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## Vibration

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Vibration is oscillatory motion. This chapter summarizes human responses to whole-body vibration, hand-transmitted vibration and the causes of motion sickness.

Whole-body vibration occurs when the body is supported on a surface which is vibrating (e.g., when sitting on a seat which vibrates, standing on a vibrating floor or recumbent on a vibrating surface). Whole-body vibration occurs in all forms of transport and when working near some industrial machinery.

Hand-transmitted vibration is the vibration that enters the body through the hands. It is caused by various processes in industry, agriculture, mining and construction where vibrating tools or workpieces are grasped or pushed by the hands or fingers. Exposure to hand-transmitted vibration can lead to the development of several disorders.

Motion sickness can be caused by low frequency oscillation of the body, some types of rotation of the body and movement of displays relative to the body.

Magnitude

Oscillatory displacements of an object involve alternately a velocity in one direction and then a velocity in the opposite direction. This change of velocity means that the object is constantly accelerating, first in one direction and then in the opposite direction. The magnitude of a vibration can be quantified by its displacement, its velocity or its acceleration. For practical convenience, the acceleration is usually measured with accelerometers. The units of acceleration are metres per second per second (m/s2). The acceleration due to the Earth’s gravity is approximately 9.81 m/s2.

The magnitude of an oscillation can be expressed as the distance between the extremities reached by the motion (the peak-to-peak value) or the distance from some central point to the maximum deviation (the peak value). Often, the magnitude of vibration is expressed in terms of an average measure of the acceleration of the oscillatory motion, usually the root-mean-square value (m/s2 r.m.s.). For a single frequency (sinusoidal) motion, the r.m.s. value is the peak value divided by √2.

For a sinusoidal motion the acceleration, a (in m/s2), can be calculated from the frequency, f (in cycles per second), and the displacement, d (in metres):

a=(f)2d

This expression may be used to convert acceleration measurements to displacements, but it is only accurate when the motion occurs at a single frequency.

Logarithmic scales for quantifying vibration magnitudes in decibels are sometimes used. When using the reference level in International Standard 1683, the acceleration level, La, is expressed by La = 20 log10(a/a0), where a is the measured acceleration (in m/s2 r.m.s.) and a0 is the reference level of 10-6 m/s2. Other reference levels are used in some countries.

Frequency

The frequency of vibration, which is expressed in cycles per second (hertz, Hz), affects the extent to which vibration is transmitted to the body (e.g., to the surface of a seat or the handle of a vibratory tool), the extent to which it is transmitted through the body (e.g., from the seat to the head), and the effect of vibration in the body. The relation between the displacement and the acceleration of a motion are also dependent on the frequency of oscillation: a displacement of one millimetre corresponds to a very low acceleration at low frequencies but a very high acceleration at high frequencies; the vibration displacement visible to the human eye does not provide a good indication of vibration acceleration.

The effects of whole-body vibration are usually greatest at the lower end of the range, from 0.5 to 100 Hz. For hand-transmitted vibration, frequencies as high as 1,000 Hz or more may have detrimental effects. Frequencies below about 0.5 Hz can cause motion sickness.

The frequency content of vibration can be shown in spectra. For many types of whole-body and hand-transmitted vibration the spectra are complex, with some motion occurring at all frequencies. Nevertheless, there are often peaks, which show the frequencies at which most of the vibration occurs.

Since human responses to vibration vary according to the vibration frequency, it is necessary to weight the measured vibration according to how much vibration occurs at each frequency. Frequency weightings reflect the extent to which vibration causes the undesired effect at each frequency. Weightings are required for each axis of vibration. Different frequency weightings are required for whole-body vibration, hand-transmitted vibration and motion sickness.

Direction

Vibration may take place in three translational directions and three rotational directions. For seated persons, the translational axes are designated x-axis (fore-and-aft), y-axis (lateral) and
z-axis (vertical). Rotations about the x-, y- and z-axes are designated rx (roll), ry (pitch) and rz (yaw), respectively. Vibration is usually measured at the interfaces between the body and the vibration. The principal coordinate systems for measuring vibration with respect to whole-body and hand-transmitted vibration are illustrated in the next two articles in the chapter.

Duration

Human responses to vibration depend on the total duration of vibration exposure. If the characteristics of vibration do not change with time, the root-mean-square vibration provides a convenient measure of the average vibration magnitude. A stopwatch may then be sufficient to assess the exposure duration. The severity of the average magnitude and total duration can be assessed by reference to the standards in the following articles.

If the vibration characteristics vary, the measured average vibration will depend on the period over which it is measured. Furthermore, root-mean-square acceleration is believed to underestimate the severity of motions which contain shocks, or are otherwise highly intermittent.

Many occupational exposures are intermittent, vary in magnitude from moment to moment or contain occasional shocks. The severity of such complex motions can be accumulated in a manner which gives appropriate weight to, for example, short periods of high magnitude vibration and long periods of low magnitude vibration. Different methods of calculating doses are used (see “Whole-body vibration”; “Hand-transmitted vibration”; and “Motion sickness” in this chapter).

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## Contents

Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVIII. Guides

## Vibration References

Alexander, SJ, M Cotzin, JB Klee, and GR Wendt. 1947. Studies of motion sickness XVI: The effects upon sickness rates of waves and various frequencies but identical acceleration. J Exp Psy 37:440-447.

American Conference of Governmental Industrial Hygienists (ACGIH). 1992. Hand-arm (segmental) vibration. In Threshold Limit Values and Biological Exposures Indices for 1992-1993. Cincinnati, Ohio: ACGIH.

Bongers, PM and HC Boshuizen. 1990. Back Disorders and Whole-Body Vibration at Work. Thesis. Amsterdam: University of Amsterdam.

British Standards Institution (BSI). 1987a. Measurement and Evaluation of Human Exposure to Vibration Transmitted to the Hand. BS 6842. London: BSI.

—. 1987b. Measurement and Evaluation of Human Exposure to Whole-Body Mechanical Vibration and Repeated Shock. BS 6841. London: BSI.

Council of the European Communities (CEC). 1989. Council Directive of 14 June 1989 on the approximation of the laws of the Member States relating to machinery. Off J Eur Communities L 183:9-32.

Council of the European Union. 1994. Amended proposal for a Council Directive on the minimum health and safety requirements regarding the exposure of workers to the risks arising from physical agents. Off J Eur Communities C230 (19 August):3-29.

Dupuis, H and G Zerlett. 1986. The Effects of Whole-Body Vibration. Berlin: Springer-Verlag.

Griffin, MJ. 1990. Handbook of Human Vibration. London: Academic Press.

Hamilton, A. 1918. A Study of Spastic Anemia in the Hands of Stonecutters. Industrial Accidents and Hygiene Series no. 19. Bulletin No. 236. Washington, DC: Department of Labor Statistics.

Hasan, J. 1970. Biomedical aspects of low-frequency vibration. Work Environ Health 6(1):19-45.

International Organization for Standardization (ISO). 1974. Guide for the Evaluation of Human Exposure to Whole-Body Vibration. Geneva: ISO.

—. 1985. Evaluation of Human Exposure to Whole-Body Vibration. Part 1: General Requirements. ISO 2631/1. Geneva: ISO.

—. 1986. Mechanical Vibration-Guidelines for the Measurement and the Assessment of Human Exposure to Hand-Transmitted Vibration. ISO 5349. Geneva: ISO.

—. 1988. Hand-Held Portable Power Tools - Measurement of Vibrations at the Handle. Part 1: General. ISO 8662/1. Geneva: ISO.

ISSA International Section for Research. 1989. Vibration At Work. Paris: INRS.

Lawther, A and MJ Griffin. 1986. Prediction of the incidence of motion sickness from the magnitude, frequency and duration of vertical oscillation. J Acoust Soc Am 82:957-966.

McCauley, ME, JW Royal, CD Wilie, JF O’Hanlon, and RR Mackie. 1976. Motion Sickness Incidence: Exploratory Studies of Habituation Pitch and Roll, and the Refinement of a Mathematical Model. Technical Report No. 1732-2. Golets, Calif: Human Factors Research.

Rumjancev, GI. 1966. Gigiena truda v proizvodstve sbornogo shelezobetona [Occupational hygiene in the production of reinforced concrete]. Medicina (Moscow):1-128.

Schmidt, M. 1987. Die gemeinsame Einwirkung von Lärm und Ganzkörpervibration und deren Auswirkungen auf den Höverlust bei Agrotechnikern. Dissertation A. Halle, Germany: Landwirtschaftliche Fakultät der Martin-Luther-Universität.

Seidel, H. 1975. Systematische Darstellung physiologischer Reaktionen auf Ganzkörperschwingungen in vertikaler Richtung (Z-Achse) zur Ermittlung von biologischen Bewertungsparametern. Ergonom Berichte 15:18-39.

Seidel, H and R Heide. 1986. Long-term effects of whole-body vibration: A critical survey of the literature. Int Arch Occup Environ Health 58:1-26.

Seidel, H, R Blüthner, J Martin, G Menzel, R Panuska, and P Ullsperger. 1992. Effects of isolated and combined exposures to whole-body vibration and noise on auditory-event related brain potentials and psychophysical assessment. Eur J Appl Physiol Occup Phys 65:376-382.

Stockholm Workshop 86. 1987. Symptomatology and diagnostic methods in the hand-arm vibration syndrome. Scand J Work Environ Health 13:271-388.