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Vibration
Vibrations are mechanical oscillations, produced by regular or irregular period movements
of a member or body about its rest position..
Vibration can be a source problems at an engineering level because they can result in damage
to equipment, loss of control of equipment, and reduction in the efficiency of operation of machines..
Vibration effects on people
Vibration can also be at a low level a source of discomfort and at a high level can
be a safety risk to personnel affected
In the context of the effect of vibration on people important factors include:
- Point of application of the vibration -
- Frequency of the vibration -
- Acceleration of the vibration -
- Duration of the vibration -
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Vibration is most normally a problem experienced in driving vehicles and in operating tools.
Vibration can affect visual perception, muscles, concentration, circulation and the respiratory system and
at certain levels can even result in physical harm to the body.
The effect of vibration on the human body is related to the natural frequency of parts of
the human body affected. Frequencies of above 30 Hz are heavily damped
by the human tissue.
The body of a sitting person responds to vertical vibration as follows:
- 3- 4 Hz ....Strong resonance in the cervical vertebrae
- 4Hz.........Peak of resonance in the lumber vertebrae
- 5 Hz........High resonance in the shoulder girdle
- 20-30 Hz....High resonance between the head and shoulders
- 60-90 Hz....Resonance in the eyeballs
- 100-200 Hz..Resonance in the lower jaw
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Higher frequency vibrating machines would have a significant effect on the body components
but this is largely mitigated by the damping effects of the tissue which tends to confine
the vibration to the point of application e.g the hands
Control Of Vibrations
The primary method of eliminating vibration is at source by design the equipment and ensuring
control over the manufacturing tolerances. Other methods of reducing the vibrations generated
by machinery are listed below
- Modify the system such that the natural frequencies are not close to the operating speed or any generated
frequencies.
- Prevent large responses by including damping
- Install vibration isolating devices between adjacent sub-systems
- Install equipment on dense heavy rigid vibration absorbent floors
- Include auxiliary mass into the equipment to reduce the response and absorb vibration
- Base design on engineering materials which have high inherent damping, such as cast iron and laminated sandwich materials
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There are a number of proprietary machine resilient support pads which provide engineering
isolation of vibration from the foundations.. Ref. Links below
Noise
Noise is primarily annoying or harmful sound....
Information on sound is found on this site... Waves - sound
Excessive or disruptive noise reduces efficiency and can damage health.
The best method of controlling noise is to eliminate or reduce the noise. However it has been proved
that a certain level of background noise is desirable and certain sound sources improve
performance and efficiency.. Music etc..
In surveys it has been proved that on many occasions it is not the level of noise which
is the problem it is the source of the sound and its frequency. Other peoples conversation,
persistent click or the sounds of insects can be very disturbing when people need to concentrate.
Peak noise levels dB(A) in Industry
| Noise Source | dB(A) value |
| | |
| Pneumatic Chisel Hammer | 115-120 |
Rocking Sieve, Chain Saw, Compressed Air Riveter
electric cutter | 105-115 |
Milling /Weaving machine,Crosscut saw,
Stamping Machine | 100-105 |
Electric Motor, Rotary Press, Wire Drawing Machine
Sawmill, Bottle filling Machine | 90-95 |
| Toolmaking machine -lightly loaded | 80 |
Human Noise Levels at 1 m (dB
| Noise Source | dB(A) value< |
| | |
| Quiet Conversation | 60-65 |
| Dictation | 65-70 |
| Speaker at Conferences | 65-75 |
| Delivery of a lecture | 70-80 |
| Loud Shouting | 80-85 |
Desirable Noise Ranges in Open Plan Office
| Noise measure | dB(A) value |
| | |
| Equivalent Noise Leq | 54-59 |
| Mean Noise level L50 | 50-55 |
| Peak Noise level L50 | 60-65 |
The noise level Leq relates to the average noise levels
The noise level L50 means that the indicated noise level may be exceeded for 50% of the
time.
The noise level L1 means that the indicated noise level may be exceeded for 1% of the
time.
Sound Deadening Effects of building items
| Item | Damping in dB(A) value |
| | |
| Normal Single Doors | 21-29 |
| Normal double Doors | 30-39 |
| Heavy Special Doors | 40-46 |
| Window - Single Glazed | 20-24 |
| Window - Double Glazed | 24-28 |
| Double Glazing with Felt Backing | 30-34 |
Dividing Wall
6-12cm brick | 37-42 |
Dividing Wall
25-38cm brick | 50-55 |
Double Wall
2 x 12 cm brick | 60-65 |
Noise Protection
Ear plugs properly used -conical plugs of synthetic material - can reduce noise levels
by up to 30 dB..
Protective Ear Caps (ear muffs)- can reduce noise levels
by 40-50dB..
It is suggested that ear plugs be used for noise levels of 85-100 dB(A) and ear muffs be used
for noise levels above 100 dB(A).. .
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