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NOISE CONTROL.
GENERAL CONSIDERATIONS.
There are two general principles in noise control which are often overlooked namely;
noise does not matter unless it is heard and
noise can be controlled if the determination exists.
The approach to noise control advocated is both logical and economical and is person centred rather than machine centred. There is nothing particularly wrong with a noisy machine if it is in an area which is seldom if ever visited. Therefore in setting the priorities for noise control in this report greatest attention is paid to noise in occupied areas i.e. noise which presents the greatest risk of hearing damage.
The following subsections provide basic guidance on various methods of noise control which should be considered for general applications latter sections of the report will deal with specific items of noise control.
NOISE REDUCTION AT SOURCE.
There are sometimes opportunities for noise reduction in the basic design and construction of machines, but built there is seldom much that the purchaser can do about the inherent noise generation. The amount of noise generated depends on two basic factors the size of the noise source and the by amplitude of vibration, these are the two factors which must be considered when tackling noise control at source. In well designed and installed machines these factors will have been dealt with. However machines wear and alter from use and to a large extent a lot of noise control at source is in fact preservation of machine in its as supplied condition. Typically wear may lead to increase bearing clearances and backlash. fans may become dirty and go out of balance and vibrate. Adequate levels of lubrication are important in reducing metal to metal contact. Obviously such items should have been part of a good programmed maintenance system and should have more recently formed part of the overall consideration of maintenance under the requirements of Provision of Workplace Equipment Regulations 1992.
Metal to metal contact should be avoided for instance by lining component collection bins, channels etc. with suitable materials i.e. plastics or rubber.
Noise can be generated by air jets and the basic controls are to ensure that the minimum air pressure is used to perform the task satisfactorily and to use porous silencers which destroy the high velocity air jet which generates the noise.
Toothed drive belts, resonating panels and also items which can be treated. The use of rubber mastics for example between panels and machine frame will reduce noise generation. Direct drives, plastic / nylon / sound deadened composite materials will all reduce transmission noise.
In summary while noise control at source is the most effective form of control, it has ideally to be tackled at the design stage. However good maintenance programmes, and simple modifications to the materials in use will help reduce the noise generated or at least maintain the level at the design value.
NOISE REDUCTION BY TRANSMISSION CONTROL.
While noise control at source is the most effective it can leave residual noise levels above the desired level, then noise reduction must be achieved by altering the routes of noise transmission. In general this can be done in three ways;
enclose the source.
enclose the receiver
modify the space between them.
Machines may be enclosed completely or barriers placed on one or more sides. Such enclosures and barriers should be constructed of heavy weight material with any spaces in construction filled with mastics or other fillers. Enclosures are typically lined with acoustic absorbent materials otherwise the noise level may be raised by reverberation. In practice enclosures are never complete, access for raw materials, ventilation etc., being need, such entry points require absorbent lined trunking etc. to limit the escape of noise energy. The use of barriers can be disappointing as lower noise frequencies will bend round barriers and walls, floors etc. may reflect sound round the barriers.
In typical workplaces the noise reaching the worker is made up of the sound energy direct from the source and that reflected from walls, ceiling, floors etc. In practice with industrial machines the direct route predominates for approximately 3m from the machine, thereafter the reflected or reverberant sounds tend to predominate. The reflected sounds can lead to "growth" in the overall noise level due to the addition effects combining the wave patterns of the sound sources so called reverberant enhancement. Treatment of walls etc. with acoustic absorbent materials will tend to reduce the reflected noise and hence reduce the overall noise levels. Typically 5 to 10 cm of glass / mineral wool applied to walls, ceilings etc. will absorb approximately 85% of the incident noise energy.
All of the above methods can be used either separately or in combination. All require careful consideration and will obviously entail commitment of resources.
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