Transmission of vertical vibration through a seat: Effect of thickness of foam cushions at the seat pan and the backrest

The perception of vehicle ride comfort is influenced by the dynamic performance of full-depth foam used in many vehicle seats. The effects of the thickness of foam on the dynamic stiffness (i.e., stiffness and damping as a function of frequency) of foam cushions with three thicknesses (60, 80, and 100 mm), and the vibration transmitted through these cushions at the seat pan and the backrest were measured with 12 subjects (6 males and 6 females). With increasing thickness, the stiffness and the damping of the foam decreased. With increasing thickness of foam at the seat pan, the resonance frequencies around 4 Hz in the vertical in-line and fore-and-aft cross-axis transmissibilities of the seat pan cushion and the backrest cushion decreased. For the conditions investigated, it is concluded that the thickness of foam at a vertical backrest has little effect on the vertical in-line or fore-and-aft cross-axis transmissibilities of the foam at either the seat pan or the backrest. The frequencies of the primary resonances around 4 Hz in the vertical in-line transmissibility and the fore-and-aft cross-axis transmissibility of foam at the seat pan were highly correlated. Compared to sitting on a rigid seat pan with a foam backrest, sitting with foam at both the seat pan and the backrest reduced the resonance frequency in the vertical in-line transmissibility of the backrest foam and increased the associated transmissibility at resonance, while the fore-and-aft cross-axis transmissibility of the backrest was little affected. Compared to sitting without a backrest, sitting with a rigid vertical backrest increased the resonance frequency of the fore-and-aft cross-axis transmissibility of the seat pan cushion and increased the transmissibility at resonance.Relevance to industryThe transmissibility of a seat is determined by the dynamic properties of the occupant of the seat and the dynamic properties of the seat. This study shows how the thicknesses of foam at a seat pan and foam at a backrest affect the in-line and cross-axis transmissibilities of the foams at the seat pan and the backrest. The findings have application to the design of vehicle seats to minimise the transmission of vibration to the body.     

Equivalent comfort contours for vertical seat vibration: effect of vibration magnitude and backrest inclination

This study determined how backrest inclination and the frequency and magnitude of vertical seat vibration influence vibration discomfort. Subjects experienced vertical seat vibration at frequencies in the range 2.5-25 Hz at vibration magnitudes in the range 0.016-2.0 ms(-2) r.m.s. Equivalent comfort contours were determined with five backrest conditions: no backrest, and with a stationary backrest inclined at 0° (upright), 30°, 60° and 90°. Within all conditions, the frequency of greatest sensitivity to acceleration decreased with increasing vibration magnitude. Compared to an upright backrest, around the main resonance of the body, the vibration magnitudes required to cause similar discomfort were 100% greater with 60° and 90° backrest inclinations and 50% greater with a 30° backrest inclination. It is concluded that no single frequency weighting provides an accurate prediction of the discomfort caused by vertical seat vibration at all magnitudes and with all backrest conditions. PRACTITIONER SUMMARY: Vertical seat vibration is a main cause of vibration discomfort for drivers and passengers of road vehicles. A frequency weighting has been standardised for the evaluation of vertical seat vibration when sitting upright but it was not known whether this weighting is suitable for the reclined sitting postures often adopted during travel.     

The application of SEAT values for predicting how compliant seats with backrests influence vibration discomfort

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.     

Influence of reading format on reading activity under uniaxial whole body vibration

In a railway vehicle, the vibrations are transmitted to the passengers through the various interfaces such as floor, seat, backrest etc. These vibrations affect the passenger comfort as well as their performance to do any work such as reading, writing, typing etc. In the present work, effects of vibration magnitude, direction of vibration, postures and reading formats have been studied on the reading activity. Thirty healthy male subjects have performed reading task, one at a time. All subjects were exposed to uni-axial whole body vibration in 1–20 Hz frequency range at 0.5, 1 and 1.5 m/s² rms vibration magnitude. The experimental task involved reading a paragraph under the different 54 experimental conditions (three magnitude, three direction, two posture and three reading format). The task performance has been evaluated in terms of time taken by the subjects to read a given paragraph and also the subjective evaluation of perceived difficulty on Borg's CR 10 scale. Perceived difficulty and performance degradation in reading have been found to increase with the increase in vibration magnitude in each direction of vibration. The perceived difficulty and performance degradation in reading have been observed to be higher in the fore-&-aft direction in with-backrest posture. In vertical and lateral vibration, perceived difficulty and performance degradation have been higher in without-backrest posture compare to with-backrest posture. The perceived difficulty and performance degradation have been lower for the triple-column format.     

Transmission of roll and pitch seat vibration to the head

A series of experiments has investigated the transmission of roll and pitch seat vibration to the heads of seated subjects. Head motion was measured in all six axes using a light-weight bite-bar while seated subjects were exposed to random motion at frequencies of up to 5 Hz at 1.0 rad.s ^?2 r.m.s. Subjects sat on a rigid flat seat in two body postures: ‘back-on’ (back in contact with backrest) and ‘back-off’ (no backrest contact). The influence of the position of the centre of rotation was also investigated.

Motion at the head occurred mostly in the lateral, roll and yaw axes during exposure to roll seat vibration and in the fore-and-aft, vertical and pitch axes during exposure to pitch seat vibration. A reduction in the magnitude of head motion occurred when the subjects sat in a 'back-off' posture compared with a 'back-on' posture. Varying the position of the centre of rotation along the lateral axis during roll seat vibration affected vertical and pitch head motion: least head motion occurred when the centre of rotation was in line with the subject's mid-sagittal plane. Varying the position of the centre of rotation along the vertical axis during roll seat vibration affected head motion in the mid-coronal plane: roll head motion decreased as the position of the centre of rotation was raised from below the seat surface to above the seat surface. Varying the centre of rotation (along the fore-and-aft and vertical axes) during pitch seat vibration altered head motion in the mid-sagittal plane. Head motion increased with increasing distance of the centre of rotation in front or behind the subject's ischial tuberosities and increased as the seat was raised from below the centre of rotation to above the centre of rotation.     

Subjective response to whole-body vibration The effects of posture

In an investigation of the effects of posture on subjective responses to whole-body vibration, 20 undergraduate subjects produced equal sensation contours adopting three postures each on different occasions. The postures adopted were standing, sitting upright and sitting slouched.

The results indicated significant differences in the contour shapes from the three postures, and the level set in the sitting postures were significantly lower than in the standing posture. No difference was obtained between the two sitting postures.

Implications of these findings are discussed regarding the role of transmissibility in subjective response to vibration, and the necessity to produce different standards for different postures.     

Objective and subjective responses of seated subjects while reading Hindi newspaper under multi axis whole-body vibration

Train passengers often read newspapers while traveling. Vibration is one of the key factors that may occasionally inhibit this activity. An experimental study was, therefore, conducted to investigate the extent of interference perceived in reading task by seated subjects in two postures under random vibration. 30 healthy male subjects were exposed to vibration magnitudes of 0.4, 0.8 and 1.2 m/s² in mono, dual and multi axis in the low frequency range 1–20 Hz. The task required subjects to read a given paragraph of Hindi national newspaper, in two seated postures (lap posture with backrest support and table posture with leaning over the table). The reading performance was evaluated by both degradation in performance in terms of time required to complete the task and subjective rating using Borg CR10 scale. Both the methods of reading performance evaluation exhibit progressive increase with an increase in vibration magnitude for both the subject postures in all the direction of vibration and are found to be higher in lateral and vertical direction among mono axes. The effects of multi axis vibration on perceived difficulty have been found to be similar to dual axes vibration and greater than mono axes vibration; however degradation in reading performance in multi axis vibration was also found to be similar to that for lateral direction. A comparison of the effect of postures by both evaluation methods revealed that the reading performance was adversely affected for table posture in all direction of vibration, however for lap posture, only the X-axis vibration effect was more severe.

Relevance to industry

Available ride comfort standards for vehicles do not include the effects of vibrations on passenger activities. Assessment of activity discomfort would be useful for vehicle design optimization to facilitate activity comfort.     

Transmission characteristics of suspension seats in multi-axis vibration environments

The multi-axis vibration transmission characteristics of selected suspension seats were investigated in the laboratory. Subjects were exposed to a flat acceleration spectrum and two low frequency signals extracted from multi-axis acceleration data recorded at the floor of a passenger locomotive. Triaxial accelerations were measured at the floor of the vibration table and at the interfaces between the subject and mounted seat (seat pan and seat back). The transmission ratios between the overall seat pan and seat back accelerations and floor accelerations provided an effective tool for evaluating the effects of measurement site, vibration direction, and posture among the selected seating systems. The results showed that the system transfer matrix, estimated using a multiple-input/single-output model, would be less than ideal for predicting low frequency operational seat vibration when using suspension seats. The Seat Effective Amplitude Transmissibility (SEAT), estimated for the tested locomotive seats, was used to predict the weighted seat pan accelerations and Vibration Total Values for assessing a 1-h operational exposure in accordance with ISO 2631-1: 1997.

Relevance to industry

Multi-axis SEAT values can be estimated for seating systems tested in the laboratory using representative operational exposures. These values can be applied to monitored vehicle floor accelerations to target potentially harmful vibration in accordance with ISO 2631-1: 1997, assuming the operational exposures have similar frequency and magnitude characteristics. The transmission at the seat back should be considered when substantial low frequency multi-axis vibration is present.     

Effect of the seating condition on the transmission of vibration through the seat pan and backrest

Changes in the seating condition may change the body posture which could affect the transmission of vibration through a vehicle seat. This study investigates the effect of different seating conditions on the transmission of vibration through a car seat. Ten male subjects sat on the passenger seat of a sedan car driven at 60 km/h adopting one of six conditions at a time. The VDV was measured on the seat and backrest. Backrest contact affected the VDV measured on the seat pan in the z- and y-axis only. Increasing the backrest angle increased the VDV at the backrest in the x-direction and reduced the VDV at the backrest in the z-direction. With the increase in the backrest angle, the total VDV at the backrest became higher than the total VDV on the seat pan. The study showed no effect of foot position and contact with a headrest on the VDVs.Relevance to industryThis research presents the effect of the seating condition on the transmission of vibration through the seat pan and backrest of a car seat. Research of this kind may help seat manufacturers recommend seating conditions that reduce discomfort caused by whole-body vibration.     

The influence of seat backrest angle on perceived discomfort during exposure to vertical whole-body vibration

National and International Standards (e.g. BS 6841 and ISO 2631-1) provide methodologies for the measurement and assessment of whole-body vibration in terms of comfort and health. The EU Physical Agents (Vibration) Directive (PAVD) provides criteria by which vibration magnitudes can be assessed. However, these standards only consider upright seated (90°) and recumbent (0°) backrest angles, and do not provide guidance for semi-recumbent postures. This article reports an experimental programme that investigated the effects of backrest angle on comfort during vertical whole-body vibration. The series of experiments showed that a relationship exists between seat backrest angle, whole-body vibration frequency and perceived levels of discomfort. The recumbent position (0°) was the most uncomfortable and the semi-recumbent positions of 67.5° and 45° were the least uncomfortable. A new set of frequency weighting curves are proposed which use the same topology as the existing BS and ISO standards. These curves could be applied to those exposed to whole-body vibration in semi-recumbent postures to augment the existing standardised methods. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This article reports new experimental data systematically investigating the effect of backrest angle on discomfort experienced. It demonstrates that most discomfort is caused in a recumbent posture and that least was caused in a semi-recumbent posture.     

Estimation of various sitting postures using a load-cell-driven monitoring system

Maintaining correct sitting posture is highly likely by simply informing seated people of their current sitting postures. Few studies have simultaneously applied load distribution measurements and sitting posture feedback to measure bodily pressure distribution. Commercialization of sitting posture monitoring systems has been difficult due to high cost. This study tested a system that measures load on the seat pan and load transferred to the backrest using four load cells installed on the seat pan. Three body weight ratios were calculated and differences in body weight ratios were tested among six sitting postures. The results were considered highly reliable based on strong correlations among three instruments’ results despite differences in force plate and dead load between the results of the monitoring system and the load-measuring system. The findings encourage commercialization and future research that includes gender and physical characteristic differences.     

Vibration and comfort I. Translational seat vibration

A series of studies of discomfort caused by multi-axis vibration at the seat, feet and back of seated persons is described. This first paper reports on studies with translational seat vibration. Two experiments concerned with the effects of level, frequency and direction of the translational vibration of a firm flat seat are reported.

At octave centre frequencies from 1 to 63 Hz the first experiment determined the levels of fore-and-aft, lateral and vertical seat vibration which caused discomfort equivalent to 0.5 and l.25m/s²r.m.s. 10 Hz vertical seat vibration. In the second experiment, comfort contours equivalent to 0.8m/s²r.m.s. 10 Hz vertical seat vibration and subject transmissibilities were determined from 18 males and 18 females at preferred third-octave centre frequencies from 1 to 100 Hz. In both studies the feet of subjects were not vibrated and there was no backrest.

It was concluded that the shapes of equivalent comfort contours need not normally depend on vibration level. The forms of both individual and group equivalent comfort contours and seat-to-head transmissibilities are presented. Significant correlations were found between subject characteristics (size and transmissibility) and subject relative discomfort. The males and females produced similar equivalent comfort contours.

Information on the computerized application of the method of constant stimuli which was developed for the series of experiments is presented together with a consideration of alternative methods of determining the central tendency of the data. A method of assessing the effect of vibrator distortion on judgements of equivalent discomfort is also defined.     

Vibration and shock exposure of maintenance-of-way vehicles in the railroad industry

The aim of this study is to investigate and compare vibration and shock measurements of maintenance-of-way vehicles used in the railroad industry for track maintenance and construction. Following international standards (i.e., ISO 2631-1: 1997) and professional guidelines the frequency weighted root-mean-square (r.m.s.) acceleration for each measurement axis, the vector sum, the seat effective amplitude transmissibility (SEAT), the crest factor (CF), the maximum transient vibration value (MTVV), the vibration dose value (VDV), the ratio and the newly proposed shock risk estimation factor ‘R’ for spinal injury according to ISO 2631-5:2004 were measured and calculated for seven different maintenance-of-way vehicles during revenue service. Furthermore, a proposed alternative spinal injury prediction method, the VibRisk model, which incorporates different typical driver postures and operator physical characteristics was included for comparison with the ISO 2631-5 risk prediction. The results of the vibration exposure measurements depended on vehicle type, track/surface conditions and seat properties, with the tamper and bulldozer showing the highest r.m.s. vibration values. The vector sum (a_v) results ranged from 0.37 to 0.99 (m/s²). Five of seven track maintenance vehicles would exceed the current Whole-body Vibration ACGIH-TLV^® guideline for an 8 h exposure duration in the vertical axis recommended by the American Conference of Governmental Industrial Hygienists (ACGIH). The measured CF, MTVV/a_w and VDV/(a_w·T^1/4) ratios were at or above the critical ratios in the majority of measurements given by the ISO 2631-1 (1997) and American industry guidelines by the American Conference of Governmental Industrial Hygienists (ACGIH-TLV). Comparing both prediction models for vibration shock risk for parts of the lumbar spine, different risk predictions and inconsistencies were found. The VibRisk model generally suggests different and higher risk of vertebral endplate failure for individual lumbar levels, whereas the ISO 2631-5 model indicated generally lower risks and did not differentiate between different disk levels and driver posture. Epidemiological studies validating the different shock risk models are lacking. Work modifications and adequate suspension seats would be beneficial for prevention of harmful exposure to vibration and shocks.     

Vibration and comfort HI. Translational vibration of the feet and back

The effects on discomfort of the frequency and direction of the translational vibration of a footrest and flat firm backrest have been studied in two experiments. At frequencies in the range 2.5-63 Hz, the first experiment determined the levels of fore-and-aft, lateral and vertical vibration of the feet of seated subjects which caused them discomfort equivalent to that from 0.8 m/s² r.m.s. 10 Hz vertical vibration of a firm flat seat. The levels of fore-and-aft, lateral and vertical vibration at the back of a seat which were equivalent to 0.8 m/s² r.m.s. 10 Hz vertical seat vibration were determined in the second experiment. The vibration of the feet or back occurred without simultaneous vibration at the seat.

Individual and group equivalent comfort contours are presented. It is concluded that the data provide a useful initial indication of the relative contribution of foot and back vibration to discomfort. Equivalent comfort contours for foot vibration were similar for all three directions of vibration. The contours for vibration of the back show a high sensitivity to fore-and-aft vibration. The results obtained from two additional studies show that vibration from a backrest and other variations in seating conditions can influence subject comfort.     

Assessments of two dynamic manikins for laboratory testing of seats under whole-body vibration

The aptness of two anthropodynamic manikins for assessing vibration isolation effectiveness of suspension seats is evaluated through laboratory measurements. The evaluations were performed using five different suspension seats exposed to idealized white noise (0.5–20 Hz) and target vehicle excitations along the vertical axis using a whole-body vehicular vibration simulator. The measurements were performed to derive acceleration transmissibility and seat effective amplitude transmissibility (SEAT) characteristics of seats loaded with: human subjects of body masses in the vicinity of 55, 75 and 98 kg; manikins configured to same masses; and equivalent rigid masses. The dynamic responses of the manikins were also measured under different magnitudes of white-noise excitations and expressed in terms of apparent mass. The relative applicability of the manikins for selected seats was evaluated by comparing the measures with those obtained for the seat–human and seat–mass systems. The comparisons suggested that the SEAT measures attained with manikins are comparable with those obtained with equivalent rigid mass, irrespective of the body mass, for the low natural frequency seats (2 Hz) considered in the study. Both the manikins and the equivalent rigid masses, however, provided an overestimate of isolation effectiveness of seats, when compared to those with human subjects. The manikins resulted in better estimates of SEAT values for high natural frequency seats than the rigid mass. The dynamic responses of manikins were also compared with the ranges of standardized values reported in ISO-5982 and DIN-45676. The results revealed considerable differences between the biodynamic responses of manikins and the standardized ranges.     

Effects of elastic seats on seated body apparent mass responses to vertical whole body vibration

Apparent mass (AM) responses of the body seated with and without a back support on three different elastic seats (flat and contoured polyurethane foam (PUF) and air cushion) and a rigid seat were measured under three levels of vertical vibration (overall rms acceleration: 0.25, 0.50 and 0.75 m/s²) in the 0.5 to 20 Hz range. A pressure-sensing system was used to capture biodynamic force at the occupant-seat interface. The results revealed strong effects of visco-elastic and vibration transmissibility characteristics of seats on AM. The response magnitudes with the relatively stiff air seat were generally higher than those with the PUF seats except at low frequencies. The peak magnitude decreased when sitting condition was changed from no back support to a vertical support; the reduction however was more pronounced with the air seat. Further, a relatively higher frequency shift was evident with soft seat compared with stiff elastic seat with increasing excitation.     

Fore-and-aft and dual-axis vibration of the seated human body: Nonlinearity,cross-axis coupling,and associations between resonances in the transmissibility and apparent mass

This study examined how the apparent mass and transmissibility of the human body depend on the magnitude of fore-and-aft vibration excitation and the presence of vertical vibration. Fore-and-aft and vertical acceleration at five locations along the spine, and pitch acceleration at the pelvis, were measured in 12 seated male subjects during fore-and-aft random vibration excitation (0.25–20 Hz) at three vibration magnitudes (0.25, 0.5 and 1.0 ms⁻² r.m.s.). With the greatest magnitude of fore-and-aft excitation, vertical vibration was added at 0.25, 0.5, or 1.0 ms⁻² r.m.s. Forces in the fore-and-aft and vertical directions on the seat surface were measured to calculate apparent masses. Transmissibilities and apparent masses during fore-and-aft excitation showed a principal resonance around 1 Hz and a secondary resonance around 2–3 Hz. Increasing the magnitude of fore-and-aft excitation, or adding vertical excitation, decreased the magnitudes of the resonances. At the primary resonance frequency, the dominant mode induced by fore-and-aft excitation involved bending of the lumbar spine and the lower thoracic spine with shear deformation of tissues at the ischial tuberosities. The relative contributions to this mode from each body segment (especially the pelvis and the lower thoracic spine) varied with vibration magnitude. The nonlinearities in the apparent mass and transmissibility during dual-axis excitation indicate coupling between the principal mode of the seated human body excited by fore-and-aft excitation and the cross-axis influence of vertical excitation.Relevance to industryUnderstanding movements of the body during exposure to whole-body vibration can assist the optimisation of seating dynamics and help to control the effects of the vibration on human comfort, performance, and health. This study suggests cross-axis nonlinearity in biodynamic responses to vibration should be considered when optimising vibration environments.     

Biomechanical response of the musculoskeletal system to whole body vibration using a seated driver model

ObjectiveThis study aimed to assess the effects of backrest inclination and vibration frequency on muscle activity in a dynamic environment using a musculoskeletal model.MethodThe muscle activity modeling method was used to analyze a full body musculoskeletal system of a seated person with a public domain rigid body model in an adjustable car seat. This model was established using AnyBody Modeling System, based on the inverse dynamic approach. And the min/max criterion in dealing with the muscle redundancy problem. Ten healthy subjects were exposed to whole body vibration (WBV) with five frequencies (3, 4.5, 6, 7, and 8 Hz) in the vertical direction in a randomized order on three separate days. The displacement of the seat-pan and head was measured using a hybrid Polaris spectra system to obtain the seat-to-head (STH) transmissibility. Muscle oxygenation was measured using near-infrared spectroscopy. The validity of the model was tested using STH transmissibility and muscle oxygenation.ResultsIncreased vibration frequency caused high muscle activities of the abdomen and the right leg with a backrest forward inclination angle. The muscle activities of the left leg decreased at a backrest backward inclination except at inclination angles of 15° and 30°. Muscle activity of the lumbar suddenly increased at a backrest inclination angle of 5° and vibration frequency of 5 Hz. Muscle activities were higher under vibration than that without vibration.ConclusionVibration frequency significantly affected the muscle activity of the lumbar area. Likewise, the inclination degree of the backrest significantly affected the muscle activities of the right leg and the abdomen. The combination of vibration and forward inclination of the backrest can be used to maximize the muscle activity of the leg, similar to the abdomen and lumbar muscles.Relevance to the industryThe musculoskeletal model established in the present study provides a method that can be used to investigate the biomechanical response of seated drivers to WBV. This model helps protect drivers from occupational injury.     

The influence of seat backrest angle on human performance during whole-body vibration

This study investigated the effects of reclined backrest angles on cognitive and psycho-motor tasks during exposure to vertical whole-body vibration. Twenty participants were each exposed to three test stimuli of vertical vibration: 2-8 Hz; 8-14 Hz and 14-20 Hz, plus a stationary control condition whilst seated on a vibration platform at five backrest angles: 0° (recumbent, supine) to 90° (upright). The vibration magnitude was 2.0 ms(-2) root-mean-square. The participants were seated at one of the backrest angles and exposed to each of the three vibration stimuli while performing a tracking and choice reaction time tasks; then they completed the NASA-TLX workload scales. Apart from 22.5° seat backrest angle for the tracking task, backrest angle did not adversely affect the performance during vibration. However, participants required increased effort to maintain performance during vibration relative to the stationary condition. These results suggest that undertaking tasks in an environment with vibration could increase workload and risk earlier onset of fatigue. PRACTITIONER SUMMARY: Current vibration standards provide guidance for assessing exposures for seated, standing and recumbent positions, but not for semi-recumbent postures. This paper reports new experimental data systematically investigating the effect of backrest angle on human performance. It demonstrates how workload is elevated with whole-body vibration, without getting affected by backrest angle.     

Developing a simplified finite element model of a car seat with occupant for predicting vibration transmissibility in the vertical direction

The transmissibility of seat depends on the dynamics of both the seat and the human body, and shows how the amplification and attenuation of vibration varies with the frequency of vibration. A systematic methodology was developed for finite element (FE) modelling of the dynamic interaction between a seat and the human body and predicting the transmissibility of a seat. A seat model was developed to improve computational efficiency before models of the seat pan and backrest were calibrated separately using load–deflection and dynamic stiffness measurements, joined to form the complete seat model, and integrated with the model of a manikin for further calibration. The calibrated seat model was combined with a human body model to predict the transmissibility of the seat. By combining a calibrated seat model with a calibrated human body model, and defining appropriate contacts between the two models, the vibration transmissibility with a seat–occupant system can be predicted.