Ind. Health, 1967, 5. 213.

EVALUATION METHODS FOR VIBRATION EFFECT

PART 3. MEASUREMENTS OF THRESHOLD AND EQUAL

SENSATION CONTOURS ON HAND FOR VERTICAL

AND HORIZONTAL SINUSOIDAL VIBRATIONS

Toshisuke MIWA

National Institute of Industrial Health, Kizuki-Sumiyoshi, Kawasaki.

(Received November 27, 1967)

The levels of threshold and equal sensation on hand for vertical and horizontal

vibrations (3-300 c/s) were determined by the same method in the previous reports and

their characteristics did not differ between both vibrations. The equal sensation curves

of hand above 10 c/s showed good agreement with those of whole body.

Then, the sensation of hand for horizontal vibration was equated to that of verti-

cal one and it was observed that the sensation for both vibrations was equal at the

same vibration acceleration level and frequency.

The vibration greatness levels (VGL) for vertical and horizontal vibrations

and the conversion of sensation for horizontal vibration into that for vertical vib-

ration were determined for the whole body in Part 1 and 2. In this report, the

same problems were studied for human hand. Because various kinds of portab-

le vibrating tools (rock drill, chipping hammer, pneumatic hammer, air grinder, ty-

tamper, chain saw and hand tractor etc) have induced various injuries in worker's

hands, for example, Raynaud' s phenomenon, degenerative change of joint bones

etc. These portable tools are being used in many industries, because of facility of

carying and setting, and of high productivity. Moreover, the new types of them

have been devised by development of midget engines.

Under such circumstances, the safty limit for vibration to handle portable vi-

brating tools without injury is needed in the industrial hygiene. Although many

predecessors in this field have investigated about this problem by various medical

method, the clear solutions have not been obtained owing to the complexity of

these injuries. The psychological evaluation method described in the previovs re-

port is considered to contribute to this problem.

With the hand, the levels of threshold and equal sensation for vertical and

horizontal vibrations were measured. Equalization of sensation for horizontal vibra-

tion to that for vertical one was also examined. The reports dealing with these

problems have not yet been published. This paper reports about these problems.

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T. MIWA

1 2

3 4

5

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EVALUATION METHODS FOR VIBRATION EFFECT ( 3 )

6 7

8 9

Fig. 1 Photographs of postures of subjects.

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T. MIWA

SUBJECTS AND POSTURES

Ten subjects tested were the same crew as previous reports. On the posture

of the subjects, their single hand was directly pushed to the vibration table and

joints of hand and elbow were bent at right angle with each other in order to im-

itate an actual working posture and to prevent the vibration transmission from

hand to head.

For the vertical vibration, the hand pressed the vibration table with the force

of 5 kg continuously. This static pressure was controlled by the subject himself

watching the strain-meter which was made by the amplifier and the strain gages

stuck on the upper leaf spring supporting the vertical vibration table.

For the horizontal vibration, the hand was vibrated to the finger direction on

the palm. In this case, the force pressing the vibration table could not be meas-

ured because of difficulty of instrumentation. Therefore, subjects were requested

to press the table in the same sensation strength as in the vertical vibration.

In preliminary experiments on three subjects, the following various cases were

tested, namely, when the subject' s hand pressed the vibration table by means of

an iron handle of a rock drill which was set on the both vibration tables, when

the both hands directly pressed the vibration table, when the horizontal vibration

was excited in the rectangular direction on the finger, and when the static pressure

of single hand added to the vibration table was changed from 5 to 10 kg. However,

the observed results of these cases were in good agreement with the data deter-

mined in the condition described above. Therefore, actual experiments were done

under the above simplified conditions and subject's postures were shown in Fig 1.

MEASURING METHOD

The same vibration tables as described in the report 1 and 2 were used for

vertical and horizontal vibrations. The frequency range examined was selected

from 3 to 300 c/s, because, with the various kinds of actual portable vibrating

tools, the spectrum affecting mainly on human hand seems to be this range. The

amplitude of vibration was taken as widely as possible by using three kinds of the

Fig. 2 Block diagram of this experiment and photographs of the measuring

apparatus.

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EVALUATION METHODS FOR VIBRATION EFFECT ( 3 )

vibration tables. Namely, the maximum vertical vibration was 80dB (VAL) at

100 c/s, and the maximum horizontal vibration 70dB (VAL) at 100 c/s.

The measurement of threshold and equal sensation was carried out by the

method of paired comparisons and vibration was given the subjects in the ascending

and descending series. The detailed procedures have been specialized in Part 1.

The equal sensation at the highest level of both vibrations was observed only by

ascending series. Because the sufficient higher level of vibration at starting of de-

scending series could not be given the subject owing to the amplitude limit of the

vibration tables. The standard frequency in the equal sensation was also 20 c/s.

The block diagram of the experiment is shown in Fig 2.

RESULTS AND DISCUSSIONS

The contours of threshold and equal sensation with hand for vertical and hor-

izontal vibration are shown in Fig 3 and 4. These show the average and the stan-

Fig. 3 Contours of threshold and equal sensation for vertical vibration with hand.

In Fig. 3 and 4 ,ordinate indicates vibration acceleration levels (VAL) (dB) (201og10(a/aref)

a; rms acceleration value, (g), aref; 10-3g, 1g=980cm/sec2) and abscissa frequencies (c/s).

The solid curves are contours connecting the average values of threshold and equal sensation

with hand. The dotted curves are contours of equal sensation curves for the vertical and

horizontal vibrations with whole body (Fig. 13 in Part 1) respectively. The group of three

circles with dots arranged lengthwise shows the standard deviations and the average values

calculated with 10 men. The center of this group indicates the average value and outside

of them the range of standard deviation.

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T. MIWA

Fig. 4 Contours of threshold and equal sensation for horizontal vibration with hand.

dard deviation of their pooled data with 10 men.

Threshold contours: The threshold curves for vertical and horizontal vibrations

are in good agreement with each other. These threshold contours consist of three

lines with the slope of velocity, jerk and displacement, and their bending points

are 15 and 100 c/s. Furthermore, the fact that these contours are similar to the

mechanical impedance curve1) of hand for vertical vibration attracts our attention,

though the bending points of the impedance curve are somewhat higher than those

of threshold curves.

The standard deviation of threshold is uniformly distributed within about •}3

dB independent of vibration frequency.

Equal sensation contours: The solid curves in Fig 3 and 4 which were approx-

imately connected with the average value of equal sensation level are composed of

three lines according with acceleration, velocity and displacement and two bending

point, 6 and 60 c/s. Above 5 c/s, the contours of the equal sensation of hand forboth vibrations well accord with each other, that is, they have good individual

congruity and group congruity whose meaning was already defined in Part 1. Besides,

above 5 c/s, the equal sensation curves on hand for both vibrations are in good

agreement with those on whole body as shown in Fig 13 in Part 1. Below 5 c/s,

the dashed curves in Fig 3 and 4 show the equal sensation contours on whole body

for vertical and horizontal vibrations respectively. The frequency characteristics

in this frequency range with hand and whole body did not agree with each other.

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EVALUATION METHODS FOR VIBRATION EFFECT ( 3 )

In order to compare the equal sensation curves of hand for both vibrations with

each other, these two figures (Fig 3 and 4) are superposed as Fig 5. From this

figure, the equal sensation contours of hand have no special different frequency cha-

racteristics for both vibrations.

The standard deviations of equal sensation show frequency dependency, that

is, they are small between 5 and 60 c/s but, except this range, they become grad-

ually larger in proportion to frequency difference between this and outside range

(•}max. 5 dB).

Fig. 5 Contours obtained by superposition of Fig. 3 and Fig. 4.

In the additional experiment, the horizontal vibration was equated in sensation

to the vertical vibration by three subjects, This equalization was made at the

same frequency of both vibrations and the measured frequency was changed at 10,

20, 60, 100, 200 and 300 c/s. One hand was set on the vertical vibration table and

another hand on the horizontal one. At this time, the joints of hand and elbow

of both hands were bent at right angle shown in Fig 1-(5). The subject was asked

to keep his posture constant as strictly as possible, because the result was consi-

derably affected by the posture. The vibrations of two directions were simultan-

eously started and stopped in both tables every 3 sec by the tester. The vertical

vibration was fixed at a certain level as the standard and the procedure giving the

subject the horizontal vibration was done in the ascending and descending series.

The level of horizontal vibration was controlled with the attenuator by the tester

untill the subject answered to obtain the equal sensation. In this experiment,

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T. MIWA

both hands were interchanged each other on the vibration tables to check the

difference of sensibility of both hands at each frequency.

The same vibration acceleration level of both vibrations induces equal sensa-

tion at each frequency. This fact may be forecasted by the result that there was

almost no difference in the threshold with hand for the vertical and horizontal vi-

brations. It is also mentioned from this result that the sensation between vertical

and horizontal vibrations with the same VAL is equally convertible with hand.

Consequently, by Fig 5, vibration greatness level is defined for the vibration

sensation of hand. This graph is commonly used in all of the following cases, nam-

ely, both vibrations (vertical and horizontal), single and both hands, the pressing

force of hand to the vibration table from 5 to 10 kg, various shapes of the handle

and every direction of horizontal vibration on the palm.

REFERENCE

1) Miwa, T. (1964). Ind. Health, 2.95.

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