Vehicle drivers receive tactile feedback from steering-wheel vibration that depends on the frequency and magnitude of the vibration. From an experiment with 12 subjects, equivalent comfort contours were determined for vertical vibration of the hands at two positions with three grip forces. The perceived intensity of the vibration was determined using the method of magnitude estimation over a range of frequencies (4–250Hz) and magnitudes (0.1–1.58ms−2 r.m.s.). Absolute thresholds for vibration perception were also determined for the two hand positions over the same frequency range. The shapes of the comfort contours were strongly dependent on vibration magnitude and also influenced by grip force, indicating that the appropriate frequency weighting depends on vibration magnitude and grip force. There was only a small effect of hand position. The findings are explained by characteristics of the Pacinian and non-Pacinian tactile channels in the glabrous skin of the hand. [Copyright &y& Elsevier]

Reductions in vehicle vibration that may contribute to improvements in overall vehicle ride could individually be too small to be detected by drivers or passengers. This study investigated the 'difference threshold' (the difference in magnitude between two stimuli which is just sufficient for their difference to be detected) required for a change in vehicle ride to be perceived and whether this was consistent with Weber's Law. Ten male and 10 female subjects sat in a car seat and were exposed to four different reproductions of the vertical vibration recorded on the seat of a car. Three of the stimuli had the same waveform recorded while the car traversed a tarmac surface. This waveform was reproduced using three different magnitudes of vibration at the seat: 0.2, 0.4 and 0.8 m s(-2) r.m.s. (Wb weighted). The other stimulus was recorded with the car traversing a 'pavé' surface that gave a different waveform that was reproduced at a magnitude of 0.4 m s(-2) r.m.s. (Wb weighted). There were significant differences in the absolute difference thresholds measured using the same waveform at the three different magnitudes. When the difference thresholds were expressed in relative terms (the proportion by which two stimuli must differ in magnitude to be discriminated), the relative difference thresholds were approximately 13%, and independent of both the vibration magnitude and the vibration waveform. The results are therefore consistent with Weber's Law. No consistent differences were observed between the responses of male and female subjects. [ABSTRACT FROM AUTHOR]

Abstract: The motions of vessels may interfere with crew activities and well-being, but the relationships between motion and the experiences of crew are not well-established. Crew responses to motions of a floating production and storage offshore vessel at a fixed location in the North Sea were studied over a 5-month period to identify any changes in crew difficulties and symptoms associated with changes in vessel motion. Ship motions in all six axes (fore-aft, lateral, vertical, roll, pitch, and yaw) were recorded continuously over the 5-month period while 47 crew completed a total of 1704 daily diary entries, a participation rate of 66–78% of the crew complement. The dominant oscillations had frequencies of around 0.1Hz, producing magnitudes of translational oscillation in accommodation areas of up to about 0.7ms−2 r.m.s., depending on the weather, and magnitudes up to three times greater in some other areas. The daily diaries gave ratings of difficulties with tasks, effort level, motion sickness, health symptoms, fatigue, and sleep. Problems most strongly associated with vessel motions were difficulties with physical tasks (balancing, moving and carrying), and sleep problems. Physical and mental tiredness, cognitive aspects of task performance, and stomach awareness and dizziness were also strongly associated with motion magnitude. There was a vomiting incidence of 3.1%, compared with a predicted mean vomiting incidence of 9.3% for a mixed population of unadapted adults. It is concluded that crew difficulties increase on days when vessel motions increase, with some activities and responses particularly influenced by vessel motions. [Copyright &y& Elsevier]