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Noise - Occupational - Noise Control Design  1


(a) Design
Noise control is best considered at the design stage; generally you get the largest and possibly the easiest noise reductions if noise control is considered at this stage. Careful design of new equipment to minimise noise propagation is one of the most effective noise control techniques. This can include machine design to minimise cutting and impact energy, fan and ductwork design to reduce noise from air turbulence and choice of component materials.

(b) Substitution
Changing an existing process can also be an effective way of reducing exposure. Examples include using an electric fork-lift truck instead of a diesel one, reaction pile driving instead of pneumatic, pressing metal into shape instead of hammering, bolting sheets together or welding instead of riveting. Also substituting soft materials for hard can reduce impact noise.

(c) Workplace Practices
Thoughtless or poor work practices, such as leaving doors open, excessive use of compressed air lines and hammers, or even staff radios can contribute significantly to high noise levels. Training and education are paramount. (Note: With environmental noise problems work practice can be critical in minimising noise nuisance.)

(d) Insulation and Absorption
Insulation provides a barrier to prevent noise energy passing through. Absorption is used to reduce the noise energy reflected from surfaces thus reducing overall (reverberant) noise within a space. These terms are often confused, probably because materials used for thermal insulation, such as mineral wool, are used for absorbing noise and not insulating noise.

Probably the best method of explaining the difference between insulation and absorption is by way of analogy - If we consider water as being equivalent to noise, when it must be prevented from passing from area to another, a barrier or dam is used; this is a massive, heavy, object with no gaps or cracks through which water would pass. On the other hand if there is a small amount of excess water within an area, a sponge could be used; this has an open texture into which the excess water is absorbed. A sponge would not be used in place of a dam nor vice-versa. Similarly with noise, heavy structures with no gaps or cracks are used to prevent noise passing from one area to the next (Insulation), and materials with open textures are used to "soak-up" smaller quantities of excess noise within an area (Absorption).

High density, low stiffness materials such as lead have the best insulation properties. However high density, high stiffness, materials such as concrete or brick work can also provide effective insulation, provided that they are not being directly excited (impact noise). Timber is the next most effective material but being less dense, you need a lot more of it for the same effect.

Steel has high density, but its stiffness tends reduces its insulation properties. However it is commonly used in enclosures [see Directory for suppliers] but in combination with other materials, eg. mastic or plasterboard which are incorporated to reduce the overall stiffness.

By contrast porous open materials make the best noise absorbers. Examples are mineral wool, unbonded fibreglass and polystyrene foam.

Sound absorption and insulation properties are frequency dependent; in general the higher the frequency the greater the absorption or insulation for any given material.

(e) Damping Materials
Damping can be used to:

(i) Reduce Impact Noise:
Very hard materials such as brickwork, steel or concrete have little inherent damping and when impacts occur, high noise levels can result. By using coverings or coatings of materials with high inherent damping, characteristics, like soft plastics and rubber, impact noise can be 'deadened'. For example a spongy carpet or specially backed linoleum can dramatically reduce footfall noise, generated on a concrete floor.

(ii) Reduce noise radiation from vibrating structures:
Large stiff panels can vibrate and radiate noise. Damping is applied to prevent vibration and hence noise [see Directory for suppliers]. For example this insides of most car body panels are lined with damping material. Options include, PVA or PVC plastic or bitchmen sprayed or stuck onto the panel or fitting stiffening ribs to change the resonant frequency of the structure away from the driving frequency and thereby reduce the noise level.

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