Measurement Of Noise and Its Reduction Techniques

8/3/2019 Measurement Of Noise and Its Reduction Techniques

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Dawood College of Engineering andTechnology, Karachi

Department: Industrial Engineering and Management

MUHAMMAD EHSAN(08-Dcet-314)

Human Factors Engineering6th Semester 3rd year

Submission Date: 20/10/2010Assignment No # 2

ASSIGNMENT: MEASUREMENT OF NOISE AND ITS REDUCTION


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Measurement Of Noise and Its Reduction

M. Ehsan (08-Dcet-314) Page 2

Measurement Of Noise:

Measurement of noise is the most important part of a work place hearing conservationand noise control program. It helps identify work locations where there are noise problems, employees who may be affected, and where additional noise measurements

need to be made.

The most common instruments used for measuring noise are:

a) Sound Level Meter

b) Integrating Sound Level Meterc) Noise Dosimeter

a) Sound Level Meter:A Sound Level Meter is an instrument designed to respond to sound in

approximately the same way as the human ear and to give objective, reproduciblemeasurement of sound pressure level. This is the basic measuring instrument for noise. Itconsist of a microphone that converts sound pressure variations into electrical signals, a

frequency selective amplifier, a level range control, frequency weighting to shape thefrequency response of the instrument, and an indicator as shown in the figure.


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b) Integrating Sound Level Meter:Typical applications for Integrating Sound Level Meters (ISLM) are identical to

those for standard sound level meters. Integrating sound level meters can however beused to measure the average sound pressure level around noisy equipment or other sound

sources where the integrating capacity can be used to determine the average sound levelin space as well as time.

Two main differences between Sound Level Meter and Integrating Sound Level Meterare:

i. Average duration for an integrating meter is usually much longer than those for a

standard sound level meter, extending to minutes, or hours.

And

ii. The integrating meter gives equal emphasis to all sounds that occurs during the

selected averaging period, while the standard sound level meter gives moreemphasis to recently occurring sounds.

c) Noise Dosimeter:A Noise Dosimeter is a sound level meter worn by the worker. It measure and

stores sound levels during an exposure period and computer the exposure as a percentage

of a criterion level such as an occupational exposure limit. The noise must be measuredusing as a weighted filter. In noise dosimeter, the microphone is attached to the worker,whose noise exposure is being measured, placement of microphone is important in

estimating exposure. The microphone is usually mounted in the ear, at the chest, or onshoulder.


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When noise levels are continuous and the worker remains essentially in one workarea during the work shift, measuring noise exposure with a sound level meter is

relatively straightforward. However, a noise dosimeter is preferred for measuring workerexposure when noise levels vary or are intermittent, when they contain components of

impulse noise or when the worker frequently moves around during the work shift.

Equipment used in the measurement of noise levels

S.No # Equipment # Specification/Area of usage #

1. Sound level meter

Type-0 : Laboratory reference standard

Type-1: Lab use and field use in specified controlled environmentType-2: General field use (Commonly used)Type-3: Noise survey

2. Noise dosimetersUsed to find out the noise levels in a working environment.Attached to the worker

3. Frequency analyzers For detailed design and engineering purpose using a set of filters.

4. Graphic recordersAttached to sound level meter. Plots the SPL as a functionof time on a moving paper chart.

5. Impulse metersFor measurement of impulse noise levels e.g. hammer

blows, punch press strokes etc

6. Calibrators For checking the accuracy of sound level meters.

.


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Reduction Of Noise/ Noise Control:

It is illegal to expose anyone to a sound level above the prescribed limit. These limits areprescribed in the Regulation of the Occupational Health, Safety and Welfare Act, 1986.The first step in controlling workplace noise is a noise assessment survey by the qualifiedpeople. This provides a detailed Noise Map of areas and an individual in the workplace

exposed to excessive noise and identifies noisy equipments. If a noise exposureassessment confirms that workers are exposed to noise in excess of the occupational

exposure limits, than the employer must develop and implement a noise control program.

The program must include the following components:

Conduct a Risk Assessment

Posting Warning Signs Take Steps to Prevent/ Control Risks

Audiometric Testing

Conduct a Risk Assessment:

Before any preventive measures are taken, it is essential to assess the risks to

workers from noise, including carrying out measurement if necessary. The method of riskassessment used will depend upon the prevailing conditions in the workplace (e.g, type,

duration, and level of exposure), and may include sampling of the personal exposure ofworkers.

o Identify the different noise-related risks organization, for example: Are some staff

more exposed to loud noises than other, with the potential risk of noise induced hearingloss? Or perhaps the nature of the noise is contributing to high stress levels with in the

organization?o Assess how the workers are affected, including temporary and part-time staff.

o Evaluate measures that are already in place to control noise level. Howsatisfactory are these?

Posting Warning Signs:

Warning signs must be posted at the periphery (outer boundary) of any work-area

where the noise level exceeds 85 dBA. The sign should include a statement that hearingprotectors must be worn while in the area. A supply of several types of hearing protectorsshould be readily accessible to those entering the area.


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Take Steps to Prevent/ Control Risks:

A noise problem can be controlled by attacking the noise at the source, along itspath from the source to the receiver, and at the receiver. Often a combination of noise

control techniques is required to achieve the desired level of abatement (interruption inthe intensity).

A straightforward approach to solve a noise problem is to examine the problem in termsof its three basic elements: the source, the conveying medium and the receiver (seeFigure). Unless a free field is present, both direct and reflected (reverberant) sound waves

that reach the receiver must be carefully looked into. The most appropriate solution to agiven noise problem requires alternation or modification of any, if or all of the three basic

elements.

Techniques/Steps taken to control the noises are:

i. Control at the Source or Reduction at the Source (To modify the source to reduceits noise output),

ii. Control along the Path or Blocking the Noise Transmission Path (To alter orcontrol the transmission path and the environment so as to reduce noise level

reaching the recipient),

And

iii. Control at the Receiver or Controlling Noise at Receptor (To provide the receiverwith personal protective device)

i. Control at the Source or Reduction at the Source:A high priority is given to the control of noise at the source, because it is much

more valuable to eliminate or reduce noise even before it is emitted from a source.

o Noise is mainly caused by vibration and therefore can be reduced by

decreasing by either the amount of vibration or the surface area of the vibrating parts. Vibration can be reduced by proper design, maintenance, lubrication, andalignment of equipment. Isolating vibrating parts from other machine parts or


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structures by use of resilient materials such as rubber or elastomers reduces thenumber, and hence the surface area of vibrating sources.

o Adding damping materials to machine parts to increase their stiffness or

mass can reduce the amplitude of vibration as well.

o Often, a potential noise problem can be adverted by selecting quieter

equipment initially. It is often more economical to pay extra for quieter equipmentthan to purchase noisier equipment that will require additional expenditure for

noise control.

o Low-frequency noise is less annoying and is tolerated better than high-frequency noise. Therefore, where possible, equipment that generates low-frequency noise should be selected over equipment that generates high-frequency

noise. For example, use of a large, low-speed blower would be preferred over asmaller, high-speed blower.

ii. Control along the Path or Blocking the Noise Transmission Path:When source control measures by themselves are not sufficient to avoid noise

impact, then path control measures are designed and implemented. This can be done inseveral ways:

o Fit sound-absorbent materials to ceiling and walls, (to absorb the sound along thepath).

o Deflect the sound in some other directions by placing a reflecting barrier in its path.

o Contain the sound by placing the source inside a sound-insulating box or enclosure.

o If a barrier is interposed between a sound source and a receiver, part of the soundwill be:

Reflected back from the barrier.

Transmitted through the barrier.

Diffracted around the barrier.


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iii. Control at the Receiver or Controlling Noise at Receptor:However there are numerous circumstances where source and path noise control

are not feasible or sufficient. In these cases, receptor control measures are necessarywhich includes:

o Window treatment: reinforcing the buildings noise reduction ability, because

window openings are typically a buildings weakest link for noise infiltration,acoustic window treatments can significantly reduce the outside-to-insidecontribution.

o Community participation: open dialog to involve affected residents.

o Noise Complaint process: ability to log and respond to noise complaints.

The following two techniques are commonly employed:

1. Alter Work Schedule:In order to limit the amount of continuous exposure to high noise levels, it is

preferable to schedule an intensive noisy operation for a short' interval of time each dayover a period of several days rather than a continuous 8-hour run for a day or two.

Inherently noisy operations, such as street repair, factory operation and aircraft trafficshould be curtailed at night or early in morning.

2. Hearing Protection Devices:Hearing protectors are generally defined as anything worn to reduce the level or

volume of sound entering the ear. Hearing protectors are subjected to many problems andshould be considered as the last resort against hazardous noise situations. Hearing

protectors can fail to provide adequate protection in many situations due to discomfort,incorrect use with other safety equipment, dislodgement, deterioration, etc.

There are two basic types of hearing protection devices:

a) Earplugs:Earplugs are inserted into ear canal. These are made of soft plastic or silicone

rubber. These plugs, when properly used, provide a high degree of sound attenuation. Theperformance of these plugs depends dynamically on the fit, maintenance, and installation.


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b) Earmuffs:Earmuffs cover the outer ear as an acoustic shield or barrier. These are basically

hard plastic cups. A cushion seal to the head is maintained by a spring-loaded headband.The effectiveness of earmuffs depends mainly on the cushion-to-head spring. The

headband must exert sufficient uniform pressure to conform the cushion to the shape ofthe head.

Earplugs, Earmuffs and other ear protectors are commercially available. They mayprovide noise reductions ranging from 15 to 35 dB. It should be aware that protective ear

devices do interfere with speech communication and can be hazardous when warningcalls from a routine part of the operation.

Audiometric Testing:

Workers exposed to noise levels exceeding the occupational exposure limits must

undergo audiometric testing. The purpose of testing is to establish a baselinemeasurement of the workers hearing and to then monitor the workers hearing at regular

intervals to detect changes in hearing ability. Audiometric testing is used to identify anddiagnose hearing loss.

(An Audiometer is used to determine the hearing loss in an individual)

Acoustic Materials:Acoustic insulation materials work by two process:

Absorption of sound energy, (which dissipates

sound as heat energy),

And

Reflection, (which reflects noise away from alocation where quieting is desired).

A single composite insulation material will beeffective as both an absorber and a reflector.

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Measurement Of Noise and Its Reduction Techniques