Modifying Noise Transmission Path

Lecture Outline Summary. Introduction. Increasing the Distance. Enclosing Noisy Machines. Full Enclosure of Noisy Machines. Partial Enclosure of Noisy Machines. Use of Barriers and Shields to Enclose Noisy Machines. Separating Noisy and Quiet Areas by Partitions. Use of Sound Absorbing Materials on Surfaces. Use of Control Rooms. Additional Noise Control Measures. Maintenance. Quieter Work Practices Administrative Controls Personal Protective Equipment. Health Surveillance. Conclusion Recommendations.

1. Summary Apart from control of noise at source, modification of noise transmission path is an effective means of controlling noise. Other methods discussed earlier include: elimination substitution isolation Modification of noise transmission path can be achieved by: increasing distance between noise sources and workstations enclosing noisy machines use of partitions sound absorbing materials control rooms

1. Summary Additional noise control measures include: maintenance quieter work practices administrative controls use of personal protective equipment health surveillance

2. Introduction Modifying the noise transmission path is seen as the second line of defense and its methods include the following: Increasing the distance between source and receiver Enclosing noisy machines by enclosures, barriers and shields. Separating noisy and quiet areas by partitions. Using sound absorbing material on surfaces Use of control rooms Plate x: Acoustic curtains separating grinding/welding section.

3. Increasing the Distance. Increasing the distance between noise sources and workstations will reduce noise. Locating high noise sources together and as far away as possible from workstations helps to reduce noise exposure of people at the workstations

3. Increasing the Distance. It is also possible in some cases to separate high noise sources from quiet ones by locating them in separate rooms or by installing partitions between them. Low noise tasks like packaging cleaning etc. should be carried out in low noise areas.

3.1. Case Study. J. & E. Hofmann Engineering Pty Ltd, a manufacturer and repairer of mining equipment, specializes in gear cutting, with various noise sources. They separated the noisy area from the quiet one by the use of acoustic curtain, allowing only overhead cranes to move larger items between them.

Plate x: Fully enclosed office 3.1. Case Study. Plate x: Fully enclosed office

3.2. Case Study. Williams Electrical Service installed special sliding doors to separate the quiet electrical workshop from noisy mechanical section. They reduction achieved was 9 dB(A). J. & E. Hoffmann Engineering installed some offices with double glazed windows that reduced noise by 5 dB (A) Plate x: Acoustic sliding door Separating quiet electrical workshop

Plate x: Double glazing enhances 3.3. Case Study. Another example of noise control along the transmission path is a fully enclosed workstation. J.&E. Hofmann Engineering installed a few of the fully enclosed offices throughout their workshops. They are fitted with full core doors and double glazed windows that have reduced the noise by 5 dB(A). For new offices overlooking the factory area it is planned to obtain more reduction by using glass panels of two thicknesses, a thicker panel on the outside and a thinner inside with a vacuum gap of 40 mm between. Plate x: Double glazing enhances noise reduction

3. Increasing the Distance. Use of remote-control systems should be a preferred option where possible because it allows the operator to be away from the noise source.

4.0 Enclosing noisy machine 4.1 Full enclosure of noisy machines Total enclosures are normally used when a high noise reduction is required. To design an enclosure that will perform well and reduce noise levels significantly the following points should be observed: For the outer shell choose stiff and heavy materials that reflect sound waves back. Common materials used include; sheet metal, plasterboard, masonry, timber, glass and loaded vinyl.

Plate x: Example of a full enclosure.

4.1 Full enclosure of noisy machines The inner shell needs to be lined with a sound absorbing material to reduce sound build-up inside the enclosure. The preferred materials are mineral wool, glass wool, foam or polyurethane. Any gaps or openings should be properly sealed off or blocked off by flexible flaps if openings are necessary. Any windows can be double-glazed for better reduction. Doors, windows, hatches or removable panels should be installed with full seals on every edge. Cooling air intakes and exhausts should be fitted with noise attenuators.

4.2 Partial Enclosure of Noisy Machines. No enclosure parts should touch any vibrating parts of the enclosed machine. In the case of pipework which goes through the enclosure, all gaps need to be sealed with a soft sealant. Plate x: Hydraulic power pack enclosure.

4.2 Partial Enclosure of Noisy Machines. This kind of enclosure can be effective in reducing noise to nearby workstations, but the noise will still escape through any openings and add to the background noise levels in the workplace.

4.2 Partial Enclosure of Noisy Machines. Plate x: Example of a partial enclosure.

4.3. Use of Barriers and Shields to Enclose Noisy Machines. Barriers and shields are used in situations where only a small reduction in noise levels of about 5 dB is needed. Barriers and shields work in the same way, redirecting the sound away from a receiver.

4.3. Use of Barriers and Shields to Enclose Noisy Machines. Plate x: Example of usage of an acoustic barrier.

4.3. Use of Barriers and Shields to Enclose Noisy Machines. Shields are usually small transparent barriers such as 5 mm thick Perspex placed between a worker and a noise source. An acoustic barrier is a larger piece of solid material, usually free- standing on the floor. Barriers and Shields are most effective when both workers and a source are close to them, and the ceiling and other nearby reflecting surfaces are lined with sound absorbing material. Common materials used to construct barriers are: sheet metal. plywood. clear plastic or safety glass.

4.3. Use of Barriers and Shields to Enclose Noisy Machines. Plate x: Linisher partially enclosed and lined with absorptive material.

5. Separating Noisy and Quiet Areas by partitions Transmission of sound to a nearby room or from outside can be minimized by increasing the sound insulation of walls, ceiling, doors and windows. The amount of sound that will pass through depends on the characteristics of the material used for partitions and most importantly on the mass per unit area. The higher the mass the higher is the sound reduction. Another way of improving the insulation of a single shell wall is to construct a second wall separated from the first.

5. Separating Noisy and Quiet Areas by partitions Plate x: Two-layer plasterboard partition.

6. Use of Sound Absorbing Materials on Surfaces The noise received by the operator consists of noise coming directly from noise sources and noise reflected from walls, floor, ceiling and other equipment. All the reflecting surfaces should be treated to reduce the noise received by the operator. This is achieved by applying sound absorbing material, which helps to reduce the reflected sound. Acoustic fables is a convenient method of employing sound absorption. Attention should also be given to machine location, as placement to reflective surfaces increases the noise levels.

Table x: Example of Sound Insulation Properties of Various Materials (EPA, 1989). 113 220 42 to 66 Material Thickness (mm) Surface Mass (kg/m2 ) Achievable noise reduction (dB(A)) 125Hz - 2000Hz Compressed strawboard 50 17 20 44 Chipboard 18 12 41 Plasterboard 13 Plywood 10 7 36 Brick wall

Plate x: Acoustic baffles.

7. Use of Control Rooms The use of automated machines happens to be the latest trend in some industries. Operators are allowed to operate from monitoring rooms in order to minimise the time spent near a noisy machinery to: starting, repairing and maintenance work. A properly designed control room can achieve up to 30 dB reduction, with the condition that the room has to be properly ventilated, with ventilated openings fitted with attenuators, acoustic louvers or a silenced air conditioner unit. Sometimes barriers or partial enclosures offer sufficient noise reduction which makes them useful at a workstation. They offer a noise level reduction of less than 10 dB.

7.1 Case Study: Company Enclosed the Workshop Office Stegbar Building Products installed an office fully constructed with 6.38 mm laminated glass panel. With all windows closed, the reduction achieved was 24 dB(A)

Case Study Plate x: Fully enclosed office constructed with laminated glass panels.

8.0. Additional Noise Control Measures. Maintenance of plant and equipment plays a very important role in overall noise control. In general, machines get noisier with use because of: Worn or chipped gear teeth - worn or chipped teeth will not mesh properly. The shiny wear marks are often visible on the teeth. Worn bearings - bearing wear will show up as vibration and noise, squealing from slack drive belts, "piston slap" in motors, air leaks, etc. Poor lubrication- this appears as squeaking noises due to friction or excess impact noise in dry and worn gears or bearings. Imbalance in rotating parts - just like car wheels, any imbalance in a fan impeller or motor shaft will show up as excess vibration.

8.0. Additional Noise Control Measures. Obstruction in airways - a build-up of dirt or a bent/damaged piece of metal in an airway or near a moving part, eg., a bent fan guard can cause whistling or other "air“ type noises. Blunt blades or cutting faces - blunt or chipped saw teeth, drill bits, router bits, etc., usually make the job noisier as well as slower. Damaged silencers - silencers for air-driven machines or mufflers for engines may become clogged with dirt, rusted or damaged, so losing their ability to absorb noise. Removal of a noise-reducing attachment such as mufflers, silencers, covers, guards, vibration isolators, etc.,

8.1.1. Effect of Maintenance on Noise Levels WorkSafe Western Australia Studied the Effect of Maintenance on Noise Levels for a Variety of Tools and Machines. Proper maintenance plays a very important part in controlling noise at workplaces, as worn and unbalanced parts of machines cause both noise and vibration. Hence it is vital to have a maintenance schedule that covers the following: lubricating/oiling all moving parts; balancing and aligning parts, especially rotating ones; replacing worn parts; and checking the machine's feed rates.

8.1.1. Effect of Maintenance on Noise Levels WorkSafe Western Australia carried out a study to see how this works in practice. A total of 11 power tools (including drills, angle grinders, sanders, a circular saw and a planer) were tested for noise "before" and "after" routine servicing at a commercial power tool service centre. The average noise level 0.5 m away was 93.9 dB(A) "before" and 92.0 dB(A) "after" whilst running free. Noise reductions ranged up to 7 dB(A). The average reduction was 1.6 dB(A). As expected, this average reduction is small in terms of decibels. However, it means that sound energy output before maintenance was 45% higher, showing that wear and tear was clearly on the rise.

8.1.1. Effect of Maintenance on Noise Levels In other studies on specific machines significant noise reductions were achieved through careful maintenance work. The noise level of a common type of reciprocating compressor was reduced by about 8 dB(A), by adjusting the valve seating to improve the seal and adding "Molyslip“ additive to the lubricating oil to reduce roughness in the piston stroke.

Effect of Maintenance on Noise Levels The noise level of a pneumatic knife used in abattoirs was reduced by about 8 dB(A), by improving the balance of the rotor vanes and replacing bearings and a worn collar that allowed parts to rattle. Also, The noise level of an electric motor and belt drive for an aluminum docking saw was reduced by 15 dB(A) when free running, by replacing squealing belts and worn motor bearings and drive pulley bearings.

8.2. Quieter Work Practices. In many workplaces material handling can be a contributing factor to the overall high background noise levels. Noise created by handling metal can be reduced by minimising metal-to-metal or metal-to-hard surface contact. Some of the techniques available are: Use of bending machines instead of hammers. Lowering materials carefully instead of dropping. Improving crane configuration, storage arrangements and operator skills. Reducing drop heights for materials which must drop. Lining of product bins or scrap bins with wear resistant rubber.

8.2. Quieter Work Practices. Plate x: Example of quiet work practices.

8.2. Quieter Work Practices. Lining of steel trestles or benches with wear-resistant rubber Lining underside of steel benches where a hard surface is required Use of wear-resistant rubber floor-mats or wall brackets in storage areas. Placement of work piece on a durable rubber mat instead of hard bench or floor.

8.2. Quieter Work Practices. Plate x: Designated noisy area where hammering takes

8.2. Quieter Work Practices. Plate x: Quieter way to use air gun.

8.2.1. Case Study: Company Involves Employees inDeveloping Quieter Practices. Macmahon Contractors recognised that part of their noise problem came from the way some of the work was carried out. To overcome this, employees received extra training on noise issues and were delegated with the responsibility of addressing the issue. The first step was to set up designated noisy areas where all hammering was done. At Macmahon Contractors when using air guns to dry machine parts, it was noticed that placing nozzles very close to the holes instead of sweeping the air stream across them decreased noise levels significantly. Measurements proved this and the noise reduction achieved was 9dB(A) from 99 to 90 dB(A).

8.2.2. Case Study: Altering a Process Significantly Reduced Employees Noise Exposure. At John's Engineering and Cranes, boilermakers used to work in a fully enclosed tank when finishing it inside. This was an extremely noisy situation as the enclosed walls were very reflective, increasing already high noise levels from grinding and hammering operations. The decision was made to finish the tank inside while it was still in two pieces so the noise can dissipate, and then weld them together. Using this method the noise levels inside are lower by at least 3 dB(A).

Plate x: Tanks are enclosed only when majority of work inside is finished

8.3. Administrative Controls. Another way of reducing an operator's noise exposure is by reducing the duration of exposure. This can be arranged in a number of ways: Rotate operators so each one spends less time being exposed to noise. Schedule noisy activities to be all done at the same time, preferably outside of normal hours or on shifts with fewer workers present. Notify other workers when noisy work is scheduled so they can organise their work around it, limiting their exposure. Provide quiet refuge rooms for paperwork tasks. Provide quiet lunch and rest areas with low background noise levels where workers can spend their breaks away from noise.

8.3. Administrative Controls. Plate x: Ear Muffs.

8.3. Administrative Controls. Train operators in effects of exposure to noise on health, methods of prevention, proper use of personal hearing protectors, maintenance, etc. Display warning signs in noisy areas to limit access to authorised personnel.

8.3. Administrative Controls. Plate x: Ear Plugs.

8.3.1. Procedures Implemented to Reduce Exposure Times. On their construction sites Macmahon Contractors delineates an 85 dB(A) contour around any noisy equipment and reschedules work so it is done outside this boundary. This method was first introduced when a contractor brought on site a drill rig that produced noise levels in excess of 90 dB(A). A noise assessment was carried out and Macmahon's employees were removed from the affected area. Work was rescheduled and was either done outside the 85 dB(A) boundary or at times when the drill rig was not operating.

8.4. Personal Protective Equipment. Personal hearing protectors offer the last possibility for prevention of noise induced hearing loss. Plate x: Canal Caps.

8.5. Health Surveillance. Medical precautions will not replace technical measures but can identify the problem early enough to prevent it from developing further. It is advisable to introduce an initial hearing test before starting at a noisy workplace, which would identify any previous noise induced hearing loss. Follow-up hearing tests during employment will identify if any of the existing problems are exacerbated or if any new ones have developed.

9. CONCLUSIONS Noise, being any sound which is often unpleasant, is unavoidable especially at workstations but can only be modified to assist operators to enhance better productivity.

10. RECOMMENDATIONS The use of automated machines should be encouraged Routine maintenance for machines should be thoroughly observed. Personal Protective Equipment should be made compulsory Machine operators should be subjected to initial hearing tests before commencement of work