implementation of noise control measure.pdf
TRANSCRIPT
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IMPLEMENTATION
OF
NOISE CONTROL MEASURES
Dr Chew Chye Heng
Associate ProfessorDepartment of Mechanical
Engineering, NUS
Hierarchy of control
1) Elimination
2) Substitution
eg. replace noisy process/machinery with
quieter process/machinery, etc
3) Engineering Control.
eg. enclosures, modification, etc.
4) Administrative Control.eg. production schedule to limit daily exposure,rotation of workers, etc
5) Personal Protective Equipment
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Substitution
metal gearwith plastic gear
roller conveyor with belt conveyor
pneumatic tools with hydraulic tools
single nozzle with dual nozzle
gear drives with belt drives
Noise control at source
Two aspects of noise sources must beexamined.
Type and manner of noise emission
Whether it is practical to reduce the noiseoutput at source may reduce the
effectiveness of source, make access
difficult or cause the source to get too hot .
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Modification of Radiating Surface
Replace solid metalcover with perforated
sheet metal and
wire mesh.
Forward movement
Positive pressureNegative pressure
Positive pressure flows to
negative pressure rather
than emits as sound.
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Elimination of Wind Tones
Symmetrical objects generate pure tones.To eliminate pure tone, object should be long
In direction of air flow, or irregular shape
Elimination of pure tone by s strip of
metal round chimney with varying pitch.
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Directivity of sound source
Siren mounted on top
of moving crane.
Siren to warn workers
that crane is moving.
Siren noise a nuisance
to nearby residents.
Original Proposed
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Types of Silencer
Attenuator / Silencer
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Enclosure test chamber
Chamber to calibrate noise sensitivity of test pieces.
Original Modified
Double diffraction
Enclosure Effect of Absorption
272320171310Effective TL, dB
0.50.20.10.50.20.1Average sound
absorption
3020TL of hood
material, dB
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Absorbing MaterialsAbsorption Coeff icient - e.g. fibrous glass, polyurethane, rock wool
to absorb the incident sound energy
ranges from 0 to 1 is higher at higher frequency high frequency noise can be
absorbed more readily thanlow frequency noise
Barrier or Shieldmax noise reduct ion (NR) = 15 to 20 dBA
length > 4 x height
TL > 5 dB + NR
receiver
source
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Sound barrier is less effective in
an indoor environment
Room Treatment
noise reduction often < 7 dBA
only reflected noise is reduced, notdirect noise
good for room with many noisymachines
at best is like removing the walls noise reduction is at some distance
from the machine
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Room Treatment
Adding sound absorbing materials to the innersurfaces will reduce the reflected noise
Hard interior surfaces Interior surfaces lined
with absorbing materials
Room Treatment - example
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Generator set
Turning Plate
Preventing short circuit
Barrier
to rain
Enclosure
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Strategy of Control
1st control the highest dBA source
Next control the next highest dBAsource,etc
Consider the surrounding noisesources
Consider the background dBA
Procedures of Control
1 Identification of sources
2 Ranking of sources
3 Establish design criteria
4 Determine the NR required
5 Selection of control options
6 Installation / fabrication