FREQUENCY RESPONSE OF THE HUMAN BODY
To understand the reason why humans are more sensitive to some frequencies than others is useful to consider the human body as a mechanical system. The system is complicated by the fact that: (a) each body part has your greater sensitivity in different frequency ranges; (b) the human body is not symmetrical, e (c) There are no two people respond to vibrations in exactly the same way. Anyway, biomechanical models were developed suitable to simulate human response to vibration.
This figure shows a mechanical model of the body greatly simplified, where each section is represented by a mass, a spring and a damper. The human body is a heavily damped system and so, When a part is excited to your natural frequency, will vibrate at a frequency range rather than a single. The human body is not symmetrical and, therefore, to your response to vibrations also depends on the direction in which vibrations are applied.
Is known the dependency, the sensitivity of the human body to mechanical vibration, the frequency and direction of excitation, as already mentioned above. These factors need to be taken into consideration when assessing the harmful effects of vibration. ISO standards offer several frequency weighting curves, that can be used to take into account the factors referred to above, When you evaluate the negative effects of the vibrations.
When the vibration is measured in a given direction the vibration level is measured in all frequencies in human sensitivity range. At frequencies that the human body is more sensitive, is given more weight than those to which the human body is less sensitive. These considerations provide a good correlation between vibration levels measured and subjective perception or impact produced by vibration. Noise levels are measured in the same way – uses a weighting filter to simulate the response of the human ear.
The vibrations that occur in the range of 1 Hz- 80 Hz are of particular interest when measuring the exposure of the whole body, and the 5 Hz- 1500 Hz to the hand-arm set.
FULL BODY WEIGHTS AND REFERENCE AXES
Whole body vibrations should be measured in the directions of the coordinates of an orthogonal system having the source in your heart. The longitudinal direction (head- ankles) is the Z axis. This way the body is more sensitive to vibrations in the range of 4 Hz-8 Hz. Human response in X-direction (front-back) and in the Y direction (left-right) not differ and the sensitivity is higher in the range 1 Hz-2 Hz.
The vibrations in the range 0,1 Hz- 0,63 Hz are considered responsible for causing discomfort and nausea to the movement, in people exposed to them. Human reactions to vibrations in this range vary greatly and depend not only on the vibration (movement), but factors such as the vision, odors and age, What makes the study of the effects of this type of vibration is particularly complicated.
Axis of the human body according to ISO 2631-1
Frequency weighting curves to the main weights according to the ISO 2631-1
|axis –z, seat surface|
|axis –x, seat surface
axis –y, seat surface
HAND-ARM WEIGHT AND AXES OF REFERENCE
For hand arm system, the frequency response is the same in all directions. So is only required a frequency weighting curve for the three orthogonal directions. Has a maximum sensitivity in the range of 12 Hz-16 Hz , as seen in the following weighting curve.
Are however used two systems of orthogonal directions to describe the direction of vibration. The basicêntrico system is defined in relation to the tool, While biodynamic system is defined in relation to the hand (ISO 5349-1 and ISO 5349-2).
Seen only exist a curve of frequency weighting to the vibrations of the hand-arm set, You can use any of the benchmarks.
Below you can see a presentation on this topic.