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.
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):
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.
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.
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.
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).