Applied Ergonomics 1978, 9.3, 155-161 Ergonomics for passenger comfort

Vibration and passenger comfort: Can data from subjects be used to predict passenger comfort? D.J. Oborne

Department of Psychology, University College of Swansea

This paper presents a review of both field and laboratory studies of human reaction to vibration, to try to answer the question whether laboratory based studies may be used to predict comfort levels for passenger vehicles. The conclusion is reached that such studies may be used, provided their restrictions are understood. Finally, tentative suggestions are made for acceptable levels of vibration in passenger transport vehicles.

I ntroduction

In previous papers (Oborne and Clarke, 1973; Oborne, 1977) the author has constantly argued that, despite the fact that many variables in a field situation are outside the control of the investigator, naturalistic field studies of passenger comfort may offer more valid information to the design engineer than do studies carried out under laboratory conditions. The justification for the thesis lies in the observation that subjects are employed (and sometimes paid) to carry out a specific task. Passengers on the other hand, are not employed (in fact they pay) and expect to receive a reasonable journey. More important, they are the people for whom the design engineer is designing his vehicle. On the other hand it must be pointed out that laboratory studies do offer the investigator more control both over the type of stimuli to be applied to his subjects and the type of subjects he employs. If the two different situations are to be employed so that usable data may be obtained, the important question to be answered is: "To what extent may results from laboratory based investigations be used to predict comfort levels for passenger vehicles?". This paper will be concerned with comparing both laboratory and field studies of the human response to whole-body vibration.

To answer the question adequately, it is necessary to assess the many attempts which have been conducted to investigate quantitatively the effect of vibration on man. The number of such investigations has increased considerably since the pioneering work carried out in Germany in the early 1930's but the published papers may be divided into three main groups. First are the studies which were instigated specifically to determine the comfort levels associated with different vibration stimuli. Although this category includes studies carried out in both laboratory and field situations, in both cases subjects (rather than fare paying passengers) were used. Secondly, and more recently, studies have been carried out to determine contours of equal subjective intensity linking frequency with amplitude - perhaps to derive a scale of human reaction to draw together some of

the existing information to provide criteria or standards for determining human reaction to vibration.

It is not the intention of this paper critically to assess the experimental validity of these results. This has been done elsewhere (Oborne, 1976). As implied above, the purpose is to consider the possibility of concordance between at least some of the experimental results to indicate possible limits for the design engineer.

1. Vibration comfort studies

a) Laboratory based studies The aim of these studies has been to attempt to describe

the manner in which human response to vibration varies over a specific frequency range, by ascribing descriptive comfort labels (eg, 'perceptible', 'comfortable', 'intolerable', etc) to contours of equal subjective intensity. The methods employed generally fall into one of three categories:

i) To present separate stimuli (of different frequency and intensity) to the subjects. After each stimulus, the subject is required to give the vibration one of a number of specified comfort labels. (eg, Reiher and Meister, 1931 ; Helberg and Sperling, 1941).

ii) To present a comfort label (eg, 'comfortable') to the subject who is asked to adjust the vibration stimulus (of either fixed frequency or intensity) until he considers the comfort label adequately describes the sensation derived from the vibration. (eg, Jacklin and Liddell, 1933; Gorrill and Snyder, 1957; Parks and Snyder, 1961; Chancy, 1964, 1965).

iii) In addition to subjective comment, recordings have been taken of the resonance of parts of the body and galvanic skin response. (Dieckmann, 1955).

The equal comfort curves produced by such investigators are shown in Fig. 1. Unfortunately, apart from those of Gorrill and Snyder (1957) and Parks and Snyder (1961),

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Equal comfort contours obtained from laboratory experiments

Table 1: Notes concerning the comfort curves depicted in Fig. 1

Investigator(s) Comfort label Notes

Reiher and Meister, 1931 Definitely perceptible

each curve has a different semantic label, thus making it extremely difficult to attempt to put curves having similar connotations on the same graph. Before interpreting the curves therefore, the notes in Table 1 should be borne in mind.

As may be seen from the data in Table t any suggestion that the descriptions represent the same level of 'comfort' must be taken with some reservation. Differences in interpretation, therefore, may account for much of the variance of curve shapes and levels shown in Fig. 1. However, this cannot explain totally the ratio of approximately 10:1 between levels produced, for example, by Reiher and Meister and by Parks and Snyder. Furthermore, it does not explain the differences in level and shape of the "definitely perceptible" curves of Gorrill and Snyder (1957)and Parks and Snyder (1961). In both cases, the same labels, equipment, subject pool and even experimenter were used.

Despite these problems, however, attempts have been made to draw such curves together to produce composite comfort bands. These studies will be discussed later.

b) F i e l d b a s e d s t u d i e s Much of the laboratory work on human response to

vibration has been performed using sinusoidal inputs. Studies carried out under field conditions, however, although still using subjects rather than passengers, will employ more 'true-to-life', multidimensional, periodic bursts of random vibration.

Field investigations of the effects of different types of vibration inputs on comfort have generally been few in number and deficient in clarity of reporting. However the paradigms used have followed closely those of the laboratory

Jacklin and Liddell, 1933 Pleasing (early)

Jacklin and Liddell, 1933 (later)

Helberg and Sperling, 1941

Dieckmann, 1955

Gorrill and Snyder, 1957

Parksand Snyder, 1961

Chaney, 1964,1965

'Comfortable'

Undeniable sensation

Well noticeable

Definitely perceptible

Definitely perceptible

Mildly annoying

10 standing subjects were used. Curve lies at the boundary of 'Weakly Perceptible' and 'Easily Perceptible'

31 seated subjects were used. No attempt made to define level to subjects

Experiment carried out in the later part of 1933. Approx 100 seated subjects were used. Curve shown arbitrarily taken to lie midway between 'Perceptible' and 'Disturbing"

25 seated subjects used. "Sensation undeniable but effect not annoying"

Said to be "Allowable in industry for any period of t ime"

5 seated subjects used. "Minimum level that can escape attention"

15 seated subjects used. "Minimum level that can escape attention"

"Lowest intensity of vibration at which any unpleasant or annoying effects are fel t" 1964 study used 10 seated subjects with harness 1965 study used 5 standing subjects Curve shown is mean of 1964 and 1965 studies

156 Applied Ergonomics September 1978

studies - namely to attempt to ascribe comfort labels to measured vibration levels in the vehicle.

The results from the four studies which have been reported are shown in Fig. 2. Zand (1932) observed passenger reaction in airplanes, but gave no details of the procedures used. Getline (1955) flew his subjects in different types of military aircraft and asked them to state, after each journey, whether they considered the journey to be 'satisfactory' or 'rough'. Best (1945) conducted his studies in grounded airplanes and used various propeller inbalances to produce different vibration stimuli. Jacldin and Liddell (1933) placed pairs of passengers in the back seat of a car which was driven at increasing speeds until the 'disturbing' level on their human reaction scale was reached. (The rather uncharacteristic cyclic curve shown in Fig. 2 possibly reflects their procedure of averaging vibration data for all directions of motion).

2. The shape of the equal sensation contour From the above discussion, it may be seen that little

agreement exists between various investigators as to which levels differentiate 'comfortable' from 'uncomfortable' levels of vibration. In later years, therefore, the trend has been to investigate human reaction to vibration simply in terms of equal sensation curves without, for the time being, reference to any one particular comfort level. The curves thus produced demonstrate, as a function of frequency, the levels of vibration intensity which produce an equivalent 'sensation'.

In essence, two methods have been used by the majority of investigators in this category. Both have involved

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Table 2: Notes concerning the methods of obtaining the equal sensation contours depicted in Fig. 2

Investigator(s) Method

Miwa, 1967

Ashley, 1970

Shoenberger and Harris, 1971

Jones and Saunders, 1972

Oborne and Clarke, 1974

Griffin, 1976

Variable stimuli (each fixed frequency) matched to be of 'equal sensation' to a 20 Hz reference signal at either 0.01, 0.032, 0.1 or 0"32 g peak (0-069, 0.22, 0-69 or 2.2 m/s 2 RMS)

Variable stimuli (each fixed frequency) matched to be of 'equal annoyance' to random vibration reference signals previously equated to 'principal' FDP boundaries at 6 Hz

Matching task: Variable stimuli (each fixed frequency) matched to be subjectively as intense as a 20 Hz reference signal at 0.32 g peak (2-2 m/s 2 RMS)

Psychophysical task: Obtained exponents (power functions) for frequencies ranging from 3.5 to 20 Hz

Variable stimuli (each fixed frequency) matched to be of 'equal comfort sensation' to a 20 Hz reference signal at either 0.1, 0.2, 0"3, 0.4, 0"5 or 0"6 g peak (0"69, 1-38, 2.08, 2.77, 3"46 or 4.16 m/s 2 RMS)

Required subjects to rate each of 75 stimuli (3-80 Hz) on 10 cm rating lines bearing the scale ends 'smooth' and 'rough'. Assuming that equal ratings along the rating line indicate equivalence of sensation, equal sensation contours obtained of which the parameters were specified distances along the rating line

Variable stimuli (each fixed frequency) matched to 'produce similar discomfort' to a 10 Hz reference signal at 0.75 m/s 2 RMS

Applied Ergonomics September 1978 157

presenting the subject with a standard reference stimulus and a second stimulus of variable intensity. For the first type, the variable stimulus is of a different frequency to that of the standard, and the subject is asked to adjust this variable until it presents to him the same sensation as the standard. Repeating the procedure with variables having different frequencies, a series of points may be placed on an intensity/frequency plot which indicate equal sensation to the standard.

In the second type of procedure, the variable is given the same frequency as the standard. In this case, the subject's task is to adjust the variable until the sensation obtained appears to be of some predetermined multiple or fraction of the standard (eg, "one half" or "twice" the sensation). Stevens (1957) maintains that, for such 'intensity' dimensions, subjective sensation (one half, one quarter, one eighth, etc) is proportional to the objective intensity raised to a power. Since this power value has been shown to be a function of frequency, equal sensation contours may be derived. (See McCullough and Clarke, 1974, for a fuller discussion of the implications of the different methods used to derive values of the power law relationship.)

The curves obtained by different investigators are shown in Fig. 3, and the appropriate methods used are described in Table 2.

One observation to be made of these curves relates to the possibility of at least two different shapes emerging. The first shape would indicate maximum sensitivity to vertical vibration to be in the range 6-15 Hz with a reduction in sensitivity below approximately 6 Hz and above 15 Hz (Ashley, 1970; Shoenberger and Harris, 1971). The second form demonstrates a much smaller range of maximum sensitivity, at around 4 - 6 Hz, with a rapid decrease above 6 Hz and below 4 Hz (Miwa, 1967; Jones and Saunders, 1972; Oborne and Clarke, 1974). The curve produced by Griffin (1976) would also tend to fall into the second category, although the reduction in sensitivity after approximately 10 Hz is not so great.

A possible reason for the occurrence of these different shapes has been suggested by Oborne and Humphreys (1976). Using a modified matching task, the authors investigated the equal sensation contours obtained from individual subjects (rather than the average, group contour). Their

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results suggested that the two types of shapes obtained by different investigators are reflected by at least two differel~t individual contour shapes. On the basis of these contours, Oborne and Humphreys strongly advocate the need to investigate individual contours in conjunction with individual variables before the human response to vibration may be understood fully.

3. Theore t i ca l and r e c o m m e n d e d c o m f o r t l imi ts

As demonstrated above, little agreement exists between the results derived from various investigators. However, a few authors have combined the results of different studies, with the hope that some value may be gained from the resultant figures. These derived contours are shown in Fig. 4 but a more detailed exposition, including information relating to their source data, is given by Oborne (1976).

As may be seen, four of the authors (Lippert, 1946; Goldman, 1948; Janeway, 1948 and Soliman, 1968)have attempted to derive some type of 'comfort bands' from the data. As a departure from this convention, however, the International Organisation for Standardization (ISO) has attempted to specify time limits for exposure to vibration, defining the curves in terms of maximum permissible exposure time to the vibration stimuli. For each time limit, ISO distinguishes three criteria:

a) the preservation of working efficiency (fatigue-decreased proficiency boundary - FDP)

b) the preservation of health (exposure limit)

c) the preservation of comfort (reduced comfort boundary = RCB).

For any exposure time, exposure limit = 2FDP, and RCB = FDP/3-15. Limits for both vertical and lateral vibration have been produced.

As a guide, the ISO document provides a useful starting point from which designers may begin to decide appropriate vibration levels. However, as Allen (1975) has pointed out, it is important to bear in mind the restrictions placed upon the application of the limits. Thus the document refers, primarily, to whole-body vibration applied to seated or standing man. It provisionally applies to recumbent or reclining man but not to local vibration. Further, it covers only people in 'normal health', and most of the evidence

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158 Applied Ergonomics September 1978

for the limits appears to be based on average laboratory data obtained from fit young men, (Jones and Saunders (1974) for example, have demonstrated some significant differences between male and female reactions to higher frequency ( > 30 Hz) vibration stimuli). Finally, and this restriction holds true for all composite curves, large individual differences exist in reaction to vibration. Curves depicting average reaction may mask these differences.

Discussion

Although a few were carried out in the field, the majority of studies considered above were conducted under laboratory conditions using sinusoidal vibration stimuli. In all cases subjects (rather than passengers) were employed. The important question to arise is whether the results obtained from laboratory subjects may reliably and validly be extended to suggest how passengers may react under actual transport conditions.

Only one major transport study relevant to this problem has been reported. Oborne (1977) obtained ratings of comfort and vibration intensity from nearly 800 passengers travelling by a cross-channel hovercraft. From vehicle vibration recordings, two primary frequency bands could be discerned: 0 -4 HZ and 8-16 Hz. Relating passenger comfort ratings to the levels of vibration measured in these frequency regions, bands of 'comfort' as a function of vibration intensity could be obtained. These comfort bands are indicated in Table 3. In relation to the present discussion, the important band for the design engineer is that described by the label 'Just Comfortable'. This represents a feeling that the passenger would only wish to endure such a vibration level for approximately ½h, and maybe viewed as being at the boundary of 'comfortable' and 'uncomfortable' levels of vibration.

To answer the question whether the results from laboratory studies may parallel adequately passenger responses, it might be a valuable exercise to compare the results obtained from this field study with appropriate levels provided by the studies described above. It must, however, be remembered that Oborne's data were obtained from hovercraft passengers only. The possibility that the responses are vehicle specific should be borne in mind.

The important bands to be considered, and the respective acceleration levels, are indicated in Table 4. In interpreting these bands three points must be remembered. First, the problem of equating semantic labels is paramount. Second, the bands represent only rough approximations. Third, no account has been taken of the levels at frequencies above approximately 20 Hz.

As may be seen from Table 4, some broad concordance does appear to exist between the 'subject' and 'passenger' comfort bands. However, there are also discrepancies. Thus, it would appear from the 'field' studies that an intensity level in the region of approximately 0-6-0.8 m/s 2 is at the border of being described as comfortable/uncomfortable.

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Fig. 4 Theoretical comfort levels derived after reviews of the literature

Table 3: Comfort bands obtained by Oborne (1977) from Hovercraft passengers

RMS acceleration in m/s 2 Label Definition 0 - 4 Hz 8 - 1 6 Hz

Very comfortable I would be willing to put up with this Below 0"48 Below 0" 15 level for a long journey

Comfortable I would be willing to put up with the 0"48-0"95 0" 15-0"85 present level for about 1½ h

Just comfortable In future I would not be willing to 0"95-1 "30 0 '85-1"30 put up with this level for more than about ½ h

Uncomfortable The level is such that I would only 1 "30-1 "80 1 "30-1 "95 tolerate i t again for a short journey

Very uncomfortable The level is so high that I would not consider travelling in any form of transport with a similar level

Above 1"80 Above 1"80

Applied Ergonomics September 1978 159

Table 4: The intensity levels of comfort bands produced by different authors, compared with the "Just Comfortable" band obtained by Oborne (1977) from hovercraft passengers

Author(s) Description Average intensity levels in m/s 2 RMS

below approx 20 Hz

Oborne, 1977

a) Theoretical Curves

Lippert, 1946

Goldman, 1948

Janeway, 1948

Soliman, 1968

ISO, 1974

Just comfortable 0"90-1" 30

Slightly disagreeable (9-16 Hz) 0" 15-1 "59

Threshold of discomfort (4-12 Hz) 0" 24-1"04

Recommended limit (6-20 Hz) 0"25

Lower level of threshold annoyance (4-22 Hz) 0"42

½ h reduced comfort level (4-8 Hz) 0"41

b) Field Studies

Zand, 1932 Threshold of unpleasantness 0"86

Best, 1945 Noticeable/objectionable 0"59

Getline, 1955 Satisfactory/rough 0"67

Jacklin and Liddell, Disturbing 0"69 1933

For the theoretical 'standards' based upon laboratory experiments, however, the limits of the band are much increased, ie, between approximately 0.2 and 1"0 m/s z . It might be, however, that the lower limit is reduced in the laboratory studies possibly because a sinusoidal waveform is more 'unpleasant' than is the same overall intensity produced by a random waveform.

Comparing the above levels with those obtained from hovercraft passengers by Oborne (1977), it appears that the nearer to a 'true-life' situation an experimenter is able to attain, the higher the levels of vibration which the 'passenger' is willing to allow. Thus, the upper level for 'comfort' obtained from the laboratory studies was approximately 0.25 m/s 2, the subjects exposed to 'field' conditions considered the intensity needed to be approximately 0.6 m/s 2 before an 'uncomfortable' level was reached, whereas passengers in the field thought that it ought to be 0.9 m/s 2 . As has been argued previously, these differences probably arise because under field conditions passengers have many other extraneous stimuli which may distract their attention.

Conclusion

It would appear, therefore, that data from laboratory studies may be used by the design engineer. Although they will not predict passenger reaction precisely, any discrepancy will cause him to tend to err on the side of reducing vibration levels below those required by the passenger.

In conclusion, it must be emphasised that the levels discussed above are only extremely rough guides. The precise level to be chosen depends, to a large extent, upon the semantic label chosen, the frequency characteristics of the vehicle, and perhaps even the type of vehicle and passengers investigated. However it would appear that, provided their limitations are realised, results from laboratory subjects may be used to help the passenger.

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160 Applied Ergonomics September 1978

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Applied Ergonomics September 1978 161