Human Response to Vibration
In layman terms, vibration can be defined as the variation of an object about a fixed position. Of course, the real definition of the characteristics of vibration is a lot more complicated, as you will need to consider the change in direction, frequency and magnitude of the vibration.
If you measure vibration as purely a mechanical form, the frequency and magnitude of acceleration during harmonic oscillation about an equilibrium position will be the key focus. Normally, acceleration will be chosen as the measurement of amplitude because it is the simplest quantity for measurement, and velocity/displacement of the vibration can be inferred in accordance with the application of basic calculus.
Vibrational frequency is the number of times the equilibrium point is passed in a period of time. One period can be defined as the motion from equilibrium (or marker point) to maximum displacement to minimum displacement, and back to equilibrium (marker point) again. A typical example of simple harmonic motion is a sine curve (y = sin x).
Ever wonder how often are you subjected to vibration in a day? Once? Twice? Definitely not! It is most likely more than you think. Vibration happens almost everywhere – from car rides, train journeys, to even your walks. Not just that, for machines and equipment in industrial areas, if they are not monitored properly, they may cause high vibration which would cause physical discomfort, reduced performance and if more untreated, it may lead to serious illness.
Which also means, it is fine to be exposed to a reasonable level of vibration, but not exceeding the safe level. How do you know if it is safe? Unfortunately, without the help of technology, you would not realise that it is unsafe until you start to experience the effects. As such, control organisations like the ISO and European Council came out with documented guidelines and standards on the safe and maximum levels of vibration. Methods of quantifying vibration with respect to human response have also been fully defined, where the procedures may be applied to all types of vibration. These include multiple-axis and multiple-input motions which are steady-state, random or transient. The procedures are applicable based on experimental research to predict the level of discomfort, annoyance, potential health risks, interference with activities and motion sickness associated with vibration measured in a wide range of environments (Griffin, 1986).
Types of human vibration
Generally, human vibration commonly experienced by us can be classified into two: whole body vibration, and hand-arm vibration. What are the differences between these two?
One would experience whole body vibration when he sits in a heavy-duty vehicle (e.g., dumper trucks, earth-moving machinery) for higher levels of vibration, whereas at lesser extent, it can be felt on a normal car or riding on a bus. Having prolonged exposure to whole body vibration may affect the human body, especially lower back pain which will affect the progress of completing a task. A high magnitude of vibration may cause difficulties in doing precise works, from pressing a button to aiming a specific point. It may also affect vision, which will in turn become a huge hazard while driving on the road. Studies have shown that people are more sensitive to whole body vibration at certain frequencies in particular, which can be figured out by using psychophysical scaling techniques like the Wk weighting network from the ISO 2631 standard series.
Standards referring to Whole-Body Vibration:
- ISO 2631 Evaluation of human exposure to whole-body vibration- general requirements
- Part 1 General requirements
- Part 2 Vibration in buildings
- Part 4 Guidelines for the evaluation of vibration and rotational motion on passenger and crew comfort in fixed guide way transport system.
- EU Physical Agents Directive (Vibration) 2002/44/EC
Another type of human vibration is hand-arm vibration. Similarly, it is something that one may not be aware of even when he is being exposed to it. In a vehicle for example, the exposure will most likely be in the form of vibration that travels from the car steering wheel to the driver’s hands. Hand-arm vibration will be labelled as a problem only when one is exposed to high levels of the vibration for a long period, for instance when workers operate hand-held power tools. A quick tip of main grip methods supported by the current ISO standards are the “Handgrip” and “Flat palm”, whereby these methods are set as being properly indicative of vibration levels.
Standards referring to Hand-Arm Vibration:
- ISO 5349-1 Measurement and evaluation of human exposure to hand-transmitted vibration – general requirements
- ISO 5349-2 Measurement and evaluation of human exposure to hand-transmitted vibration – practical guidance for measurement at the workspace
- EU Physical Agents Directive (Vibration) 2002/44/EC
Hand-Arm Vibration Syndrome
As mentioned earlier, there may be some unwanted effects when you experience a prolonged hand-arm vibration exposure. The main effect that is associated with this would be the Hand-Arm Vibration Syndrome (HAVS). In general, though not all, most of the HAVS victims are workers, due to the high exposure of vibration at work. Workers who use hand-held/hand-guided power tools like concrete breakers, hammer drills, chainsaws etc. are the ones who are highly at risk of HAVS (HSE, 2021).
So, what are the symptoms of HAVS?
- Buzzing – a feeling of vibration within the exposed extremities after using vibratory equipment. At a more serious stage, it may gradually progress until the point where the buzzing sensation can be felt even after a long time from using the equipment
- Raynauds phenomenon – also known as finger blanching, this is usually triggered by exposure to cold. The skin on the fingers will turn pale, and sometimes accompanied by numbness
- Discolouring of fingers – when the fingertips start to appear as a dusky cyanotic colour, it indicates extremely reduced blood flow to the fingers
- Necrosis of the skin – This stage is similar to gangrene, which is the starve of blood, where the flesh in fingers start to die and may begin to fall off
Studies and educational research have been done sufficiently such that the methods to reduce risk of exposure have been known. Therefore, these extreme symptoms’ occurrence has been greatly minimised thanks to legislations that emphasise on employee’s occupational health and safety.
To further lower the risk of facing HAVS, employers need to play their role by obeying the guidelines as stated by the law. Safety representatives or other employee representatives can be very useful in communicating problems, inspecting documents and consulting employers over measures to meet these regulations. Of course, one can also seek advice from an occupational health professional in the medical field.
How to measure vibration?
The methods depend on what type of application needs to be evaluated, whether it is for human response, building, machine etc., and which standard is being employed. In most cases, vibration is measured with an accelerometer, where more specific applications may require a Geophone that measures velocity instead.
There are also vibration meters and software for vibration report and analysis available in the market where you can use them in accordance with standards like ISO 2631-1 for whole body vibration and ISO 5329-1 for hand arm vibration.
AS, N. (n.d.). Human Vibration. Retrieved from Norsonic AS: https://web2.norsonic.com/application/vibration/
Griffin, M. J. (1986). Evaluation of Vibration with Respect to Human Response. SAE Transactions, 95, 323-346. Retrieved September 13, 2021, from www.jstor.org/stable/44721661
HSE. (2021, May 25). Worried about your hands? Retrieved from Health and Safety Executive: https://www.hse.gov.uk/vibration/hav/yourhands.htm
Written by Khei Yinn Seow
Posted on September 24, 2021