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Vibration - Noise

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:
  1. Point of application of the vibration -
  2. Frequency of the vibration -
  3. Acceleration of the vibration -
  4. Duration of the vibration -
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<
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
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 SourcedB(A) value
  
Pneumatic Chisel Hammer115-120
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 loaded80


Human Noise Levels at 1 m (dB

Noise SourcedB(A) value<
  
Quiet Conversation60-65
Dictation65-70
Speaker at Conferences65-75
Delivery of a lecture70-80
Loud Shouting80-85


Desirable Noise Ranges in Open Plan Office

Noise measuredB(A) value
  
Equivalent Noise Leq54-59
Mean Noise level L5050-55
Peak Noise level L5060-65


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

ItemDamping in dB(A) value
  
Normal Single Doors21-29
Normal double Doors30-39
Heavy Special Doors40-46
Window - Single Glazed20-24
Window - Double Glazed24-28
Double Glazing with Felt Backing30-34
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)...


Links on Vibration Analysis/ Reduction
  1. Vibrotec ...Articles on Vibration Analysis
  2. Maintenance Resources ...Using Vibration Analysis for maintenance
  3. EAR_Online...E A R Speciality Composites- Including Useful Article
  4. Taylor Devices...Taylor Devices Free Techical Papers Literature on Vibration, Shock, Damping, Isolation..
  5. Euro Bearings...Product information on Vibrations Dampers..
  6. Acoustics for Project Engineers...Tony woolf Acoustics.Excellent guide

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Please Send Comments to Roy@roymech.co.uk

Last Updated 06/05/2007