In-vehicle vibration study of child safety seats

This paper reports experimental measurements of the in-vehicle vibrational behaviour of stage 0&1 child safety seats. Road tests were performed for eight combinations of child, child seat and automobile. Four accelerometers were installed in the vehicles and orientated to measure as closely as possible in the vertical direction; two were attached to the floor and two located at the human interfaces. An SAE pad was placed under the ischial tuberosities of the driver at the seat cushion and a child pad, designed for the purpose of this study, was placed under the child. Four test runs were made over a pave' (cobblestone) surface for the driver's seat and four for the child seat at both 20 km h(-1) and 40 km h(-1). Power spectral densities were determined for all measurement points and acceleration transmissibility functions (ATFs) were estimated from the floor of the vehicle to the human interfaces. The system composed of automobile seat, child seat and child was found to transmit greater vibration than the system composed of automobile seat and driver. The ensemble mean transmissibility in the frequency range from 1 to 60 Hz was found to be 77% for the child seats systems as opposed to 61% for the driver's seats. The acceleration transmissibility for the child seat system was found to be higher than that of the driver's seat at most frequencies above 10 Hz for all eight systems tested. The measured ATFs suggest that the principal whole-body vibration resonance of the children occurred at a mean frequency of 8.5, rather than the 3.5 to 5.0 Hz typically found in the case of seated adults. It can be concluded that current belt-fastened child seats are less effective than the vehicle primary seating systems in attenuating vibrational disturbances. The results also suggest the potential inability of evaluating child comfort by means of existing whole-body vibration standards.     

Some observations regarding the vibrational environment in child safety seats

A growing issue in the area of vehicular ride comfort is that of child safety seats. Postural, thermal and vibrational comfort considerations are finding their way into child seat design. This paper makes some observations regarding the current state of child safety seat design, then goes on to present the results of vibration tests performed over two road surfaces using two child seats and two children. The vibration levels measured at the interfaces between the children and their seats were found to be higher than the vibration levels between the driver and the driver's seat. Calculated power spectral densities and acceleration transmissibility functions showed that the vibration transmission characteristics of the coupled system consisting of the automobile seat, child seat and child were different from those of the driver/seat system. Whereas, automobile seats normally reduce vibrational disturbances at most frequencies, the child seats tested amplified vibration at most frequencies up to 60 Hz.     

Vibration attenuation performance of suspension seats for off-road forestry vehicles

Four different types of vertical suspension seats were evaluated in the laboratory and in the field in order to measure their adaptability for attenuating whole-body vibration in log skidders used in the forest industry. Laboratory testing first consisted of determining the static and dynamic characteristics of the seats such as the static stiffness of the cushions and suspension systems and the hysteresis parameters and damping properties of the cushions. The vibration attenuation characteristics of the seats were then measured using a laboratory test rig simulating a driver work station. The influence of amplitude of excitation and the variations in seat height on the vibration attenuation performance of the suspension seats was evaluated for sinusoidal excitations in the frequency range of 0.2–8.0 Hz. The seats were then field tested during normal skidding operations to determine their vertical transmissibility characteristics and to compare the vibration exposure that results from operating a skidder while being equipped with a suspended seat, as opposed to having an unsuspended one. There was generally good agreement between the transmissibility characteristics measured in the laboratory and in the field. The results of vibration transmissibility and exposure are helpful in identifying one of the suspension seats as being the most appropriate for attenuating vertical whole-body vibration on skidders, while conforming at the same time to the basic dimensional characteristics and stability required for safe operation of such vehicles.     

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.     

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.     

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.     

Whole-body vibration: Measurement of horizontal and vertical transmissibility of an agricultural tractor seat

The seats may significantly reduce the exposures levels transmitted to the driver, but the European Directive 2002/44/EC (2002) requires only tests on the damping seat capacity along the vertical direction, whereas nothing is required for the longitudinal and transversal directions.Field tests were carried out using a 93 kW tractor to verify the vibrational comfort values given by seat with pneumatic suspension. The tests were executed with the tractor running on different surfaces, at two different forward speed and tire pressures and with different tractor masses. Three repetition were carried out for each configuration. Accelerations were always measured on both the seat and the cabin platform and the calculations were done using the ISO 2631 standard suggestions. The vibration total values and the acceleration transmissibility along the 3 perpendicular axes were calculated and analysed.Despite different boundary conditions (surface, tire pressure, forward speed and tractor mass distribution), along the Z axis the transmissibility was constantly around 0.7, to confirm that the seat worked well to damp the vertical exposures. Different were the situations for the X and the Y axes. Excluding the asphalt, on the other crossed surfaces high transmissibility values were observed (never less than 1), especially along the X axis.Relevance to industry. This paper describes the vibration transmissibility of an agricultural tractor seat. Tests were carried out with the tractor running on different surfaces and with different configurations. The seat transmissibility along the three orthogonal directions was acquired.Results suggest that the tractor manufacturer should consider, during the machine design, also the rolling and pitching movements, because the seat accelerations along the X and Y axes are influenced by them. The seat manufacturer could reduce the rolling and pitching effects using specific suspension systems along the horizontal and lateral directions.     

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.     

Haptic perceptions in the vehicle seat

The perceptual overloads of visually and auditorily based information and their interference phenomena within vehicles led to research for the applicability of haptically based information and the haptic interfaces to intelligent vehicles. Because seats are the interface that touches the largest area of the driver's body, the driver's seat in vehicles has been the focus of a promising haptic interface that can improve the safety of drivers and the effectiveness and efficiency of the information transfer between vehicles and drivers. This study aims to provide practical guidelines as a building block for designing the haptic (or vibrotactile) interface in a vehicle's driver's seat by investigating, through four experiments, 1) proper intensity of vibration, 2) minimum distance of spatially distinguishable vibrations, 3) proper position and direction of vibration, and 4) proper rhythm of vibration. Twenty participants took part in the experiments, which were conducted in driving simulation environments. These environments consisted of a real car seat, commercial vibration actuators (i.e., the eccentric motors), and a monitor that showed scenes of the road while driving. This study recommended the proper intensity (approximately 26 to 34 Hz and 2.0 to 3.4 G), position (seat pan or back support), direction (horizontal or indirect), intervibration distance (8 to 9 cm), and rhythm of vibration (3‐s duration with 0.5‐s interval), and showed how the characteristics of drivers, such as gender and age, had effects on setting the design variables of the haptic interface in the vehicle seat. © 2010 Wiley Periodicals, Inc.     

Characterisation of the human-seat coupling in response to vibration

Characterising the coupling between the occupant and vehicle seat is necessary to understand the transmission of vehicle seat vibration to the human body. In this study, the vibration characteristics of the human body coupled with a vehicle seat were identified in frequencies up to 100 Hz. Transmissibilities of three volunteers seated on two different vehicle seats were measured under multi-axial random vibration excitation. The results revealed that the human-seat system vibration was dominated by the human body and foam below 10 Hz. Major coupling between the human body and the vehicle seat-structure was observed in the frequency range of 10–60 Hz. There was local coupling of the system dominated by local resonances of seat frame and seat surface above 60 Hz. Moreover, the transmissibility measured on the seat surface between the human and seat foam is suggested to be a good method of capturing human-seat system resonances rather than that measured on the human body in high frequencies above 10 Hz.Practitioner Summary: The coupling characteristics of the combined human body and vehicle seat system has not yet been fully understood in frequencies of 0.5–100 Hz. This study shows the human-seat system has distinctive dynamic coupling characteristics in three different frequency regions: below 10 Hz, 10–60 Hz, and above 60 Hz.     

Whole-body vertical biodynamic response characteristics of the seated vehicle driver: Measurement and model development

The vertical driving-point mechanical impedance characteristics applicable to seated vehicle drivers are measured in the 0.625–10 Hz frequency range with excitation amplitudes ranging from 1.0 to 2.0 m s⁻² using a whole-body vehicular vibration simulator. The measurements are performed for seated subjects with feet supported and hands held in a driving position. Variations in the seated posture, backrest angle, and nature and amplitude of the vibration excitation are introduced within a prescribed range of likely conditions to illustrate their influence on the driving-point mechanical impedance of seated vehicle drivers. Within the 0.75–10 Hz frequency range and for excitation amplitudes maintained below 4 m s⁻², a four-degree-of-freedom linear driver model is proposed for which the parameters are estimated to satisfy both the measured driving-point mechanical impedance and the seat-to-head transmissibility characteristics defined from a synthesis of published data for subjects seated erect without backrest support. The parameter identification technique involves the solution of a multivariable optimization function comprising the sum of squared magnitude and phase errors associated with both the mechanical impedance and seat-to-head transmissibility target values, subject to limit constraints identified from the anthropometric and biomechanical data. The model response, however, is found to provide a closer agreement with the mechanical impedance target values than that with the seat-to-head transmissibility. From the model, the main body resonant frequencies computed on the basis of both biodynamic response functions are found to be within close bounds to that expected for the human body.

Relevance to industry

The development of an appropriate analytical seated vehicle driver model should provide means of estimating the forces and motions being transmitted within the body under specific vehicular vibration environments. Furthermore, its use in conjunction with a corresponding model for the vehicle seat should allow the prediction of the driver's vibration exposure levels and the seat's ability to attenuate the vibration in particular vehicles.     

The effect of posture and vibration magnitude on the vertical vibration transmissibility of tractor suspension system

The efficiency of suspension seat can be influenced by several factors such as the input vibration, the dynamic characteristics of the seat and the dynamic characteristics of the human body. The objective of this paper is to study the effect of sitting postures and vibration magnitude on the vibration transmissibility of a suspension system of an agricultural tractor seat. Eleven (11) healthy male subjects participated in the study. All subjects were asked to sit on the suspension system. Four (4) different sitting postures were investigated – i) “relax”, ii) “slouch”, iii) “tense”, and iv) “with backrest support”. All subjects were exposed to random vertical vibration in the range of 1–20 Hz, at three vibration magnitudes - 0.5, 1.0 and 2.0 m/s² r.m.s for 60 s. The results showed that there were three pronounced peaks in the seat transmissibility, with the primary resonance was found at 1.75–2.5 Hz for every sitting postures. The “backrest” condition had the highest transmissibility resonance (1.46), while the “slouch” posture had the highest Seat Effective Amplitude Transmissibility (SEAT) values (64.7%). Changes in vibration magnitude for “relax” posture from 0.5 to 2.0 m/s² r.m.s resulted in greater reduction in the primary resonance frequency of seat transmissibility. The SEAT values decreased with increased vibration magnitude. It can be suggested that variations in posture and vibration magnitude affected the vibration transmission through the suspension system, indicating the non-linear effect on the interaction between the human body and the suspension system.Relevance to industry: Investigating the posture adopted during agricultural activities, and the effects of various magnitudes of vibration on the suspension system's performance are beneficial to the industry. The findings regarding their influence on the human body may be used to optimize the suspension system's performance.     

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.     

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.     

The effect of posture and seat suspension design on discomfort and back muscle fatigue during simulated truck driving

Several studies have shown a relationship between low-back problems and exposure to seated whole-body vibration. The amount of vibration transmitted to the operator is influenced by the posture of the subject in the vehicle. The aim of this study was to determine whether a truck seat with a gas spring in its suspension is superior to the standard spring seat in slowing the onset of muscle fatigue and reducing the level of discomfort experienced during road vibrations while maintaining typical driving postures. The experiment used a 2 x 3 (2 seats x 3 postures) repeated measures design. It was conducted on six males free from low-back pain. Subject comfort was rated before and directly after exposure to typical vibrations. Muscle fatigue using centre frequency was determined during vibration exposure, and the magnitude and phase of acceleration transfer were calculated from the base plate to the seat pan and from the seat pan to the bite bar. None of comfort, fatigue rate or fatigue average were affected by seat type or seat suspension design in the short term, 10 min vibration exposure. Fatigue and comfort measures could continue to be used to detect postural defects, but the more sensitive measures of seat/driver interactions remain mechanical ones using motion-measuring techniques such as accelerometry and correcting for the heavily damped nature of the system. Until more sophisticated manikins are available the characteristics of vibration-attenuating seats should be confirmed using live humans.     

Equivalent comfort contours for fore-and-aft,lateral, and vertical whole-body vibration in the frequency range 1.0 to 10 Hz

Abstract

Standards assume vibration discomfort depends on the frequency and direction of whole-body vibration, with the same weightings for frequency and direction at all magnitudes. This study determined equivalent comfort contours from 1.0 to 10?Hz in each of three directions (fore-and-aft, lateral, vertical) at magnitudes in the range 0.1 to 3.5?ms^?2?r.m.s. Twenty-four subjects sat on a rigid flat seat with and without a beanbag, altering the pressure distribution on the seat but not the transmission of vibration. The rate of growth of vibration discomfort with increasing magnitude of vibration differed between the directions of vibration and varied with the frequency of vibration. The frequency-dependence and direction-dependence of discomfort, therefore, depended on the magnitude of vibration. The beanbag did not affect the frequency-dependence or direction-dependence of vibration discomfort. It is concluded that different weightings for the frequency and direction of vibration are required for low and high magnitude vibration.

Practitioner summary: When evaluating whole-body vibration to predict vibration discomfort, the weightings appropriate to different frequencies and different directions of vibration should depend on the magnitude of vibration. This is overlooked in all current methods of evaluating the severity of whole-body vibration.     

Whole-body vibration and visual performance: an examination of spatial filtering and time-dependency

Two experiments have examined the effects of whole-body vibration on visual performance. The first experiment concerned alphanumeric reading performance and contrast thresholds for gratings subtending 7-5, 10 and 12-5 cycles per degree (c deg)^?1. Seated subjects were exposed to vertical sinusoidal whole-body vibration (4 Hz, 2-5 ms^?2 r.m.s.). Greatest reading errors occurred with characters exhibiting a high spatial complexity in their vertical axis. Reductions in contrast sensitivity due to vibration increased with increasing spatial frequency, the greatest loss occurring with horizontally orientated gratings.

In the second experiment, contrast thresholds for horizontally orientated gratings subtending 1-5 and 12-5cdeg^?1 were obtained from ten subjects at five-minute intervals during a 60-minute whole-body vibration exposure (20 Hz I -7 m s ^?2 r.m.s.), a 20-minute pre-exposure and a 60-minute post-exposure period. There were no significant changes in contrast thresholds for gratings subtending 1-5cdeg^minus;1 during or after vibration exposure. A large variation was found in the effect of vibration upon performance with the higher spatial frequency grating both during and after vibration exposure. Significant correlations between vertical head motion and contrast sensitivity were obtained with five of the ten subjects, suggesting that time-dependent changes in seat-to-head transmissibility were partly responsible for the results. Other time-dependent changes were found with the high spatial frequency grating. Possible explanations are discussed.     

Influence of tyre inflation pressure on whole-body vibrations transmitted to the operator in a cut-to-length timber harvester

The influence of tyre inflation pressure on whole-body vibrations transmitted to the operator during the movement of a cut-to-length timber harvester was evaluated. Vibration measurements were taken in three orthogonal (x, y, z) axes at tyre pressure settings of 138, 345 and 414 kPa. Vibration was predominant in the vertical (z) direction with the peak rms acceleration value for the operator seat (0.281 ms(-2)) occurring at approximately 3.2 Hz.The corresponding peak value for the operator cabin chassis was 0.425 m s(-2) at 4 Hz.At 414 kPa, there was potential health risk on the operator for exposures above 8h duration. The vibration total values recorded for the operator seat at the maximum tyre inflation pressure setting were classed as "fairly uncomfortable" (ISO standard 2631-1), and vertical seat vibration transmissibility was highest between 4 and 8 Hz at the 345 kPa tyre pressure setting. The recorded values of WBV were significantly reduced by a reduction in tyre inflation pressure which may therefore be used to moderate the magnitude of WBV on wheeled timber harvesters.     

Study of human–seat interface pressure distribution under vertical vibration

The influence of whole-body vertical vibration on the dynamic human–seat interface pressure is investigated using a flexible grid of pressure sensors. The ischium pressure and the overall pressure distribution at the human–seat interface are evaluated as functions of the magnitude and frequency of vibration excitation, and seated posture and height. The dynamic pressure at the seat surface is measured under sinusoidal vertical vibration of different magnitudes in the 1–10 Hz frequency range. Two methods based on ischium pressure and ischium force are proposed to study the influence of seat height, posture and characteristics of vibration. The results of the study reveal that the amplitude of dynamic pressure component increases with an increase in the excitation amplitude in almost entire frequency range considered in this study. The dynamic components of both the ischium pressure and the ischium force reveal peaks in the 4 to 5 Hz frequency band, the range of primary resonant frequency of the seated human body in the vertical mode. The mean values of the dynamic ischium pressure and the ischium force remain constant, irrespective of the excitation frequency and amplitude. The magnitudes of mean pressure and force at the human–seat interface, however, are dependent upon the seat height and the subject's posture. The inter-subject variability of the static ischium pressure and effective contact area are presented as functions of the subject weight and subject weight-to-height ratio. It was found that heavy subjects tend to induce low ischium pressure as a result of increased effective contact area.

Relevance to industry

Pressure distribution at the human–seat interface has been found to be an important factor affecting the seating comfort and work efficiency of various workers. The study of human–seat interface pressure distribution under vibration is specifically critical to the comfort, work efficiency and health of vehicle drivers, who are regularly exposed to vibration. The results reported in this paper will be useful to study dynamic response of the interface pressure and design vehicle seats.     

The effects of the duration on the subjective discomfort of a rigid seat and a cushioned automobile seat

The subjective discomfort caused by the seat would affect the judgements of discomfort for the seated subjects. However, there have been few studies concerned with the discomfort on the rigid seat in static states, especially for a relatively long duration. This paper investigated the subjective discomfort caused by a rigid seat and a cushioned automobile seat for an hour. Twelve students (eight males and four females) rated the overall discomfort on a category-ratio scale and the local body discomfort on a 6-point rating scale every 10 min caused by two seats in two separate days. The static discomfort increased with increasing time, and the rigid seat caused greater discomfort than the cushioned seat. The local discomfort on the back dominated on the automobile seat, whereas the local discomfort on the buttock area dominated on the rigid seat. We established the empirical equations to predict the relations between the discomfort and duration of the two types of seats, for benchmark in the future studies on vibration and noise discomfort.