Duration of whole-body vibration exposure its effect on comfort

Journal of Sound and Vibration (1976) 48(3), 333-339

DURATION OF WHOLE-BODY VIBRATION EXPOSURE: ITS EFFECT ON COMFORT M. J. GRIFFIN AND E. M. WHITHAM

Institute of Sound and Vibration Research, University of Southampton, Southampton S09 5NH, England (Received 16 February 1976, and in revisedform 24 May 1976) Most human response to vibration standards imply that low vibration levels are acceptable for longer periods than higher levels. In such standards it is usually assumed that the relationship between exposure duration and vibration level is of a similar form for a wide range of different types of motion. The experiment described in this paper was conducted to determine whether the relative discomfort produced by 4 Hz and 16 Hz sinusoidal wholebody vertical (a,) vibration was dependent on the duration of the vibration exposure. Each of eight seated subjects was exposed to two 36-minute vibration sessions. Both sessions consisted of ten-second periods of 4 Hz and 16 Hz vibration alternating continuously. In one session the 4 Hz motion was set at the "standard" level of 0"75 m/s 2 r.m.s. while the level of the 16 Hz "test" motion could be adjusted by the subjects. In the other session the 16 Hz motion was the standard at 0"75 m / s 2 r . m . s , and the level of the 4 Hz motion could be adjusted. The subjects were required to control the intensity of the test motion to compensate for periodic changes in its intensity made by the experimenter and so to maintain it at a level which produced similar discomfort to that caused by the standard motion. It was found that the relationship between the average levels of the two motions when adjusted to produce similar discomfort was independent of the vibration duration. The findings are discussed in relation to other laboratory research and the need for a better understanding of the effects of the duration of a vibration on its acceptability.

1. INTRODUCTION It is natural to assume that the duration of exposure to an environmental stress will have some effect on human reaction to that stress. There are reasons for proposing particular time-dependent characteristics for some environmental hazards: e.g., ionising radiations, cold water immersion and sounds which damage hearing. However, the exposure limits proposed for some stresses, including whole-body vibration, are often expressed as a function of time because this is reasonable rather than proven. In this paper the methods employed by researchers attempting to determine the effect of vibration duration on discomfort are reviewed. It is suggested that the considerable difficulties inherent in determining a solution to the problem may be partly resolved by considering the related question of whether different motions have different time-dependencies. A knowledge of whether duration produces a change in the relative discomfort of different motions has practical value to vehicle designers and if an appreciable change in the relative discomfort were found the consequence would be considerable. It would provide proof that the discomfort of some motions is a function of exposure duration and would indicate the need for considerable restrictions on the application of all previous laboratory studies of discomfort. Exposure duration would need to be incorporated as a most cumbersome variable in many future experimental studies and the guidance given to designers would become exceedingly complex. 333

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This paper describes a method for determining the relative discomfort of two motions throughout a prolonged vibration exposure and illustrates the application of the method to two commonly experienced motions. The vibration levels studied are similar to those in many' surface transport systems and the duration was typical of many commuter journeys. The two vibration frequencies (4 Hz and 16 Hz) tend to excite motion in different parts of the body. 2. BACKGROUND The current International Standard on human exposure to whole-body vibration (ISO 2631-1974) [1] proposes a time-dependent effect alter four minutes exposure to vibration. Figure 1 shows the manner in which the vertical (a:) 4 Hz and 16 Hz vibration Reduced Comfort Boundaries are alleged to decrease with increasing exposure duration. Minor modifications of the time dependency of the limits in this Standard have recently been proposed by Maslen to greatly increase the convenience of vibration evaluation [2]. rO'O

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There are few data that support the suggestion that the degree to which vibration reduces comfort depends on the duration of the vibration exposure. Sperling [3] proposed some fatigue times for exposure to railway vibration which are of a similar form to part of the International Standard ISO 2631-1974. However, the experimental basis of the Sperling proposals is not fully documented. Miwa et al. [4] claim that the form of the time dependency in ISO 2631-1974 is confirmed by some data they obtained by asking subjects to rate motion on a five-point semantic scale at intervals throughout two or four hour vibration exposures. The statistical basis for this conclusion appears to be very weak and many differing interpretations of their results are possible. Other authors have proposed specific time-dependent forms (e.g., Notess [5] and Pradko et al. [6]) but these also lack sufficient experimental support to be generally accepted. Some experimenters have asked subjects to estimate the periods for which they would be prepared to accept exposure to a type of vibration (e.g., Simic [7], Jones and Saunders [8], Oborne and Clarke [9]). It is interesting to discover how a subject will respond when asked how long he could tolerate being exposed to a given motion. However, if he states a duration longer than that for which he is exposed he produces a prediction of what will happen rather

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than a rating of what has happened. The experiment does not establish whether his predictions are correct and it is possible that an experienced experimenter will possess more information upon which to make such a prediction than his subjects! It has been shown by Fothergill and Griffin [10] that if subjects adjust two vibration stimuli so that they produce similar discomfort their settings have greater precision than when they categorise the discomfort of the motions with a semantic scale. Relatively large changes in discomfort due to exposure duration would therefore be required for them to be detected by a semantic category production or category selection method. Further, even if instructed to consider only the discomfort produced by the vibration, the subjects' ratings are likely to be influenced by boredom, backache and the stress of sitting in one posture for a prolonged period. To determine whether a change in rating was due to vibration duration a control condition of the same duration with no vibration is required. However, asking subjects to rate discomfort due to vibration for a prolonged period of no vibration is obviously not helpful. While rating methods tend to be imprecise and somewhat uncontrolled, relative measures of discomfort (especially intensity matching techniques) are both more accurate and more easy to control. The present experiment required that subjects adjust one motion to produce similar discomfort to another motion throughout a 36-minute vibration session. While there are many reasons why the subjects may have become more or less uncomfortable as the session progressed, the method, if carefully applied, will primarily detect whether there was a change in the relative discomfort associated with the two motions. 3. APPARATUS Whole-body vertical (az) vibration was produced by a Derritron VP 85 electrodynamic vibrator powered by a 1"5 kW power amplifier. The subjects sat on a fiat, horizontal wooden seat 360 mm by 360 mm firmly attached to an aluminium plate 12 mm thick secured to the vibrator table. The feet and upper legs of each subject were positioned horizontally and the lower legs vertically by means of a non-vibrating adjustable footrest. Subjects were not restrained but were required to sit in a comfortable upright posture and were kept under the observation of the experimenter so that any gross posture changes could be corrected. An automatic sequencing switch was used to expose subjects to "test" and "standard" vibrations alternately such that the stimuli were " o n " for ten seconds and " o f f " for one second. The switch had exponential on-off characteristics with a rise and fall time of two seconds. As the switch was turned off the vibration level began to decay. After one second the level of the other stimulus began to increase and was summed with the decreasing level of the first stimulus. This procedure continued throughout the experiment so that the motion was always present. The subjects controlled the level of the "test" motion by means of a ten-turn potentiometer mounted on a control box and a red light on the control box lit up when this motion was being presented. The level of the "standard" vibration was set by the experimenter and remained constant throughout the experimental session. The vertical (as) vibration of the seat was measured by means of an accelerometer mounted within the aluminium plate beneath the wooden seat surface. The accelerometer output was displayed on an oscilloscope and measured by a digital true r.m.s, meter. The meter was used to check the levels of the predetermined standard motions and measure the levels of the test vibration as adjusted by the subjects. During the experiment the laboratory was darkened, noise levels at the subjects' position were about 55 dB(A) and independent of the vibration condition. The temperature was in the range 18 to 24°C. Acceleration distortion on the vibrator was about 10~o with the 4 Hz motion and appreciably less with the 16 Hz motion.

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4. EXPERIMENTAL DESIGN AND PROCEDURE A discomfort matching procedure required each of eight subjects to adjust the level of the "test" vibration so that it produced a similar amount of discomfort to that produced by the "standard" vibration at a level of0-75 m,s 2 r.m.s. (see the Appendix). Each subject attended the laboratory for two sessions of 36-minutes duration. Four Hz vertical sinusoidal vibration was the "standard" motion with 16 Hz sinusoidal vibration as the "test" motion in one session and vice versa in the other session. Half the subjects were exposed to the 4 Hz as the standard motion in their first session while the other half began with 16 Hz as the standard. The two vibrations alternated continually for the duration of each session. Alter every four repetitions of the test-standard cycle the experimenter noted the level to which the subject had adjusted the test motion. Then, while the standard motion was being presented, the gain of the system producing the test motion was altered so that with two turns of the subjects' ten-turn potentiometer the vibration level would be one of four levels (0.6, 0.9, 1.2 or 1-5 m/s e r.m.s.). The subject was required to re-adjust the potentiometer during the next four cycles before a further reading and gain alteration were made. The order of the four gain settings and the direction of the magnitude changes were balanced across subjects and each subject was exposed to each setting six times. The same procedure was adopted in both sessions. The subjects were eight male research w o r k e r s - f o u r of whom had previously participated in a vibration experiment. Their ages and physical characteristics are shown in Table 1. TABLE I

Characteristics of experimental subjects Subject

Age (yrs)

Height (cm)

1 2 3 4 5 6 7 8

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170 178 178 173 180 180 175 175

Weight (kg) 61 65 82 63 71 91 67 68

Prior to being exposed to the vibration all subjects were shown a list of medical conditions [11] that would render them unfit for the experiment. 5. RESULTS AND DISCUSSION Figure 2 shows the levels of the two test motions matched against the two 0.75 m/s z r.m.s. standard motions by each of the eight subjects. (The points plotted are the means of four sequential matches corresponding to one of each of the four gain settings of the system producing the "test" motion.) It may be observed that, while two subjects considered that approximately equal levels of 4 Hz and 16 Hz vibration produced similar discomfort, three required higher levels of 4 Hz and three required higher levels of 16 Hz. Similar large differences between individuals have been demonstrated in previous work (e.g., by Griffin [12], and by Fothergill and Griffin [13]). In the present experiment the standard deviations across subjects were generally in the range 20 to 50 ~ of the mean values while the within subject variability was in the range 10 to 20 ~,~. This compares with the values of 35 and 15 ~ determined previously by Fothergill and Griffin for two similar frequencies. Figure 3 shows the mean levels of the two test motions matched against the two 0.75 m/s 2 r.m.s, standard motions by the group of eight subjects. (The points plotted are the means

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across subjects for each o f the 24 matches. Every mean is therefore associated with two o f each o f the four gain settings o f the system producing the "test" motion. Except for the first match each mean is also associated with four gains which have increased and four which have decreased since the previous match.) The analysis o f variance which was performed on the data (see Table 2) shows that there was no significant interaction between frequency and

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conditions (i.e., exposure duration) and no significant main effect of either vibration frequency or conditions. The absence of a significant frequency-conditions interaction indicates that for the frequencies considered there was no significant difference in their time trends over their exposure periods. Any change in discomfort associated with exposure duration was therefore similar for both the 4 Hz and 16 Hz stimuli. It is possible that a single mechanism may have been affected by the exposure duration and that this mechanism may be equally important in determining the discomfort produced by the different vibration frequencies in different parts of the body. Alternatively it is possible that there was simply no effect of exposure duration. As explained earlier the distinction between these two possibilities presents considerable additional difficulties. The objective of the present experiment was merely to determine whether the relative discomfort of the two vibration frequencies changed during the exposure duration. The absence of a main effect of vibration condition indicates that there was no significant tendency for subjects to increase or decrease the level of the test motion with exposure time. It therefore appears that as the experiment progressed subjects did not attempt to decrease their total vibration "dose" by gradually reducing the levels of the two test motions below those required for a match. Over the entire experiment the mean level of the 4 Hz "test" motion was 0.70 m/s 2 r.m.s, while the mean level of the 16 Hz "test" motion was 0.73 m/s z r.m.s. These values suggest that there was a consistent bias towards setting the test motion to a slightly lower level than that truly required to match the 0.75 m / s ~- r.m.s, standard---this is particularly evident in the data for subject 4 (see Figure 2). Six of the eight subjects showed this trend towards lower than expected test values while two showed the opposite effect. According to the Wilcoxon Matched-Pairs Signed-Ranks test the mean effect was not significant over the group of subjects. The absence of a significant effect of vibration frequency conflicts with the common suggestion that a 4 Hz a= sinusoidal motion is more uncomfortable than a 16 Hz a: sinusoidal motion of the same acceleration level. (See, for example, ISO-2631 .) However, such general conclusions are often based on several criteria in addition to comfort. Further, it is well established that there are large differences between subjects in their response to vibration frequency. The present results therefore also serve to emphasise yet again the importance of intersubject variability in human response to vibration. 6. CONCLUSIONS The experiment has shown that when using the method of adjustment to obtain equivalent discomfort between a 4 Hz and 16 Hz vibration the equivalence is independent of time for vibration exposures up to 36 minutes. If the prolonged exposure altered the discomfort due to either stimulus it altered the discomfort of the other to the same extent. The motions employed in the experiment have levels and frequencies similar to those in many transport situations. It is therefore concluded that the shape of the frequency weightings employed to assess the vibrations experienced in such cases need not be a function of the journey duration. The results also suggest that the results of many previous laboratory whole-body vibration experiments employing intensity matching methods have probably not been greatly affected by the total duration of the subjects' vibration exposures. The present experiment has employed a very limited range of vibration conditions and different conclusions might have emerged from an experiment with other motions. The results presented in this paper may therefore be considered to be a small, but necessary, step towards a better understanding of the effects of vibration duration on comfort. The method can be applied further and it may be that among the large number of possible corn-

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binatioias of vibration level, frequency, axis and duration there are types of motion for which the equivalent discomfort does depend on exposure duration. Finding such conditions would confirm the assumption in ISO 2631 that the effect of vibration on comfort is dependent on the duration of the vibration. However, it would simultaneously destroy the assumption that all types of vibration have the same time-dependency characteristics. REFERENCES 1. INTERNATIONALORGANIZATIONFOR STANDARDIZATION1974 IS0-2631-1974(E). Guide for the

evaluation of human exposure to whole-body vibration. 2. K. R. MASLEN 1975 Royal Aircraft Establishment Technical Memo FS 50. An anomaly in the time-dependency of human exposure to vibration in present standards. 3. E. SPERLING1956 Ch. Betzhold, Glasers Annalen, 314-317. (R.A.E. Library Translation No 1630.) Contribution to judging the riding comfort of railway wagons. 4. T. MIWA, Y. YONEKAWAand A. KOJIMA-SuDO1973 lallu~wiQltl~tl~ It~ |S5-196. Me~uremettt and evaluation of environmental vibrations. Part 3. Vibration expo0utre criterion. 5. C. B. NOTESS1963 Cornell AeronauticalLabOratOry Inc. Full-Scale D~i~ion Memo 343. Flexible airplanes, gusts, crew: A triangle. 6. F. PRADKO,R. LEE and V. KALUZA1966 AD 634-632, Theory of human vibration response. 7. D. SIMIC 1970 Dissertation D.38 der Technischen Universitdt, Berlin (R.A.E. Library Translation No. 1707). Contribution to the optimisation of the oscillatory properties of a vehicle: physiological foundations of comfort during oscillations. 8. A. J. JONESand D. J. SAUNDERS1974 Journal of Soand and Vibration 35, 503-520. A scale of human reaction to whole body, vertical, sinusoidal vibration. 9. D. J. OBORNE and M. J. CLARKE 1974 Ergonomics 17, 769-782. The determination of equal comfort zones for whole-body vibration. 10. L. C. FOTHERGILLand M. J. GRIFFIN 1976 Ergonomics (in press). The subjective magnitude of whole-body vertical vibration. 11. BRITISHSTANDARDSINSTITUTION1973 BSI DD 23. Draft for development: guide to the safety aspects of human vibration experiments. 12. M. J. GRIFFIN 1975 U.K. Group Meeting on Human Response to Vibration, University of Southampton, 18 and 19 September. A study of the subjective equivalence of sinusoidal and random whole-body vibration. 13. L. C. FOTHERGILLand M. J. GRIFFIN 1976 Ergonomics (in press). The use of an intensity matching technique to evaluate human response to whole-body vibration.

APPENDIX INSTRUCTIONS TO SUBJECTS

You will be presented with two stimuli alternately: You have control over the strength of the second stimulus and a red light on your control box indicates when you have control. Please concentrate on the first stimulus and, when the light comes on, adjust the second until you consider T H E D I S C O M F O R T IT CAUSES IS S I M I L A R TO T H E DISC O M F O R T P R O D U C E D BY T H E F I R S T S T I M U L U S . To make the match, turn the knob on the control box. The two stimuli will continue to alternate and vary in level throughout the experiment. You are asked to make the above adjustments as necessary when the red light is on. Your posture will affect your reaction to the stimuli. Please sit in the same comfortable upright posture throughout the experiment, and keep your feet and legs as still as possible. I f you want to alter your position please do this when the red light on the control box is off. You may stop the experiment at any time by pressing the red STOP button.