Whole-body vibration perception thresholds

Journal of Sound and Vibration (1988) 121(2), 237-258

W H O L E - B O D Y VIBRATION PERCEPTION THRESHOLDS K. C. PARSONSt AND M. J. GRIFFIN

Institute of Sound attd Vibration Research, Unirersity of Southantpton, Southampton S09 5Nil, England ( Receired 8 January 1987, and in revised form 30 April 1987) This paper presents the results of a series of laboratory experiments concerned with perception thresholds for whole.body vibration. The nature of absolute perception thresholds is discussed and a method of determining vibration thresholds, based upon signal detection theory, is proposed. Thresholds of subjects exposed to x-, y- and z-axis sinusoidal vibration were determined for sitting and standing subjects (from 2 to 100 Hz). Perception thresholds have also been determined for supine subjects exposed to vertical (x-axis) sinusoidal vibration (10-63 Hz). In additional experiments the effects of complex (e.g., random) vibration and the effects of duration on the perception thresholds were investigated. The relation between perception thresholds and vibration levels, said by subjects to be unacceptable if they occurred in their own homes, was investigated as well as the effects of subjects' personality and the visual and acoustic conditions in the laboratory. For the vertical vibration of seated subjects no significant differences were found between the responses of male and female subjects. Significant differences were found between perception thresholds for sitting and standing postures. The median threshold was approximately 0.01 m/s 2 r.m.s, between 2 and 100 Hz. Perception thresholds for x-axis and )'-axis vibration were not significantly different in either sitting or standing subjects but significant differences in thresholds were found between sitting and standing positions for both x-axis and )'-axis vibration. Subjects tended to be more sensitive to vibration when lying than when sitting or standing. The results suggested that the perception of random vibrations can be predicted from a knowledge of the perception of its component vibrations. The number of cycles of vibration did not affect perception thresholds for vibration durations of more than about 0.25 s. Some assessments suggested that vibration at more than twice the perception threshold may not be acceptable if it occurs in the home. I. INTRODUCTION

Sources o f potentially disturbing vibration in buildings include blasting, road and rail traffic, aircraft and industrial machinery. T h e type o f source, the g r o u n d characteristics and the structural response o f the building determine the nature o f the vibration in a building and the vibration to which occupants are exposed. SubJect reaction to the vibration will d e p e n d on its magnitude relative to the vibration perception threshold. Most investigations o f perception thresholds have involved subjects exposed to steady state single frequency, single axis vibration. Building vibration is often more complex; mul.tiple frequency or r a n d o m in nature and o f varying amplitude and duration. The vibration can enter the b o d y at several positions and in various directions. The b o d y m a y be sitting, standing, or lying. The individual effects o f these variables must be c o m b i n e d so as to provide a general procedure for predicting the perceptibility o f complex building vibration. The disturbance caused by building vibration (and whether or not an individual will complain) depends on psychological factors in addition to the physical characteristics o f -~Now at the Department of Iluman Sciences, Loughborough UMvershy of Technology, Loughborough LEII 3TU, England. 237

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238

K. C. P A R S O N S

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the vibration. These may include fear of building collapse or structural damage, emotional state, perceived source of vibration and the attitude of the person to the source. Predictions o f t h e responses of individuals are therefore difficult and not likely to be highly accurate. The series of laboratory experiments described in this p a p e r was conducted to determine absolute perception thresholds for the whole-body vibration of seated, standing and lying subjects exposed to a range of vibration conditions (see Table i). One application of the results was expected to be in the assessment of building vibration. 2. EXPERIMENT ONE: WHOLE-BODY VIBRATION PERCEPTION THRESHOLDS OF SITI'ING AND STANDING SUBJECTS EXPOSED TO VERTICAL (z-AXIS) SINUSO1DAL VIBRATION 2.1. AIMS The aims of Experiment One were to determine perception thresholds by using the signal detection theory method and to compare results from males and females obtained in the standing and sitting positions with sinusoidal vibration. 2.2. APPARATUS Whole-body vibration was produced by a Derritron VP30 electrodynamic vibrator powered by a 300 W amplifier. An aluminium plate, 400 m m by 400 mm and 12 mm thick, was attached to the vibrator to provide a horizontal surface which vibrated vertically. Subjects sat or stood on a rigid flat wooden surface which was of similar dimension and firmly attached to the aluminium plate. The vibrator was surrounded by a rigid frame which supported four adjustable springs. A single spring was attached to each of the four corners on the undersurface of the aluminium plate. This provided a means of supporting the stati~ weight of the subject. Eight horizontal spring steel flexures attached the rigid frame to the aluminium plate so as to reduce cross-axis coupling of the vibrator. Vibration was measured by using a Shaevitz A220 translational accelerometer. The vibration signals were generated and measured by using a digital computer. Tests conducted with a seated subject revealed that for z-axis vertical vibration there was no significant cross-axis coupling in roll, pitch or yaw over the frequency range 1.0 to 100 Hz, with the exception of 63 Hz where there was some pitching of the seat. It was found that, for the condition with no subject at 63 Hz, the vibration was not uniform over the seat, the range being in some cases four to one between the centre and the edge of the seat. It was concluded that for frequencies other than 63 Hz, cross-axis coupling would not significantly affect results. Values at 63 Hz, however, must be regarded with caution. The presence of a subject reduced the effect but it is possible that some subjects were receiving vibration at magnitudes greater than those measured. Hence, perception thresholds at 63 Hz may be somewhat higher than those reported here. Background vibration on the vibrator table was less than 4 x 10 -3 m / s 2 r.m.s, with a frequency of 50 Hz and greater. The acoustic noise level at the position of the subject's head was 56 dB(A) and was not significantly correlated with the vibration signals. The noise was mainly caused by the vibrator cooling fan and did not vary. For the magnitudes of vibration employed in the experiment the acceleration distortion was 20% or less. 2.3. METIIOD 2.3.1. Vibration perception threshold The absolute perception threshold for a vibration might be defined as the magnitude of the vibration above which a subject will detect the vibration and below which he will not detect the vibration. Practical investigation, however, reveals that for a vibration near

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threshold, subjects will sometimes report that they can feel motion and sometimes report that they cannot. A perception threshold is therefore more usefully reported as the vibration magnitude which results in detection on, say, 50% of occasions. Experimental investigation also reveals that there is an interval of uncertainty. That is, a range of vibration magnitudes where subjects are neither certain that they can, nor certain that they cannot, feel vibration (see Appendix 1). Whether a subject reports that he can feel a vibration which falls within his range o f uncertainty depends upon his attitude. For example, consider two subjects with identical sensitivities to vibration. The first subject is inclined to report that a vibration is present even if he is uncertain whether or not he can feel the vibration. The second subject, however, is inclined only to indicate that he can feel the vibration when he is sure that it is present. If 50% detection is used as the definition o f the absolute perception threshold, the first subject will be reported as having a lower absolute perception threshold to vibration than the second, even though their sensitivities to vibration are the same. "Signal detection theory" provides a method of determining a subject's absolute perception threshold independent of his inclination to report that a stimulus is present (i.e., his criterion (/3)). The signal detection model is one of detecting a signal above background noise. The signal in the experiments reported in this paper is vibration and the background noise is anything which is not the signal (e.g., physiological noise, background vibration, acoustic noise, etc.). The subject was presented with periods when the vibration stimulus was present and periods when the vibration stimulus was not present. The subject indicated whether he could or could not feel vibration. By using the proportion o f hits (i.e., the subject indicated that he could feel the vibration stimulus when it was present) and the proportion or false alarms (i.e., the subject indicated that he could feel the vibration stimulus when it was not present), the subject's discriminability (d') of the vibration could be determined [1, Licklider 1964]. Although the signal detection theory model implies a continuous change o f d' values with vibration magnitudes it is convenient to define a single magnitude as the subject's perception threshold. In this study, the perception threshold is defined as the discriminability value (i.e., d') which, for a criterion o f unity (i.e.,/3 = 1.0), the subject reports 75% hits and 25% false alarms. Assume that the perception of the noise is a normal distribution and that the perception of the signal plus the noise is also a normal distribution (see Figure 1). The discriminability

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WHOLE-BODY VIBRATION PERCEPTION THREStIOLDS

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value, d', is the magnitude o f the ditterence between the deviations of the normal curves. For 75% hits and 25% false alarms these values are shown in Table 2. Hence the corresponding discriminability is given by d ' = 0 . 6 7 4 - ( - 0 . 6 7 4 ) = 1.348, and so a value of d ' = 1-35 was used in the experiment. TABLE 2

Definition of dbration perception threshold when using signal detection theory Experimental data

Normal curve deviation

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2.3.2. Experimental method Eighteen male and 18 female subjects took part in the first experiment. Subjects were shown a list of medical conditions which would render then unfit for vibration exposure and asked to read an instruction sheet giving the experimental procedure (see Appendix 4). A computer produced the vibration stimuli, controlled a series of lights which informed the subject when to respond, recorded the vibration and recorded the subject's response. The first instruction to the subject was that the experimenter was "'setting up". This was the period when the experimenter ensured that when the vibration was presented it would be at the desired vibration level. The subject was asked to keep as still as possible during this period. This period also demonstrated to the subject the type of vibration (at a low level) which he would later be asked to detect. After the "setting u p " was complete, a light labelled "starting" came on. This was followed by a light labelled "stimulus". The stimulus light was on for a four second period then a light labelled "response" came on. The subject then pressed one of two buttons on a hand-held box: the button labelled " y e s " if he felt vibration, or the button labelled " n o " if he did not feel vibration during the period when the stimulus light was on. The stimulus light then came on again, then the respond light, and so on. After the stimulus light had been on 10 times (i.e., 10 responses) the "setting up" light came on again and a ditterent vibration stimulus was investigated. The start and end of the period in which the stimulus light was on was also indicated by a short bleep sound when the l!ght came on and a longer bleep when the light went ott. In five of the 10 periods when the stimulus light was on the vibration was present and in five of the periods the vibration was not present. The order of presentation of "vibration" and " n o vibration" periods was random. Seven frequencies o f sinusoidal vibrations were investigated: 2, 4, 8, 16, 31.5, 63 and 100 Hz. There were 11 possible vibration magnitudes: 0.002, 0.004, 0.0056, 0.008, 0.010, 0.014, 0.028, 0.040, 0.056, 0.080 and 0.160 m / s 2 r.m.s. The actual vibration magnitudes to which subjects were exposed were within 5% of the desired values. At each frequency the initial stimuli were at a magnitude of 0.010 m / s : r.m.s. Subsequent stimuli were presented in a random order from a magnitude by frequency matrix and depended upon the d ' values obtained. I f the subject's d ' value was greater than or equal to 2.0 the next presentation of that vibration frequency was at the next lower vibration magnitude in the magnitude by frequency matrix. If the subject's d ' value was less than or equal to 1.0 then the next presentation at that vibration frequency was at the next

242

K.C.

PARSONS A N D M. J. G R I F F I N

higher vibration magnitude in the matrix. If the subject's d' value was greater than !.0 and less than 2.0 then the subject was subsequently exposed to the next higher and the next lower vibration magnitude in the matrix. The order of presentation of subsequent vibration stimuli was random. The experiment continued until a d' value of greater than 2.0 and a d' value o f less than 1.0 had been obtained for each vibration frequency. Subjects attended two experimental sessions. The sessions were conducted, for each subject, at the same time o f day on separate days. In each experimental session a vibration perception threshold curve was determined. Half of the subjects (nine males and nine females) adopted a sitting posture in their first session and the other half of the subjects adopted a standing posture in their first session. Subjects sat with their ischial tuberosities on a line which was drawn across the centre of the wooden seat surface. They sat in a comfortable upright posture with their eyes open, looking forward, with their hands on their lap, their thighs horizontal and their feet flat on a stationary footrest. There was no backrest. Subjects stood with their eyes open looking forward with their feet fiat on the line across the centre of the wooden surface. They stood in a comfortable upright posture with the centre of their feet 200 mm apart. Standing subjects wore socks (tights, etc.) but no shoes. During the experiment the subjects looked forward at the instruction lights which were placed in front of a black curtain 700 mm from the subjects' eyes. Pilot experimentation and discussion .with subjects revealed that, for the conditions employed in this experiment, subjects did not detect the stimuli using auditory or visual cues (see Appendix 2). The temperature in the laboratory ranged from 19~ to 25~ over the period of the experiment. 2.4. RESULTS For each stimulus a d' value was calculated from the proportion of hits and the proportion of false alarms made by the subject. The d' values were used in a sine function interpolation procedure to provide an estimate of the vibration magnitudes for which d ' = 1-35 at each vibration frequency. With the single exception of 8 Hz vibration in the sitting posture, there were no significant differences (p > 0.05) between the perception thresholds determined for male and female subjects, in either the sitting or the standing postures (Mann-Whitney U test). The data for males and females were therefore combined for subsequent analysis. Figure 2 shows the 25th, 50th (i.e., median) and 75th percentile perception thresholds and the maximum and minimum perception thresholds of 36 subjects in sitting and standing postures. With the single exception of 63 Hz vibration, significant diflerences Slond~ng

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WtlOLE-BODY VIBRATION PERCEPTION THRESHOLDS

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(p < 0.05) were found between sitting and standing postures at all vibration frequencies (Wilcoxon matched pairs signed ranks test) (see Table 3). 2.5. DISCUSSION OF RESULTS Subjects were more sensitive to low-frequency vibration when sitting and more sensitive to high-frequency vibration when standing. This might be attributable to differing transmission o f vibration to the body in the two positions and the larger amount of postural sway with standing subjects, making low frequency vibration more difficult to detect. Different conclusions may have been reached if subjects had bent their legs when standing. However, the differences between the responses of sitting and standing subjects were small compared to the differences between the responses o f individual subjects. In the context o f human perception of building vibration it therefore appears reasonable to use the same analysis procedures for both sitting and standing persons and for both male and female subjects. These findings agree with the experimental results of McKay [2] who reported only small effects of subject gender and posture on perception thresholds. The results are in accord with lSO DIS 2631/2 [3] and BS 6472 [4] in so far that both standards propose identical procedures fo r male and female, sitting and standing persons in buildings. TABLE 3

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Figure 3 compares the median threshold for the standing subjects with the median threshold (48 sitting plus standing subjects) reported by McKay [2]. The two curves are similar: the threshold is approximately 0.01 m/s 2 r.m.s for all vibration frequencies up to 63 Hz. The results of the present experiment are also compared with those reported by Reiher and Meister [5] in Figure 3. The curve shown is the line of constant velocity which is said to be the lower boundary of the category described, by 10 standing subjects, as "barely perceptible". This curve has sometimes been used by those assessing the vibration of buildings. It can be seen that, compared with the results o f the present experiment, this line o f constant velocity greatly overestimates the response to vibration below about l0 Hz and greatly underestimates the response to vibration at higher frequencies. In Figure 2 the results of the present experiment may be compared with the base curve for z-axis vibration as defined in 1SO DIS 2631/2 and BS 6472. The median curve from the present experiment is at a higher vibration magnitude than this base curve for vibration frequencies below about 25 Hz. The 25th percentile threshold curve is above the base curve at frequencies below about 20 Hz. This suggests that use of the curves from these

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3. EXPERIMENT TWO: WHOLE-BODY VIBRATION PERCEPTION THRESHOLDS OF SITTING AND STANDING SUBJECTS EXPOSED TO x- AND )'-AXIS SINUSOIDAL VIBRATION 3.1. AIM The aim o f Experiment Two was to determine perception thresholds for sitting and standing subjects exposed to x- and y-axis sinusoidal vibration. 3.2. APPARATUS The apparatus was similar to that described for Experiment One. The vibrator was positioned such that it produced horizontal vibration o f the aluminium plate. This produced fore-and-aft (x-axis) vibration if the subject sat facing forward and lateral ()'-axis) vibration if the subject turned through 90 degrees. Four spring steel flexures were attached between the rigid frame and the aluminium plate so as to allow horizontal motiun without appreciable cross-axis coupling. The subjects' feet were not vibrated and there was no backrest. The vibration stimuli were produced by an oscillator and the magnitude of the vibration was controlled by the subjects using a 10-turn potentiometer. Background vibration on the vibrator table was less than 4 x l0 -3 m / s ~ r.m.s, with a frequency of 50 Hz and greater. The acoustic noise at the position o f the subjects" heads was 56dB(A) and was not significantly correlated with the vibration signals. For the vibration employed in the experiment the acceleration distortion was 20% or less. 3.3. METilOD Twelve male subjects adjusted the vibration stimuli to magnitudes which they could "just feel". Seven frequencies of sinusoidal vibration were used as stimuli (2,4,8, 16,31-5,63 and 100 Hz). Each subject sat (with a stationary footrest), or stood (over the centre of the wooden seat) holding the potentiometer (which had no cue marks), facing forward at a black curtain with eyes open. Subjects wore socks but no shoes. Each subject attended one experimental session of approximately one hour duration. They were initially exposed to vibration in one of four conditions (x-axis sitting, x-axis

WHOLE-BODY

VIBRATION

PERCEPTION

245

THRESHOLDS

standing, ),-axis sitting, )'-axis standing). For each condition, the subject's perception thresholds were determined for all seven frequencies ofvibration. The subject then changed to another condition and the perception thresholds were again determined. The orders of presentation o f both the vibration frequencies and the conditions were random. 3.4. RESULTS The median, 25th and 75th percentile perception thresholds are presented in Figures 4 and 5 for all four conditions. A Wilcoxon matched-pairs signed ranks test showed that there were no significant differences (p > 0.05) between perception thresholds for x-axis and ),-axis vibration in either sitting or standing postures, except at 16 Hz for sitting subjects and 31-5 Hz for standing subjects (see Table 4). Significant differences (D < 0.05) were found between sitting and standing postures for both x-axis and y-axis vibration. The results of the Wilcoxon matched-pairs signed ranks test showed that in both axes, for vibration frequencies of less than 16 Hz, subjects were more sensitive to vibration when sitting than when standing (see Table 4). This can also be seen in Figure 6 in which the median perception thresholds for all four conditions are compared.

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3.5. DISCUSSION

The results indicate that the sensitivity ofsubjects to x- and )'-axis acceleration decreases TABLE 4 Results o f Wilcoxon matched-pairs signed ranks test on x. and y-axis thresholds from standing and seated subjects (Experiment Two): * = significant difference (p < 0.05) Vibration frequency (Hz) 2 Sitting (x-axis vs. y-axis)

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greater than 31.5 Hz the sensitivity o f subjects to acceleration changed by a relatively small amount. In both sitting and standing postures perception thresholds derived from x- and y-axis vibration were similar. However, there were significant differences between sitting and standing postures at frequencies less than about 16 Hz. The above findings are in partial agreement with the proposals in ISO DIS 2631/2 and BSI 6472 which i m p l y that there is no difference between the perception of x- and ),-axis vibration. However, the sensitivity to vibration frequency proposed in these standards

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differs from the results o f the present study. The " b a s e " curve in the Standard is at vibration magnitudes lower than the 25th percentile perception threshold determined in the present experiment at frequencies below 31.5 Hz. The difference is particularly great if the perception thresholds for standing subjects are compared with the base curve in the Standard. This suggests that the proposed procedure for evaluating the effects of vibration on the occupants of buildings is "over-sensitive" to x-axis and )'-axis vibration. As a consequence, for standing subjects exposed to low-frequency horizontal vibration, the standards may suggest unacceptable vibration conditions when the vibration is imperceptible.

WHOLE-BODY VIBRATION PERCEPTION TIIRI~SIIOLDS

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4. EXPERIMENT THREE: WHOLE.BODY VIBRATION PERCEPTION THRESHOLDS FOR LYING SUBJECTS EXPOSED TO VERTICAL (x-AXIS) SINUSOIDAL VIBRATION 4.1. AIM The aim ofthis experiment was to determine vibration perception thresholds for subjects lying supine and exposed to x-axis (vertical) sinusoidal vibration. 4.2. APPARATUS Whole-body vibration was generated by using a Derritron VPI80LS electrodynamic vibrator powered by a 1500 W amplifier. The vibration was produced by the reaction of the vibrator with a suspended concrete block on which it was placed. Subjects lay on the concrete block. The magnitudes of vibration over the area of the body were within 10% of the nominal magnitude. The vibration stimuli were generated using an oscillator and the vibration magnitude was controlled, by the subject, using a 10-turn potentiometer. 4.3. M E T H O D Each o f eight male subjects lay, in a supine position with their feet, legs, back and head supported by the concrete block. They adjusted the magnitude of the stimulus vibration, using a 10-turn potentiometer which had no cue marks, until they could "just feel" the vibration. Four frequencies of vertical sinusoidal vibration were investigated ( 10, 16, 31.5 and 63 Hz). Each stimulus was presented to every subject twice and the mean of the two readings was taken as the perception threshold. The order of presentation o f stimuli was random. Subjects attended one experimental session o f about 30 minutes duration. After each stimulus had been presented, subjects indicated on which part o f their body they first detected vibration. Subjects wore their normal clothing with socks but no shoes. 4.4. RESULTS The median and range o f the perception thresholds are presented in Figure 7. Subjects detected vibration over various parts of the body for 10, 16 and 31.5 Hz but all subjects first detected 63 Hz at the head (see Table 5). It can be seen that subjects' sensitivity to vibration acceleration decreased slightly with increasing vibration frequency from i0 to 31-5 Hz (see also Table 6).

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4.5. mSCUSStON The results of this relatively small scale experiment show that, at lower frequencies, ~ubjects" sensitivity to vibration was not dominated by the perception of vibration in the

248

K. C. PARSONS AND M. J. GRIFFIN TABLE 5

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TABLE 6

Results of Wilcoxon matched-pairs signed ranks test for supine subjects exposed to x-axis vibration (Experiment 3): * = significant difference (p < 0 . 0 5 ) Vibration frequency (Hz) l0 16 31.5

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head. in buildings, the vibration o f supine persons usually occurs when they are lying on a soft bed with their heads resting on a pillow. A n y vibration measured on the building structure (e.g., floor) is modified by the vibration transfer function o f the bed before it reaches the subject. As beds are usually soft, magnitudes o f 63 Hz vibration on the floor will often be greatly attenuated. It is therefore likely that supine persons will often be first disturbed by vibration at p a n s o f the b o d y other than the head. Again, the frequency d e p e n d e n c e o f the perception thresholds differs from those in the standards (ISO 2631/2 and BS 6472). The x-axis thresholds for supine subjects are appreciably lower than the x-axis thresholds for standing and seated subjects. However, they are broadly similar to those for the z-axis vibration o f standing and seated subjects.

WIIOLE.BODY VIBRATION PERCI-P'I-IONTIIRESIIOLi)S

249

5. EXPERIMENT FOUR: WHOLE-BODY VIBRATION PERCEPTION THRESHOLDS OF Si'I'riNG SUBJECTS EXPOSED TO VERTICAL (z-AXIS) MOTIONS WITH COMPLEX SPECTRA 5.1. Atr,l The aim ofthis experiment was to investigate methods ofpredicting vibration perception thresholds for random motions from a knowledge of vibration perception thresholds for sinusoidal vibration. 5.2. APPARATUS The apparatus used was as described for Experiment One. Subjects sat over the centre of the seat (which had no backrest) with their feet on a stationary footrest and faced a black curtain l m to the front. 5.3. METHOD Eight male subjects took part in the experiment with each attending one session o f approximately one hour's duration. Subjects were presented (via a computer) with a number o f four second, z-axis, vibration stimuli. During each four second stimulus the subject adjusted the magnitude of the vibration, using a 10-turn potentiometer which had no cue marks. The same four second stimulus was repeatedly presented until the subject said that the vibration was at the level at which he could "just feel it". The vibration was then recorded (via the computer). The vibration stimuli were: sinusoidal vibration at frequencies of 2, 4, 8, 16, 31-5 and 63 Hz; one-third octave bands of random (Gaussian) vibration with centre frequencies of 2, 4, 8, 16, 31-5 and 63 Hz; octave bands of Gaussian random vibration with centre frequencies 4, 8, 16, 31.5 and 63 Hz and a Gaussian random vibration stimulus over the frequency range 2.8-89.6 Hz. 5.4. RESULTS

The root-mean-square (r.m.s.), root-mean-quad (r.m.q.), maximum and minimum acceleration magnitudes were calculated from each of the recorded vibration signals for each subject and stimulus without any frequency weighting. A Wilcoxon matched-pairs signed ranks test compared perception thresholds for sinuosoidal vibration with the perception thresholds for the unweighted one-third octave, octave and five-octave vibration over all eight subjects, (see Table 7). It can be seen that for the conditions considered TABLE 7

Comparison of perception threshold acceleration magnitudes for sinusoidal, l / 3-octave, octave and 5-octave vibrations by using the Wilcoxon matched.pairs signed ranks test (Experiment Fo,lr) (Centre) frequency (Hz)

Prediction method "Sine vs. I/3-octave Sine t,s. octave Sine vs. 5-octave r.m.s, r.m.q, max. rain. r.m.s, r.m.q, max. min. r.m.s, r.m.q, max. min.

2

+

+

+

+

4

+

+

+

+

+

+

+

8

+

+

+

+

+

*

*

*

16

+

+

*

*

+

*

*

*

3l'5

+

+

*

*

+

+

*

*

63

+

+

*

*

+

+

*

*

+

+

*

,

9

+, Fail to reject the null hypothesis: no significant dillerence (p>0-05) *, Reject null hypothesis:significant ditterence (p<0.05).

250

K. C. PARSONS AND M. J. G R I F F I N

in this experiment, acceleration perception thresholds for one-third octave and sinusoidal vibration are similar when they are assessed by either the r.m.s, or the r.m.q, methods. For the octave and five-octave stimuli there was no significant difference in acceleration magnitudes only when they were evaluated by the r.m.s, method. The average r.m.s. acceleration perception thresholds for sinusoidal and one-third octave random vibration are shown in Figure 8.

0"1 E

001 o

O00s

%

IOO

Frequency (Hz}

Figure 8. Mean vibration perception thresholds for sinusoidal ( vibration of eight male subjects.

) and one-third octave random ( - - - )

5.5. D I S C U S S I O N The results of this experiment suggest that root-mean-square measures of acceleration provide reasonably accurate indications o f the perceptibility of both sinusoidal and random vibration over the frequency range considered. It was noticeable, however, that, for the five-octave band analyzed with no frequency weighting, the r.m.s, values were consistently larger than those for sinusoidal vibration. This difference, which would have been significant at the p<0.1 level, was probably due to the decreased sensitivity of subjects to vibration acceleration at higher frequencies. A perception threshold function given by constant acceleration below 16 Hz and constant velocity above 16 Hz (as in BS 6841 [6]) provides a weighting function which appropriately reduces the importance of magnitudes at higher frequencies. Similarly, it may be argued that frequency-weighted peak and r.m.q, values would provide more accurate results than unweighted acceleration values. Future experimentation should evaluate these methods more comprehensively. It can be concluded from the results of this experiment that, for the conditions investigated, the r.m.s, acceleration (without frequency weighting) provides a reasonably accurate measure of the perceptibility of both sinusoidal and random vibration. For vibrations with a frequency content greater than 16 Hz, the use of a suitable frequency weighting would improve the prediction.

6. EXPERIMENT FIVE: THE EFFECT OF THE NUMBER OF CYCLES OF SHORT DURATION VIBRATION ON VIBRATION PERCEPTION THRESHOLDS 6.1. AIM The aim of this experiment was to determine whether or not a vibration perception threshold depends upon the number o f cycles of the stimulus vibration.

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251

6.2. APPARATUS The apparatus was as described for Experiment One. Subjects sat over the centre o f the seat (which had no backrest) with their feet on a stationary footrest and faced a black cui'tain 1 m to the front. 6.3. METttOD "Twelve seated male subjects took part in the experiment. Each subject attended one session o f approximately one hour's duration. The method of signal detection (see Experiment One) was used to determine subjects' perception thresholds to 16 Hz, sinusoidal, z-axis vibration. Seven vibration stimuli were investigated: 1, 2,4, 8, 16, 32 and 64 cycles of the 16 Hz vibration. The stimuli were presented about the centre of a 4-second time period during which the stimulus light was on. Subjects indicated whether they could or could not feel vibration at any time while the stimulus light was on. The dynamic response of the vibrator was such that approximately half a cycle of vibration was added to the end o f each stimulus. 6.4. RESULTS The median perception thresholds from the 12 subjects are presented in Figure 9. A one-way analysis of variance showed that there were significant differences ( p < 0 . 0 5 ) between the mean threshold values over all vibration conditions. The results of Fisher's LSD (least significant difference) test, for differences between individual pairs o f means, are presented in Table 8. It can be seen that perception thresholds derived from 1, 2 and 4 cycles o f vibratiofi are significantly different from those derived from vibration with 8, 16, 32 and 64 cycles. The thresholds for 1, 2 and 4 cycles of vibration are higher. 0.1

E =

0-01

.9

o-oo,

~

~

~

,'o

~2

6~

Number of cycles

Figure 9. Median perception thresholds for 12 male subjects exposed to different numbers of cycles of 16 ttz vibration.

6.5. DISCUSSION The results show that for 16 Hz sinusoidal vibration, higher magnitudes of vibration are required for perception if the vibration duration is less than 0.25 to 0.5 seconds (i.e., 4-8 cycles). Further experimentation, involving a wide range of vibration frequencies is required to investigate whether this is a " d u r a t i o n " effect or a " n u m b e r of cycles" effect, or both. Consideration of the median results suggest that the magnitude required for

252

t<. c .

PARSONS

AND

M.

J. G R I F F I N

TABLE 8

Results of Fisher's LSD test for differences between individual pairs of mean perception levels ( Experinlent Five) No. of cycles of 16 Hz vibration 1

I

2 +

2 4 8

16 32 64

4

8

16

32

64

+

*

*

*

*

+

* +

* *

* *

* *

+

+

+

+

+ + +

+, Fail to reject null hypothesis: no significant difference (p > 0.05) *, significant difference between pairs of means (p < 0.05).

perception at the shortest duration is only 1.5 times the magnitude required to produce perception at durations greater than about 0.5 seconds. Other research suggests that the time-dependency at supra-threshold magnitudes has a slightly greater slope than that found here [7, Griffin and Whitham 1980; 8, Howarth and Griffin 1987]. When assessing building vibration the supra-threshold time-dependency is likely to be more important than any change in the relevant threshold of perception with changing durations of vibration. 7. D I S C U S S I O N

The median, 25th and 75th percentiles o f x-, y- and z-axis whole-body vibration perception thresholds for sitting, standing and lying subjects for sinusoidal vibration are presented in Figure 10. The relevant "base" curves from ISO DIS 2631/2 and BSI 6472 are presented for easy comparison. It can be seen that for sitting and standing subjects the base curves are generally below the perception thresholds at frequencies below about 31-5 Hz. This is particularly true for standing subjects exposed to x-axis and y-axis vibration. However, for lying subjects the curve in the standard is at a higher level than the perception threshold. The results of the present research may be used to define a method of determining whether or not vibration will be perceptible to the occupants of buildings. For single frequency, muhiple frequency or random building vibration it is suggested that the reciprocal of the relevant median perception threshold is used as a frequency weighting function. Alternatively the experimental contours can be used to determine which frequencies of a vibration spectrum are most likely to be perceived and the percentiles o f a population which may feel vibration. For seated and standing subjects the perception thresholds for z-axis acceleration have far less frequency dependence than the contours in ISO DIS 2631/2 and BS 6472. The z-axis weighting in BS6841 is more appropriate (i.e., constant acceleration 5-16 Hz, constant velocity 16-80 Hz). Alternatively, at frequencies below about 63 Hz, constant acceleration could be used and a reasonable approximation to the perceptibility of vibration may be obtained directly from the r.m.s, value, with no frequency weighting. l f t h e vibration exceeds the perception threshold the disturbance produced by the vibration may become more dependent on vibration frequency [9, Griffin et al. 1982; 10, Corbridge and Griffin 1986; 11, Howarth and Griffin 1987].

%VHOLE-ROI)Y

253

VIBRATION PERCEPTION T I I R E S I I O L I ) S

1.0 X--(]XI~

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F'tequenc ( H z ) Figure 10. Comparison of x-, .1- and z-axls perception thresholds for sitting, standing and lying subjects (medians and inter-quartile range).

The results of the experiment conducted to determine perception thresholds for different numbers of cycles of vibration implied that, if vibration duration is greater than about 0.25 or 0-5 seconds, no correction for duration is required to predict whether a motion is perceptible. Again, when the vibration magnitude is well above the threshold a different time-dependency is likely to apply (see, for example, [12, Woodroof etal. 1983]; [8, Howarth and Griffin 1987]). If vibration occurs simultaneously in more than one axis the thresholds may be different from those shown here. In the absence or experimental data, it would seem a reasonable first approximation to suggest that the most perceptible axis of vibration will dominate the response. The application of perception threshold curves to the prediction or the disturbance caused by vibration in buildings was investigated in an experiment described in Appendix 3. In the absence of more substantial data, it is tentatively concluded that vibration may become unacceptable when the threshold is exceeded by a factor of about two (i.e., at a multiplying factor of 2). Much higher magnitudes may be required in some circumstances. Additional research should be conducted to establish the relationship between vibration perception thresholds and the incidence o f complaints. The concept of multiplying factors is simplistic when attempting to represent the mechanism by which perceptible vibration becomes disturbing and produces complaints. The reasons why people are disturbed and complain arc not understood but are certainly complex. It can be seen in Appendix 3 that a single personality measure (extrovert/introvert) did not contribute to an understanding of these mechanisms.

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PARSONS ANt)

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The experiments have provided additional information on human perception of wholebody vibration, but further research is required to identify how people respond to real building vibration. The laboratory experimentation should be extended to include the simulation o f recorded building vibration [8, 12]. Although studies o f c o m m u n i t y response to building vibration have been conducted [13, Fields and Walker 1980; 12, W o o d r o o f and Griffin 1987; 14, Watts 1984], field studies are not well suited to the evolution of a comprehensive model defining how response depends on the magnitude, frequency, direction and duration of vibration. Field studies involve stimuli which are difficult to control or quantify and human response in the home is dependent on many additional variables. The present results should provide some indication o f the lower boundary at which complaints will occur in buildings and so assist the interpretation of data from field studies. They may also assist the design of laboratory experiments of human response to simulations o f supra-threshold building vibration. REFERENCES I. J. C. R. LICKLIDER 1964 in Signal Detection and Recognition by Human Obsercers 3. (J. A. Swets, Editor). New York: John Wiley. Theory of signal detection. 2. J. R. MCKAY 1972 Ph.D. Thesis, Uni~'ersity of Southampton. Human response to vibration: some studies of perception and startle. 3. ! NTERNATIONALSTANDARDSORGANISATION1985 ISO/DIS 2631/2. Evaluation ofhuman exposure to vibration and shock in buildings (l to 80 Hz). 4. BRITISH STANDARDS INSTITUTION 1984 BS 6472: 1984. The evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz). 5. H. REItIER and F. J. MEISTER 1931 Translation of Report No. F-TS-616-RE (1946), Forschung (VDI) 2(If), 381-386, Headquarters Air Material Command, Wright Field, Dayton, Ohio. The sensitiveness of the human body to vibrations (Empfindlichkeit des menschen gegen erschutterung). 6. BRITISH STANDARDS INSTITUTION 1987 BS 6841:1987. Guide to the measurement and evaluation of human exposure to mechanical vibration and expected shock. 7. M. J. GRIFFIN and E. M. WI|ITllAM 1980 Journal of the Acoustical Society of America 68, 1277-1284. Discomfort produced by impulsive whole-body vibration. 8. H. V. C. HOWARTil and M. J. GRIFFIN 1988 Journal of Sound and Vibration 120, 413-420. Human response to simulated intermittent railway-induced building vibration. 9. M.J. GRIFFIN, K. C. PARSONSand E. M. WHITIIAM 1982 Ergonomics 25, 721-739. Vibration and comfort: IV. Application of experimental results. 10. C. CORBRIDGE and M. J. GRIFFIN 1986 Ergonomics 29, 249-272. Vibration and comfort: vertical and lateral motion in the range 0.5 to 5-0 Hz. l 1. H. V. C. HOWARTH and M. J. GRIFFIN 1987Journal of the Acoustical Society of America (awaiting publication). The frequency dependence of subjective reaction to vertical and horizontal whole-body vibration at low magnitudes. 12. H.J. WOODROOF,C. H. LEWISand M. J. GRIFFIN 1983 ORE Report DT 159 (D 151) Utrecht, O~icefor Research and Experiments of the International Union of Railways. Experimental studies of subject response to railway-induced building vibration. 13. J. M. FIELDS and J. G. WALKER 1980 I.S. ER. Technical Report No. 102. Reactions to railway noise: a survey near railway lines in Great Britain. 14. G. R. WATTS 1984 Laboratory Report 1119, ISSN 0305-1293, Transport and Road Research Laboratory. Vibration nuisance from road traffic,--results of 50 site survey. APPENDIX 1 EXPERIMENT SIX: INVESTIGATION OF METt|ODS OF DETERMINING VIBRATION PERCEPTION THRESHOLDS A I . I . INTRODUCTION Pilot investigations confirmed that there was no single vibration magnitude above which a subject will always detect a vibration and below which he will not. There is an "interval

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THRESHOLDS

of uncertainty" such that the probability that a subject will detect a stimulus increases as the stimulus intensity increases. In addition, the subjects' task is one of detecting the vibration above background noise (physiological, acoustic, etc.). Whether a subject reports that he can feel a vibration, or not, also depends upon his inclination to do so. These considerations suggested that a signal detection theory model would provide a realistic basis on which to determine vibration perception thresholds.

AI.2. ArM The aim of the experiment was to identify subjects' intervals of uncertainty and to determine whether the method of signal detection could provide meaningful values of a subject's "detectability" of vibration within his range of uncertainty.

A1.3. METHOD Method 1. Two vibration frequencies of z-axis sinusoidal vertical vibration were investigated (8.0 and 31.5 Hz). Twelve seated male subjects took part in the experiment. The interval o f uncertainty of seated subjects was obtained by asking them to adjust a magnitude of z-axis sinusoidal vibration until it was at: (A) "the lowest level of vibration which you are certain you can feel" and (B) "the highest level of vibration which you are certain you can not feel". Method 2. The signal detection method involved subjects being presented with 20, four second stimuli. On half of the occasions vibration was present and on half of the occasions vibration was not present (a random presentation order was used). The task o f the subjects was to indicate whether or not they felt vibration when a stimulus light was illuminated (see Experiment One).

AI.4. R E S U L T S The results presented in Figure AI show the vibration magnitudes obtained from method 1 for all 12 subjects and the discriminability "perception threshold levels" (i.e., condition for which d ' = 1.35) obtained by means of the signal detection theory (method 2) for both vibration frequencies.

100

3 1 . 5 HZ

8 . 0 Hz

A

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2

3

4

5

6

7

8

9

I0

II

12

2

3

4

5

6

7

8

9

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Subject Figure AI. Vibration perception thresholds determined by two methods.

II

12

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K.c. PARSONS AND M. J. GRIFFIN

AI.5. CONCLUSIONS Three main conclusions can be drawn: (I) when considering the perception of wholebody vibration there is a range of uncertainty between vibration magnitudes which subjects can "definitely not feel" and those which they "definitely can feel"; (2) the discriminability values obtained from the method of signal detection provide measures of a subject's detectability of vibration which is generally within the range o f uncertainty; (3) logical consideration suggested that a d ' value of 1-35 could be used as a realistic definition o f a perception threshold.

APPENDIX 2 EXPERIMENT SEVEN: TIlE EFFECT OF VISUAL AND ACOUSTIC CONDITIONS ON VIBRATION PERCEPTION TttRESHOLDS A2.1. AIM The aim of this experiment was to investigate the effects o f the visual and acoustic conditions of the laboratory on vibration perception thresholds. A2.2. METIIOD The method of signal detection was used to determine vibration perception thresholds of seated subjects. This involved 20, 4-second stimuli as described previously. The three conditions investigated were as follows: (i) "'normal" (eyes open, no ear muffs--subjects looked at a black curtain at I m distance, background noise 55 dB(A) which was not correlated with the vibration); (it) "eyes closed" (no ear muffs); (iii) "ear mult's on" (eyes open). Perception thresholds were determined for 12 male subjects (with normal hearing) for 4 Hz and 63 Hz z-axis vertical sinusoidal vibration. After the perception thresholds had been determined, the subjects were asked how they detected the vibration. They were then asked to listen for the vibration and their perception thresholds were re-determined at 63 Hz. A2.3. RESULTS The perception thresholds determined at each vibration frequency for each condition are presented in Table A2.1. A Wilcoxon matched-pairs signed ranks test showed no significant differences ( p < 0 . 0 5 ) between perception thresholds determined for each condition. Consideration of individual data suggested that there may have been small effects of visual condition at a vibration frequency of 4 Hz. The investigation of auditory TABLE A L l

Vibration perception thresholds (m/s 2 r.m.s, x I O-J) for conditions "tlormal" (N--eyes open, no ear muffs), "'eyes closed" (EC) a M "'ear muffs on" (EM) (Experiment Set, en)

4Hz

63 Hz

Subject

I

2

3

4

5

6

7

8

9

10

II

12

N

5

12

9

14

II

6 7

10 9

12 13

15 13

12 7

12 II 12

l0 It 9

12 16 31

II 10 19

13 7 15

13 13 16

9

EC EM N EC EM

9 9 6

6 7 9

16 12 4

I1 7 5

7 8 8

15 12 5

6 6 6

13 12 17

9 9 l0

12 7 II

10 9 2

20 17 18

8 8

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257

cues at 63 Hz suggested that, for the condition with ear muffs on, three of the 12 subjects could hear the "vibration" at lower vibration magnitudes than those at which they could feel the vibration. A2.4. CONCLUSION Perception thresholds determined under the laboratory conditions were not significantly influenced by the visual or auditory conditions in the laboratory. APPENDIX 3 EXPERIMENT EIGHT: THE RELATION BETWEEN VIBRATION PERCEIrFION THRESttOLDS AND MAGNITUDES OF ACCEPTABLE BUILDING VIBRATION A3.1. AIM The aim of this experiment was to determine the relation between the lowest magnitude of vibration which people would find unacceptable in their own home and the lowest magnitude of vibration which people can just feel. A3.2. METHOD Eighteen male and 18 female seated subjects took part in the experiment with 16 Hz z-axis vertical vibration. Each subject attended one experimental session of approximately 15 minutes duration. Subjects adjusted the vibration magnitude, using a 10-turn potentiometer which had no cue marks, to two vibration conditions: (a) "The lowest level o f vibration you can JUST F E E L " ; (b) "The lowest level of vibration which you would find U N A C C E P T A B L E IF IT O C C U R R E D IN YOUR OWN H O M E " . Half o f the subjects first made adjustments to level (a) and half commenced with level (b). The potentiometer was turned to zero between setting conditions (a) and (b). When subjects had reached the required position, the vibration magnitude was measured. After the experiment each subject completed an Eysenck personality inventory to provide an E (Extrovert) score. A3.3. RESULTS The mean magnitude (over all 36 subjects) of the 16 Hz vibration which subjects could just feel was 0.033 m/s 2 r.m.s. The mean magnitude which subjects indicated they would find unacceptable if it occurred in their own homes was 0.070 m/s 2 r.m.s. The ratio o f the two values is 2.12. No significant differences ((p < 0.05) Mann-Whitney U test) were found between the results of male and female subjects for perception thresholds, unacceptable levels, ratios between unacceptable levels and perception thresholds or E scores. A significant correlation ((p < 0.05) Spearman's re) was found between perception thresholds and vibration magnitudes found to be unacceptable. No significant correlations were found between vibration perception thresholds, unacceptable levels, or the ratio o f unacceptable and perception levels, and the personality score. A3.4. DISCUSSION For the vibration investigated in this experiment, subjects judged that, on average, they would find vibration at a magnitude 2.12 times their threshold value unacceptable if it occurred in their own homes. This is in agreement with the British Standard 6472 and the proposed International Standard (ISO DIS 2631/2 (1985)) which give a multiplication factor of between 2 and 4 above the base curve for residential properties. The highly significant correlation ( r , = 0 . 8 ) between subjects' perception thresholds and magnitude which they would find unacceptable in their own homes, provides s o m e

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supporting evidence for the method of using a base perception threshold curve with multiplication factors for predicting the acceptability of vibration to occupants of buildings. It also suggests that an individual with a low threshold may be more annoyed by building vibration than an individual with a high threshold. The very low correlation coefficients between vibration magnitudes and the Extrovert scores suggests that the large individual variability in the perception of vibration cannot be explained by this simple personality measure. APPENDIX 4 SUBJECT INSTRUCTIONS (EXPERIMENT ONE) The aim of this experiment is to determine whether you can detect various vibrations. You will be presented with a number of stimuli. Each time a stimulus is presented a light labelled " S T I M U L U S " will come on. On half of the occasions when the stimulus light is on, a vibration will be present. On half of the occasions when the stimulus light is on, no vibration will be present. The period of time over which the stimulus light is on will be indicated by "bleep" sounds. A "bleep" will occur at the beginning of the period and a longer "bleep" will occur at the end o f the stimulus period. You will be given a box with buttons on it. When the light labelled " R E S P O N D " comes on (i.e., after the Ionger"bleep*' defining the end o f t h e stimulus period) I N D I C A T E W H E T H E R YOU C O U L D FEEL VIBRATION OR W H E T H E R YOU C O U L D NOT FEEL VIBRATION by pressing the button labelled: Yes--you did feel vibration, N o - - y o u did not feel vibration.. You will be asked to either sit or stand on the vibrator platform: please position yourself looking forward, in a comfortable upright posture over the centre of the table and maintain this posture throughout the experiment. Please indicate to the experimenter if you have had a cigarette (cigar, pipe, etc.) or an alcoholic drink within the last eight hours or if you have consumed coffee (or other drink) within the last hour.