PREVENTION AND CONTROL OF NOISE HAZARDS

The source, transmission path and the receiver constitute the chain of sound transmission. Hence for any noise control strategy, the following three form the fundamental element;

Reduction of Noise at Source;

Reduction of noise by change in path; and

      Reduction of Noise at Receiver`s end.        

  

Reduction of Noise at Source: There are at least three areas in which control of noise generated by a source may be initiated. They are proper design, proper equipment operation (by changing method of operation) and equipment maintenance. The change in design shall be aimed at;

• Decreasing energy for driving vibrating system;
• Changing coupling between this energy and acoustic radiating system;
• Changing structure so that less sound is radiated.

Reduction of Noise by change in Path: In addition to source control, it is possible to obtain significant noise reduction at the receiver by controlling the noise along the path of transmission. This can be achieved by;

• Increasing the distance between source and receiver.
• Acoustic treatment of ceiling, walls, floor to absorb sound and reverberation.

Reduction of Noise at Receiver: There is little opportunity for noise control at the receiver. Normally, the permissible noise levels are set for the receiver, and engineering techniques must be used on the source and or path in order to limit the exposure of the receiver. Some techniques that can be adopted are as follows;

• Enclosing/isolating the workers;
• Rotational of Personnel to reduce exposure time;
• Changing job schedule of the affected employee.

  

ENGINEERING CONTROL MEASURES: The following are the examples of applying engineering principles to reduce noise level;

      1. Maintenance:

• Replacement or adjustment or worn and loose or unbalanced parts of machine.
• Lubrication of machine parts and or used of cutting oil;
• Properly shaped and sharpened cutting tools. 

      2. Substitution of Machines:

•  Larger, slower machines for smaller and faster ones.
•  Step dies for single operation dies;
• Rotating shears for square shears;
• Hydraulic for mechanical process;
• Belt drivers for gears.

3.     Substitution of Process:

• Compression for impact riveting;
• Welding for riveting;
• Processing for rolling and forging.

4.     Driving force of vibrating surfaces may be reduced by;

• Reducing the force;
• Minimizing rotation speed;
• Isolating.

5.     The response of vibrating surface may be reduced by;

• Damping;
• Additional supports;
• Increasing the stiffness of the material;
• Increasing mass of vibrating members.

6.     The sound radiation from vibrating surface can be reduced by;

• Reducing the radiating area;
• Reducing overall size;
• Perforating surfaces.

7.     Reduce sound transmission through solids by using;

• Flexible mountings; flexible sections in pipe runs;
• Flexible shaft coupling;
• Fabric sections in ducts;
• Resilient flooring.

8.     Reducing sound produced by gas flow;

• Intake and exhaust mufflers;
• Fan blades designed to reduce turbulence;
• Large low speed fan for smaller high speed fans;
• Reduced velocity of fluid flow;
• Increased cross-sections of streams;
• Reduced pressure and turbulence.

9.     Reducing noise by reducing its transmission through air;

• Use of sound absorbing materials on walls and ceilings in work-area.
•  Use of sound absorption material along the transmission path; complete enclosure of individual machine.
• Use of baffles;
• Confining high noise machine to insulated room.

      10.  Isolating operator by proving a relative sound proof both for the operator or attendant.

MEDICAL CONTROL MEASURES:

Industrial Audiometry:  In an Industrial audiometry, the testing of workers’ hearing acuity, may well identify hearing disabilities. Hearing is regularly tested, preferably from the time the worker joins the workplace, to observe if there is any discernible deterioration in hearing above that which is expected from loss due to ageing (Presbycusis).

A health practitioner trained in conducting hearing test, will test the worker`s baseline auditory threshold for both ears. The frequencies used for both reference and monitoring audiometry are 500 Hz, 1000 Hz, 1500 Hz, 2000 Hz, 3000 Hz, 4000 Hz, 6000 Hz and 8000 Hz.

Reference audiometry must be conducted as soon as possible after commence of employment, but should ideally be conducted before exposure to a noisy workplace occurs.

Monitoring audiometry should be performed within 3 months and then 12 months of the initial work exposure for comparison with the results of the reference audiometry.

If there is no significant change in the threshold shift or in the work situation, it may then be sufficient to retest at yearly intervals or as prescribed by OHS legislations. For workers exposed to high exposure levels, >100dB (A), more frequent audiometric testing may be required.

• Where significant hearing impairment is detected at the initial hearing audiometric test, the person should undergo a medical examination if a repeat test conducted on another day, confirms the original findings.
• Audiometric testing can be of benefit to both employers and workers in excessively noisy industries (foundries, canneries, metal industries, air transportation), but only if it is an integral part of a rigorous hearing conservation program.
• Audiometric testing in isolation from other elements of a hearing conservation program only serves to record the deterioration in hearing.
• In combination with other elements it can detect the early onset of NIHL and enable counter measures to be put in place.

 Hearing Conservation Program

Hearing conservation programs are designed to prevent long-term hearing impairment, principally by maintaining noise exposure within the limits required by the legislation.

Any of the technical control processes (Engineering Control Measures) discussed above might be used in such program. Certainly, the best are noise reduction program.

However, if noise sources cannot be quieted sufficiently, the workers must be provided with hearing protective devices.

Management`s commitment to training and proper education of the workforce in the purpose, use and benefits of protective devices must be matched by the rigid enforcement of such program.

The hearing conservation program aims;

• To prevent Noise Induced Hearing Loss (NIHL)
• To reduce cost of claims for workmen compensation.

A well-organized successful hearing conservation program shall have coordination and integration of the following three phases;

• Physical evaluation the noise exposure.
• Medical evaluation of the hearing of personnel exposed to noise; and
• Control of the noise exposure.

Team approach for Hearing Conservation Program

Function	Members involved
Identification of noise source	Industrial hygienist, supervisors, workers.
Reduction of noise exposure	Design/Plant Engineer, Industrial hygienist, supervisor, workers, audiologist.
The hearing measurement	Audiologist, ontologies, workers

           Note:  

       1. The medical personnel shall be the team leader.

       2. Equipment requirement includes audiometer / Evoke system; Noise level meter / sound level meter; Noise dosimeter.

STUDY OF SOME NOISE CONTROL MEASURES

Acoustic Enclosures for Diesel Generator: Lot of engineering techniques has been developed to bring down the noise level of the diesel generator set. One of such is the “Acoustic Enclosure” for the set. In some models, the sets are completed enclosed in an acoustic enclosure (Shown in figure). Whereas in other case, the walls and floors of the DG room are lined with noise absorbing materials, thereby bringing down the noise level.

Personal Protective Equipment:

Where it has been confirmed that the workplace noise levels are in excess of any regulatory noise limit and other noise reduction programs such as those above cannot be fully implemented, personal protective equipment must be used until such time that the noise exposure has been reduced to below the regulatory limits. Hence, PPE are considered as a last line of defense as it cannot eliminate the hazard but can only limit the effects of hazards.

Hearing protection should be used compulsorily if the noise or sound level at workplace exceeds 85 decibels (A)though the allowable limit is 90 dB(A).

Various PPE are available with varying degree of Noise Reduction Rating (NRR).

[NRR is a standard measure of sound blocking capabilities of any hearing protective device.]

There are basically four types of hearing protectors available,

• Acoustic Helmet;
• Ear Canal Caps;
• Ear Plugs; and
• Ear Muffs.

Of the above four, only two (ear plugs and ear muffs) are in widespread use.

Ear Plugs: Ear plugs are inserted to block the ear canal. They may be performed or moldable (foam ear plugs). Ear plugs are available both as disposable products or reusable plugs.

Ear plugs are soft, comfortable and provide un-obstructive hearing protection. They are recommended for high noise situations (up to 100 dBA TWA). They are most suited when exposed to continuous exposure. Other advantages of ear plugs are:

• Can be combined with ear muffs for additional protection.
• Can be used with other Personal Protective Equipment.
• Comfortable for a wide range of ear sizes.

Ear Muffs: Ear muffs consist of sound-attenuating material and soft ear cushions that fit around the ear and hard outer cups. They are held together by a head band.  Ear muffs are ideal for user who needs intermittent hearing protection against noise. Ear muffs are recommended for situations where people move in and out of noise often during the work day.

Combined usage: The combined usage of ear plugs with ear muffs is recommended for sound level more than 105 dBA. The dual protection however only adds 5 dB to the noise reduction provided by the more protective device of the two.

INDUSTRIAL NOISE REDUCTION PLAN- RECOMMENDATIONS

The industrial noise can be controlled at three different levels i.e. at source, along the transmitting path or at the receiver`s end, as discussed earlier. Here are the recommendations in a logical series of actions, which if followed, should lead to effective noise control.

1. Identify the noise problem areas in the plant. This can be done by conducting health surveys to measure the sound level at different areas of plant.
2. Establish the noise exposure of the employees in the potential problem areas as identified above. This can be measure by timing the exposure at each noise level for the employee.
3. Get to know the problem. Analyze the data and information collected above and identify the major noise frequencies. This in turn will help to identify the source or sources generating the major portions of the noise.
4. Use proper operations and maintenance to decrease noise generation. This can be done by applying the engineering control measures at source.
5. Adopt control measures to reduce the noise in the transmission path. Provide barriers, enclosures, or other absorption media in the transmission path to effectively control the noise.
6. Use administrative measures like change in the work assignment for the affected employees to effectively limit his or her noise exposure.
7. Provide Personal Protective Equipment to limit the noise exposure to the individual.