5.2 Engineering controls - existing plant and workplaces
Once a noise assessment has been carried out and the need to reduce the noise exposure is established, the task of controlling the noise can be addressed. Priority should be given to those noise sources that contribute the highest noise exposure levels to the largest number of persons.
The need for noise control should be taken into account when deciding production methods or processes.
There are three basic engineering noise control measures for managing noise levels:
- engineering treatment of the source
- engineering treatment of the noise transmission path
- engineering treatment at the receiver
Engineering treatment of the source
Engineering treatment of the source is the preferred method of permanently removing the problem of noise exposure due to machinery or processes at the workplace. All noise-emitting objects generate airborne energy (noise) and structure-borne energy vibration. The treatment of these noise problems may require modification, partial redesign or replacement or the noise emitting object.
Subjective inspection or acoustical measurement of the source can identify how and where the noise is generated. Some problems can be solved by relatively inexpensive and simple procedures.
Noise sources which are more difficult may require advice from specialists. This approach could provide the most satisfactory results. A person who understands noise and the operation of the machine or process is able to consider a range of options for treating noise at the source. Engineering noise control measures can be specifically targeted at the machine and its parts, or towards the actual processes, including material handling systems.
General noise control solutions and examples of particular engineering noise control measures that can be carried out on machines include:
- Eliminating or replacing the machine or its operation by a quieter operation with equal or better efficiency, for example, by replacing rivets with welds.
- Replacing the noisy machinery by installing newer equipment designed for operation at lower noise levels. Machinery power sources and transmissions can be designed to give quiet speed regulation, for example, by using stepless electric motors. Vibration sources can be isolated and treated within the machine. Cover panels and inspection hatches on machines should be stiff and well damped.
- Correcting the specific noise source by minor design changes, for example, avoiding metal-to-metal contact by using plastic bumpers, replacing noisy drives with quieter types or using improved gears.
- Maintaining a high standard of plant and equipment maintenance to reduce noise levels to as low as practicable. Badly worn bearings and gears, poor lubrication, loose parts, slapping belts, unbalanced rotating parts and steam or air leaks all create noise which can be reduced by good maintenance. Plant and equipment resulting in excessive noise should be repaired immediately.
- Correcting the specific machine elements causing the noise by a local source approach. This is preferable to considering the entire machine as a noise source. Examples of this include the addition of noise barriers, noise enclosures, vibration isolation mountings, lagging to dampen vibrating surfaces, mufflers or silencers for air and gas flows, or reducing air velocity of free jets. These may be considered as a solution for the individual noise producing elements of the total operation.
- Separating the noisy elements that need not be an integral part of the basic machine. For example, moving pumps, fans and air compressors that service the basic machine.
- Isolating the vibrating machine parts to reduce noise from vibrating panels or guards.
Noise exposure can also be reduced by adopting less noisy processes, for example, mechanical pressing rather than drop forging. Where possible, metal-to-metal impact should be avoided or reduced. Vibration of the surfaces of the machine or the material being processed can be reduced by adequate stiffness and damping, by careful dynamic balancing where high speed rotation is used and by using suitable materials.
Material handling processes can also be modified to make sure that impact and shock noise sources during handling and transport are minimised as far as possible. This may be achieved by:
- minimising fall height of objects on to hard surfaces;
- stiffening and/or fixing damping materials to tables, walls, panels or containers where they are struck by materials or items during processing;
- absorbing shocks by providing wear resistant rubber or plastic coatings;
- using conveyor belts rather than rollers, which are more likely to rattle;
- controlling the speed of processes to match practical or realistic production rates. This can result in a smoother flow of production with less noise created by stop start impact noise.
Engineering treatment of the noise transmission path
It may not be possible to change or modify the noise-generating equipment or processes by engineering treatment of the source. In this case, engineering treatment of the noise transmission path between the source and the workers or other persons should be investigated.
Engineering treatment of the noise transmission path includes isolating the noise-emitting object(s) in an enclosure, or placing them in a room or building away from the largest number of workers, and acoustically treating the area to reduce noise to the lowest possible levels.
Consider the following when carrying out engineering treatment of the noise transmission path:
- Noise levels do fall over distance without any engineering treatment. Distance is often the cheapest solution. However, it may not be effective in reverberant conditions.
- Erect a noise barrier between the noise source and the listener.
- Sometimes a partial barrier can be effective. Where either area has a false ceiling, make sure the dividing wall extends to the true ceiling. All air gaps in the wall should be closed and airtight.
- Once a noise barrier is erected, further treatment may be necessary. This could include the addition of absorbing material on surfaces facing the noise source.
- Materials that are good noise barriers, for example, lead, steel, brick and concrete, are poor absorbers of sound. The denser and heavier the material, the better the noise barrier will be.
- Good sound absorbers, for example, certain polyurethane foams, fibreglass, rockwool and thick pile carpet, are poor barriers to the transmission of sound. Absorbers work because they allow sound to travel through their material turning the sound energy into heat energy.
- Walls and machine enclosures should be designed to minimise resonances that will transmit acoustical energy at the resonant frequency to the protected area. This can be achieved by placing reinforcement or bracing in strategic areas during construction or modification.
- Reduce the reverberation of the room where noise is generated as far as possible. This is best achieved by the introduction of acoustically absorbent material(s). The presence of reverberation in a room shows the need for absorbing material. Excessive reverberation produces unpleasant and noisy conditions that can interfere with speech communication.
Note: Reduction of the reverberation of a room may not lead to significant reduction of the noise exposure level of people close to the source but will be useful for people further away from the source.
Examples of engineering control measures
Diagram 1 – Engineering treatment at the source
Installation of a perforated sheet metal and wire mesh cover, to replace previous solid metal cover over belt drive and flywheel. Sound radiation is reduced at the source.
Diagram 2 – Engineering treatment of the noise transmission path
Hanging of sound absorbing baffles in the noise path provide low frequency absorption and are easy to install.
Diagram 3 – Engineering treatment at the receiver
When noise cannot be contained at the source or the path it may be necessary to provide a quiet control room. A well-insulated room is built and the floor plate is isolated from machinery vibrations.
These principles can be put to use by:
- using a sound-reducing enclosure which fully encloses the machine(s)
- separating the noisy area and area to be quietened by a sound reducing partition
- using sound-absorbing material on floors, ceiling and/or walls to reduce the sound level due to reverberation
- using sound-absorbing baffles between workers and the noise source
- using acoustical silencers in intake and exhaust systems associated with gaseous flow activity, for example, internal combustion exhaust systems or air conditioning systems.
Engineering treatment at the receiver
Where noise cannot be adequately reduced at the source, the environment in which the worker conducts work should be looked at to see if it is desirable to protect the worker(s) instead of enclosing the sound sources. In this case, design of a sound-proof room or sound reducing enclosures should follow the same control principles as for the control of noise at the noise transmission path, or improving the acoustic qualities of the structure between a noisy workplace and an administration area or supervisor’s office etc.
Inspection and maintenance of controls
Vibration mountings, impact absorbers, gaskets, seals, silencers, barriers, absorptive materials and other equipment controlling noise levels should be inspected and maintained regularly to make sure they continue to be effective.
Last updated 22 July 2008