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.
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
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
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
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but in combination with other
materials, eg. mastic or plasterboard which are incorporated to reduce the
By contrast porous open materials make
the best noise absorbers. Examples are mineral wool, unbonded fibreglass and
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
Large stiff panels can vibrate and
radiate noise. Damping is applied to prevent vibration and
hence noise [see
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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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