Frequency weighting for the evaluation of steering wheel rotational vibration

The human perception of rotational hand–arm vibration has been investigated by means of a test rig consisting of a rigid frame, an electrodynamic shaker unit, a rigid steering wheel, a shaft assembly, bearings and an automobile seat. Fifteen subjects were tested while seated in a driving posture. Four equal sensation tests and one annoyance threshold test were performed using sinusoidal excitation at 18 frequencies in the range from 3 to 315 Hz. In order to guarantee the generality of the equal sensation data, the four tests were defined to permit checks of the possible influence of three factors: reference signal amplitude, psychophysical test procedure and temporary threshold shift caused by the test exposure. All equal sensation tests used a reference sinusoid of 63 Hz at either 1.0 or 1.5 m/s² r.m.s. in amplitude. The four equal sensation curves were similar in shape and suggested a decrease in human sensitivity to hand–arm rotational vibration with increasing frequency. The slopes of the equal sensation curves changed at transition points of approximately 6.3 and 63 Hz. A frequency weighting, called W_s, was developed for the purpose of evaluating steering wheel rotational vibration. The proposed W_s has a slope of 0 dB per octave over the frequency range from 3 to 6.3 Hz, a slope of −6 dB per octave from 6.3 to 50 Hz, a slope of 0 dB per octave from 50 to 160 Hz and a slope of −10 dB per octave from 160 to 315 Hz. W_s provides a possible alternative to the existing W_h frequency weighting defined in International Standards Organisation 5349-1 (2001) and British Standards Institution BS 6842 (1987).

Relevance to industry

For the manufacturers of tyres, steering systems and other vehicular components the proposed W_s frequency weighting provides a more accurate representation of human perception of steering wheel rotational vibration than the W_h weighting of ISO 5349-1 and BS6842.     

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.     

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.     

A micro electromagnetic low level vibration energy harvester based on MEMS technology

This paper presents a micro electromagnetic energy harvester which can convert low level vibration energy to electrical power. It mainly consists of an electroplated copper planar spring, a permanent magnet and a copper planar coil with high aspect ratio. Mechanical simulation shows that the natural frequency of the magnet-spring system is 94.5 Hz. The resonant vibration amplitude of the magnet is 259.1 μm when the input vibration amplitude is 14 μm and the magnet-spring system is at resonance. Electromagnetic simulation shows that the linewidth and the turns of the coil influence the induced voltage greatly. The optimized electromagnetic vibration energy harvester can generate 0.7 μW of maximal output power with peak–peak voltage of 42.6 mV in an input vibration frequency of 94.5 Hz and input acceleration of 4.94 m/s² (this vibration is a kind of low level ambient vibration). A prototype (not optimized) has been fabricated using MEMS micromachining technology. The testing results show that the prototype can generate induced voltage (peak–peak) of 18 mV and output power of 0.61 μW for 14.9 m/s² external acceleration at its resonant frequency of 55 Hz (this vibration is not in a low ambient vibration level).     

Subjective response of standing persons exposed to fore-aft,lateral and vertical whole-body vibration

The aims of this study were to propose multiply scale factors for evaluation of discomfort of standing persons and to investigate whether there exist differences between multiplying factors used for evaluation of discomfort of standing persons and those of seated persons exposed to WBV. Twelve male subjects were exposed to twenty-seven stimuli that comprise three acceleration magnitudes (0.2, 0.4, and 0.8 m/s² r.m.s.) along fore-aft (x), lateral (y) or vertical (z) direction. The subjects with seated or standing posture on the platform of the vibration test rig rated the subjective discomfort for each stimulus that has frequency contents ranging from 1.0 Hz to 20 Hz with a constant power spectrum density. The order of presentation of the test stimuli was fully randomized and each stimulus was repeated three times. The subjective scale for discomfort was calculated by using the category judgment method. The best combinations of multiplying factors were determined by calculating correlation coefficients of regression curves in-between subjective ratings and vibration magnitudes. In all the directions, body posture significantly influenced on subjective discomfort scales. Particularly in the fore-aft and lateral direction, the upper limit of all the categories for the standing posture resulted in higher vibration acceleration magnitudes than those for the seated posture. In contrast, in the vertical direction, only the upper limit of category “1: Not uncomfortable” for standing posture was observed to be higher than that for seated posture. The best agreement for ISO-weighted vibration acceleration occurred at x factor of 1.8 and y factor of 1.8 in the standing posture and x factor of 2.8 and y factor of 1.8 in the seated posture. The results suggest that seated people respond more sensitively and severely in perception of discomfort to fore-aft and lateral vibration than standing people do while standing people respond more sensitively and severely to vertical vibration than seated people do. Thus the effects of body postures on multiplying factors should be considered in evaluation of discomfort caused by whole-body vibration.Relevance to industryThis study reports differences in subjective response of standing persons to fore-aft, lateral and vertical whole-body vibration. The results obtained in this study propose the fundamental data on the sensitivity to whole-body vibration exposed with standing posture.     

Subjective response to seated fore-and-aft direction whole-body vibration

Subjective response to seated, fore-and-aft direction, whole-body vibration of the type experienced in automobiles was investigated. Fore-and-aft acceleration was measured at the seat guide of a small automobile when driving over two representative road surfaces, and was replicated in a laboratory setting using a whole-body vibration test rig and rigid seat. A single 15 s section of each of the two acceleration time histories was band-pass filtered to the frequency interval from 0.5 to 50.5 Hz, and was used as a base stimulus. Thirteen test stimuli were then constructed for each base stimulus by rescaling to BS 6841 W_d frequency-weighted r.m.s. amplitudes from 0.01 to 0.86 m/s². Two groups of 16 participants (8 male and 8 female in each case) rated the discomfort of the test stimuli. The first group was asked to use the psychophysical method of magnitude estimation while the second used a Borg CR-10 scale. The order of presentation of the test stimuli was fully randomised and each was repeated three times. For each group of participants, regression analysis was used to determine both the individual and the group mean Stevens’ Power Law exponent describing the relationship between stimulus amplitude and subjective response. All mean power exponents were found to be less than unity, with the CR-10 scale having produced smaller exponents than magnitude estimation. The power exponents ranged from 0.66 to 0.91, corroborating the value of 0.84 obtainable from the guidelines of standard BS 6841.The results suggest that the numerical response scale provided in the BS 6841 guidelines is appropriate for use in the case of automobile fore-and-aft vibration, but that the semantic labels under-represent the actual human subjective response in this direction. Psychophysical test method, vibration stimulus range and test participant gender were all found to affect the Stevens’ Power Law exponent achieved from subjective testing. Each factor may therefore require control when attempting to compare human responses to vibration originating from different automobiles.     

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.     

A demonstration of level-anchored ratio scaling for prediction of grip strength

Level-anchored ratio scaling, such as the Borg CR10 scale^® and the Borg CR100 scale^®, uses verbal anchors in congruence with numbers to give ratio data together with natural levels of intensity. This presupposes that the anchors possess natural positions in the subjective dynamic range and also “numerical” inter-relations. In an experiment, subjects had to produce a force of handgrip corresponding to their conception of “Strong”, followed by a “Maximal” performance. By using the previously found relationship between “Strong” and “Maximal” of 1:2 together with knowledge of the exponent in the power S-R-function (R = c × Sⁿ) for grip strength, n = 1.8, predictions of individual maximal performances were obtained. The predicted values correlated 0.76 with, and deviated only 3% (ns) from, actual maximal performances of grip strength. This result –as previously also found for aerobic capacity–gives a strong support for the use of verbal anchors, so common in category scaling, also in “ratio scaling” and that the Borg CR-scales fulfill the requirements for ratio scales. For estimation of muscular strength, such as grip strength, this present study points to the value of using submaximal determinations as a compliment to maximal performances (e.g., to obtain measures of functional capacity). The results also support the increasingly common use of the CR-methodology in other ergonomic settings concerning suitable design of tools and equipment.     

Power hand tool vibration effects on grip exertions

Operation of vibrating power hand tools can result in excessive grip force, which may increase the risk of cumulative trauma disorders in the upper extremities. An experiment was performed to study grip force exerted by 14 subjects operating a simulated hand tool vibrating at 9.8 m/s² and 49 m/s² acceleration magnitudes, at 40 Hz and 160 Hz frequencies, with vibration delivered in three orthogonal directions, and with 1.5kg and 3.0kg load weights. Average grip force increased from 25.3 N without vibration to 32.1 N (27%) for vibration at 40 Hz, and to 27.1N (7%) for vibration at 160 Hz. Average grip force also increased from 27.4 N at 9.8 m/s² acceleration to 31.8 N (16%) at 49m/s². Significant interactions between acceleration x frequency, and frequency x direction were also found. The largest average grip force increase was from 25.3N without vibration to 35.8N (42%) for 40 Hz and 49 m/s² vibration. The magnitude of this increase was of the same order as for a two-fold increase in load weight, where average grip force increased from 22.5N to 35.0N (56%). A second experiment studied hand flexor and extensor muscle responses using electromyography for five subjects holding a handle vibrating at 8 m/s² using ISO weighted acceleration, with frequencies of 20 Hz, 40 Hz, 80 Hz and 160 Hz, and grip forces of 5%, 10% and 15% of maximum voluntary contraction. Muscle responses were greatest at frequencies where grip force was affected, indicating that the tonic vibration reflex was the likely cause of increased grip exertions.     

Micromachined PZT cantilever based on SOI structure for low frequency vibration energy harvesting

A PZT piezoelectric cantilever with a micromachined Si proof mass is designed and fabricated for a low frequency vibration energy harvesting application. The SiO₂ layer in the SOI wafer promotes accurate control of the silicon thickness that is used as a supporting layer in the cantilever beam structure. The entire effective volume of the fabricated device is about 0.7690 mm³. When excited at 0.75g (g = 9.81 m/s²) acceleration amplitude at its resonant frequency of 183.8 Hz, the AC output measured across a resistive load of 16 kΩ connecting to the device in parallel has an amplitude of 101 mV. The average power and power density determined by the same measurement conditions are, respectively, 0.32 μW and 416 μW/cm³.     

Bedside safety rails: assessment of strength requirements and the appropriateness of current designs

This second paper in a series of studies of the discomfort produced by multi-axis vibration is concerned with rotational seat vibration. The effects of level, frequency and direction of the roll, pitch and yaw vibration of a firm flat seat have been studied in two experiments. At octave centre frequencies in the range 1-31.5 Hz the first experiment determined the levels of roll, pitch and yaw 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 0.8 m/s² r.m.s. 10 Hz vertical seat vibration were determined from 18 males and 18 females at preferred third-octave centre frequencies from 1 to 31.5 Hz. In all cases the axis of rotation passed through the centre of the seat surface. There was no vibration of the feet and no backrest.

It was concluded that the shape of equivalent comfort contours need not normally depend on vibration, level. Both individual and group equivalent comfort contours are presented. Although there were significant correlations between subject size and subject relative discomfort it is not thought that these correlations have much practical application. In all three axes the median contours of vibration acceleration increase in proportion to vibration frequency. Sensitivity is greatest for roll vibration and least for yaw vibration of the seat.     

Effects of whole-body vertical shock-type vibration on human ability for fine manual control

The effects of vertical (z-axis) whole-body shock-type vibration on the ability for fine manual control were examined. The amplitudes and frequency of the shocks was varied, but a constant frequency-weighted acceleration of 1·25 m/s² r. m. s. was maintained. The examination of the shock's effects was carried out using an experimental system that simulated the actual workplace of earth-moving machinery. Control was measured using a first-order pursuit tracking-test, in which a seated subject was asked to use both hands to direct a cursor on a monitor using a steering wheel. Although the magnitude of shocks (peak amplitude of 6-10 m/s²) and the number of shocks per unit time (shock cycle of 10-40 s) were varied, and two types of shock (symmetric and asymmetric) used, no shock effect could be found by calculating an integrated square of tracking error during the whole exposure time. The tracking error only increased significantly during the moments that the subjects were exposed to a symmetrically shaped shock that reached the highest peak value (for the experiment) of 10 m/s². The results suggested that shocks with peak amplitudes below defined value induce no evident effect on the steering of vehicles.     

Variations in response to whole-body vibration Intensity dependent effects

When equal sensation contours were obtained from 24 subjects starting at four different intensities (0.5, 1.0, 2.0 and 3.5 m s^?2r.m.s.), significant differences in the shapes of the average contours were obtained. As the overall intensity range increased (from 0.5 to 3.5 m s^?2 r.m.s.), the contours became significantly less linear in shape. Implications of these results for the International Standard on human exposure to whole-body vibration are discussed.     

The whole-body vibration in operation of wheeled and tracked harvester IN PINUS thinning

Technological advances available in wood harvesting machines have provided productivity improvements and more comfort in forestry operations. However, operators may be exposed to whole-body vibration which can lead to a wide variety of health disorders and industry losses. This research aimed to evaluate the exposure of workers to whole-body vibration in thinning harvesting operations comparing two models of harvesters (tracked and wheeled) working in Pinus taeda stands submitted to the first commercial thinning. Whole-body vibration was determined by the three orthogonal axes (X, Y, and Z) using a 3-Axis integrator vibration meter, and the data were analyzed according to the criteria established by the ISO 2631–1:1997 and the European Directive, 2002/44/EC, concerning the resulting acceleration from normalized exposure A (8) and value of the resulting vibration dose value (VDV). The acceleration and vibration levels obtained by the tracked harvester presented mean values of A (8) of 0.6 m/s² and VDV 11.2 m/s^1.75, while by the wheeled harvester, mean values were A (8) of 0.4 m/s² and VDV 9.3 m/s^1.75, respectively. It was possible to verify the expressive vibration from the tracked harvester analysis, the operators were exposed to the whole-body vibration above the limits recommended by the regulating norms.     

An ergonomics evaluation of the vibration backpack harness system in walking

Many studies in backpack design have been focused on reducing trunk muscle activity and improving overall comfort while the wearers (college students and outdoor enthusiasts) were walking. However, little work has done on combining the vibration with harness system design. The purpose of the present study was to evaluate the effect of the vibration backpack harness system on trunk muscle activity and overall comfort in walking. There were four vibrators sewn in the four different positions of our harness system. Subjects were asked to support a load (20% body weight) on their backpack while performing 5-min walking trials on the treadmill (speed = 1.6 m/s) with different frequencies of vibration (0 Hz, 28 Hz, 35 Hz, 42 Hz). The objective measures of trunk muscle activity (electromyography) were obtained during the walking task. Subjects also were asked to complete subjective ratings of comfort. The results of the objective measures in this study had shown that the vibration function had a positive effect on reducing muscle activity for upper trapezius (UT), but not for erector spinae (ES). From the data of the two subjective surveys in our study, the comfort level of no-vibration state (0 Hz) was worse than vibration state (28 Hz, 35 Hz, 42 Hz) for both muscles, and when the frequency was 35 Hz, the comfort of the harness system was higher than the other three frequencies. The findings of the present study support that backpack with low frequency vibration has a positive effect on reducing trunk muscle activity and improving overall comfort level for wearers in walking.Relevance to industryObservations of present study is beneficial in assisting wearers to reduce muscle activity and improve overall comfort in walking according to the vibration backpack harness system. New backpack design criteria for harness systems are discussed to optimize production strategies. The wearers could be students, outdoor enthusiasts and old people.     

R2: Drilling into concrete: Effect of feed force on handle vibration and productivity

Approximately 1.6 million commercial construction workers in the US use rotary hammer drills for drilling into concrete to insert anchor bolts or rebar. The exposure to vibration may lead to hand-arm vibration syndrome and other musculoskeletal disorders depending on handle vibration acceleration level, hand grip force, and duration of exposure. There is little information on the relationship between feed force (FF), e.g., the push force applied by the worker, and handle vibration. A robotic test bench for rotary hammer drills was used to evaluate the effects of different FF on handle vibration and productivity (e.g., penetration rate and holes drilled). Increasing FF from 95 to 163 N was associated with an increase in total weighted handle vibration (a_hv) of 7.2–8.5 m/s² (slope, p < 0.001) but from 163 to 211 N there was no change in vibration level (slope, p = 0.17). Increasing FF from 95 to 185 N was associated with an increase in penetration rate of 7.2–8.5 m/s² (slope, p < 0.001) but from 185 to 211 N there was no change in penetration rate (slope, p = 0.49). Based on the maximum allowable duration of exposure to hand vibration, specified by the ISO and ACGIH Action Limits, and the penetration rate, the drilling productivity, in m drilled per day, is greatest for the lowest FF tested. Contractors and construction workers should be informed that when drilling into concrete, the lowest exposure to harmful hand vibration and the best overall productivity occurs when the lowest operational FF is applied during hammer drilling.     

Vibration perception and excitatory direction for haptic devices

Vibration feedback is one of the most popular ways to communicate between human and haptic interfaces nowadays. In order to deliver a wider variety of information accurately and efficiently, significant design factors of the vibration need to be investigated and applied to haptic devices. In this study, the excitatory direction was examined as a design factor of the vibration in terms of sensitivity and emotion. We conducted two experiments. In the first experiment, the sensitivities of three excitatory directions—X (lateral), Y (fore-and-aft) and Z (vertical) axes were estimated by the absolute thresholds of the vibration perception with two frequency levels (150 and 280 Hz). Based on ten participants’ estimated absolute thresholds, we conclude that the vibration with X axis is less sensitive than Z axis at the frequency of 150 Hz, while the vibration with Y axis is less sensitive than Z axis at the frequency of 280 Hz. In the second experiment, the agreeability of 29 emotional expressions to the vibrations was measured by a 7-point scale with a total of 12 conditions (2 frequencies × 2 amplitudes (i.e., 50 × 10⁻³ and 500 × 10⁻³ g) × 3 excitatory directions). Based on 20 participants’ responses, it is concluded that at the frequency of 150 Hz and the amplitude of 50 × 10⁻³ g, the vibration is perceived as ‘light’, and as even ‘lighter’ if the vibration is with Y axis rather than with Z axis. Likewise, at the frequency of 150 Hz and the amplitude of 500 × 10⁻³ g, the vibration is perceived as ‘repulsive’, and as even ‘more repulsive’ if the vibration is with Y or Z axis rather than with X axis. Therefore, three excitatory directions can be selectively utilized to design the distinguishable vibration by its sensitivity and emotion.     

Biodynamic response analysis of semi-supine human under varying vertical excitations

The biodynamic responses of semi-supine humans exposed to varying vertical vibration magnitudes (0.125–1.0 m/s² r.m.s.) are studied employing a multi-body modeling approach. The model comprises five rigid segments: the head, upper torso, lower torso, thigh, and leg. The viscoelastic property of tissues at joints and body-support interface are incorporated using the Kelvin-Voigt model. The dynamic model parameters identified through optimization are employed to capture the transmissibility responses of different body segments at varying vibration magnitudes. The Monte-Carlo simulation is performed to ascertain the effect of uncertainty of the model parameter and body mass on the biodynamic responses at different vibration magnitudes. The calibrated model accurately predicts the decrease in the primary resonance frequency with the increase in vibration magnitude. This nonlinearity is also apparent in vertical transmissibility responses of all the body segments. The effect of uncertainty of model parameters and body mass on the transmissibility responses is prominent near resonance frequency, while their effect on the apparent mass response is consistent across the entire frequency spectrum. The Monte-Carlo simulation illustrates higher dispersion in the transmissibility responses of the head and thorax at 1.0 m/s² r.m.s. compared to at 0.125 m/s² r.m.s. Therefore effective restraint systems are required at the head and thorax to counter the impact of high vibration magnitudes experienced during spaceflight.     

Equivalent comfort contours for vertical vibration of steering wheels: effect of vibration magnitude, grip force, and hand position

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 (4-250 Hz) and magnitudes (0.1-1.58 ms⁻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.     

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.