Implementation of Noise Control Measure.pdf

7/29/2019 Implementation of Noise Control Measure.pdf

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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

Implementation of Noise Control Measure.pdf

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