Specification of Industrial Noise & Vibration Systems:
Specification of Industrial Noise: The effect of Noise on men; measurement and evaluation of noise, vibration damping, noise isolation, noise absorption – silencers, practical aspects of control of noise- case studies on impact of noise from Compressors and Generators.
Effect, Measurement and control. Noise is a sound which is unwanted, unagreeable and annoying interfering with working efficiency at times causing accidents and may at a certain level cause damage to hearing. Sound is produced by the vibration of material objects in air which in turn vibrates in sympathy causing the formation of pressure waves. These sound waves are transmitted through the air at a finite speed (1122 m/ sec or 342 meters/sec.)
Sound waves have two characteristics: the amplitude of vibration and the frequency as shown below:
Amplitude of a sound wave determines loudness for the average human being the audible range of pressure amplitude is from the threshold of hearing at 20 u pa to 200 pa (Pascal’ s) where the intensity noise is proportional to the pressure become painful. This is a ratio of 1 to 106. The pressure squared. Hence the range of intensity covers a ratio of 1 to 1012. With such a range it becomes more convenient to express the intensity of pressure amplitude on. a logarithmic base. The intensity level is proportional to the square of the pressure. Thus the sound pressure level (Lp) can be defined as
Lp = 1010910 (p1/p0)2
Where Lp = Sound Pressure Level in Decibels (dB)
P1 = Pressure amplitude of sound
Po = Reference pressure 20 u Pa
Specification of Industrial Noise:
It may be noted here that for all practical purposes doubling the sound intensity increases the sound pressure level by 3 dB. For all practical purposes doubling the sound intensify increases the sound pressure level by 3dB.
For example,90 dB + 90 dB = 93.1 dB
Similarly, 103 dB + 90 dB = 103.2 dB .
This shows that the effect of a small source of noise when added to a large source the Resultant noise level would be relatively unchanged. Of course the converse is true and this “is important when considering the control of noise form number of different sources, since treatment of minor source may-not result-in any change in overall levels.
Sound by environment factors in dB
|SOUND PRESSURE LEVEL IN dB||ENVIRONMENT|
|140||Threshold of pain, Loud speaker (near) Crackers, Riveting. No. of groups at a time.|
|130||Pneumatic Drill. Chipping Hammer|
|120||Loud motor car horn (dist. Im) Diesel Loco Hern|
|110||Rush Hour traffic freely flowing. Grinding noise.|
|100||Inside Metro “Machine Shop noise.|
|90||Inside Bus Normal Workshop noise|
|80||Average traffic on Street|
|70||Conversational Speech Meeting|
|60||Typical business office|
|50||Living Room (City I/TV)|
|20||Bedroom at night|
Specification of Industrial Noise
The rate at which the pressure fluctuations take place is called frequency and is measured in hertz (Hz) or Cycles/Sec. The human ear is capable of hearing over a range from 20 Hz to 16000)tz (16 KHz) although this range. Can be considerably reduced at the high frequency end for older people and for those buffering from hearing impairment.
The human ear does not respond equally to all frequencies. To obtain the same subjective loudness at low frequencies as at higher frequencies requires a larger physical amplitude (greater L) since the ear is less sensitive ‘at low frequencies. Curves can be drawn at equal loudness defined in dB phony. The difference between physical amplitudes at different frequencies required to produce the same loudness curve reduces as the loudness increases. As an approximation an Increase in sound pressure level of 8 -10DB corresponds to a subjective doubling of loudness. It is apparent that the subjective response to noise is extremely complex arid these complexities should always be borne in mind when dealing with individual people. A noise or noise level which is acceptable to one individual may not be to another.
The methods of assessing sound and developing noise criteria are complex. The physical characteristics of sound over the frequency range on a meter can measured; and then corrected from the ear. The unit of measurement used is the decibel (dB). A Weighted Curve. This curve gives the most widely used unit (dBA) for quantifying a noise level. The other curves B, C and D are used for other work.
Specification of Industrial Noise
Sound pressure level
The sound pressure level (LP) is always defined at a specific point in relation to the noise source. It is dependent on the location relative to the source arid the environment around source and the receiver.
Equivalent noise level
The noise level in the community of inside a factory will vary with time. The equivalent noise level (Leg) is defined as the notional steady noise level which over a given period of time would deliver the same amount of sound energy as the fluctuating level.
|NOISE LEVEL dBA||EXPOSURE TIME(h)|