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OPTIONS - continued
(f) Anti-Vibration Mounts or Isolation
Mechanical vibrations pass easily through
rigid structures. Isolators can be used as vibration breaks to prevent
structural vibrations reaching the surfaces which can radiate noise [see
directory
for suppliers].
Various types of isolator are available
from rubber in shear, metal springs and even cork.
Isolators must be selected carefully; a
poor choice can result in resonance - which make matters far, far worse. For
example car engines are mounted on anti-vibration mounts to reduce cabin, and
environmental noise; if the mounts were too soft, the engine could begin to
resonate and shake itself violently eventually to destruction! Suppliers of this
type of equipment can be found by searching the
directory.
(g) Noise Enclosures
Noise enclosures are commonly used to
control noise exposure, and employ a combination of insulation, absorption,
damping and isolation to reduce noise.
Basic features include:
- Shell - the basic insulating shell
should be composed of heavy insulating material with an inner absorptive
lining to minimise reverberant sound as follows. Commonly galvanised sheet
steel possibly lined with plasterboard is used for the outer layer, then
mineral wool held in place by the inner layer of perforated galvanised steel
or expomet etc.
- Materials may need to be fire or
chemical resistant or even non-magnetic depending on the environment in
which they are to be placed
- The enclosure should be as
"airtight" as possible, i.e. doors and panels should be sealed
with no gaps or cracks through which noise could pass.
- Openings for services and
entry/egress of materials should be acoustically treated such as by means of
an acoustic tunnel or baffle.
- Windows should be double-glazed
with large gaps and unequal thickness panes.
- Generally acoustic enclosures are
also provide very good thermal insulation and the heat build up can be very
significant. Therefore, ventilation is likely to be required and should be
provided via acoustically treated ducting.
- Noise at source should be reduced
wherever possible, eg. close shielding of noisy components of machines
inside the enclosure.
- Vibration - isolation of the
machine itself and service connection may be needed.
- Other requirements include adequate
lighting, reasonable access points for maintenance, as well as adequate
space for operation.
In some instances a separate enclosure
may not be necessary as it may be possible to incorporate the noise enclosure
into the machinery guarding.
Noise enclosures can be very effective
depending on the source characteristics, reductions up to 25 dBA can be
achieved; unfortunately a number do not work as well as they might because
insufficient thought has been given to selection of materials and minimising
gaps and holes.
(h) Acoustic Screens/Barriers
These can be effective when placed close
to the source or close to the recipient. Good sound insulating materials provide
the best screens. As noise levels will increase with reflections, and to avoid
the canyon effect where multiple reflections cause the noise to pass over the
top of the screen, it is sensible to line the reflecting surface with sound
absorbing material.
Noise can pass around the side of the
screen either by reflection from ceiling or walls (therefore treat with
absorbing material) or refraction. This limits the benefit to about 10 dBA but
with extensive screening (i.e. making a partial enclosure) 15 dBA is possible.
For environmental noise, very large barriers or even earth mounds can be used,
and the practical maximum for such barriers is probably between 20 and 25 dBA.
(i) Silencers and Air Turbulence
Silencers are used to reduce noise
contained in an air stream by two different processes, the first being more
common that the second.
Absorptive Silencers
In its simplest form, attenuation is
achieved by lining the inside of the duct with absorption material such as
mineral wool. More sophisticated designs use sound absorbing baffles to split
the duct into sections; the geometry of the baffles being related to the
dominant frequency of the noise. It should be noted that silencers or
attenuators will probably restrict the airflow, which will impose extra work on
the fans, and in extreme circumstances the restricted and turbulent airflow
could lead to noise generation! Like most forms of noise control, the higher the
frequency the more effective the silencer.
Reactive Silencers
Reactive silencers work by forcing the
air to oscillate through narrow constrictions in a resonant chamber so that the
sound waves coincide and cancel each other out. Reactive silencers can be tuned
to deal with specific frequencies, and therefore they are particularly useful if
low frequency or tonal noise is a problem.
(j) Distance
For a point source, every time the
separation distance is doubled the noise level falls by 6 dBA. So that if you
can position the noise source further from the noise sensitive location, no
"real" noise control need be done at all. However the "6 dBA per
doubling" applies to noise from a point source in open field conditions;
however, noise is rarely from a point source and the 6 dB doubling only starts
from 2-3 metres from the source. Reflections inside buildings also reduce the
effect. Therefore increasing separation distance is probably more appropriate to
environmental rather than occupational problems.
(k) Sound Havens
This applies solely to occupational
exposure. Instead of putting the machine in an enclosure, the operator spends
most of his time in an enclosure away from the process and the exposure is
reduced. The design factors are similar to a noise enclosure. To be effective,
operators should spend most of their time inside the haven; therefore, the
location of controls and monitors inside the haven is as important as the
construction of the enclosure.
(l) Maintenance
Noise is often created through lack of
maintenance; unbalanced motors, worn bearings and loose panels all being common
problems. They are easy to resolve technically, but unless the underlying
problems are addressed, the noise will continue to occur.
(m) Anti Noise
This noise control technique is still
being developed. Essentially, a noise is measured via a microphone and a
computer works out the opposite signal to the real noise, and feeds this to a
series of loudspeakers, the "real" signal and the "anti"
signal cancel each other out to a large extent, and the overall level is
significantly reduced. Anti-noise works best a low frequencies, and therefore it
is particularly suitable for problems where conventional noise control cannot be
used. The
anti-noise FAQ can be read elsewhere.
(n) Insulating
the noise sensitive location
For environmental noise, in some
instances a barrier can be erected to screen housing or residents/landlords may
accept the offer of secondary glazing in a separate heavy frame with a large air
gap [up to 100mm] which can reduce noise levels inside dwellings, although
provision may need to be made for forced ventilation so that windows are not
left open. More information on
insulating homes can be found here.
Secondary
glazing is most likely to be practicable if only the front of the property is
affected, or there is only a nighttime noise problem.
For occupational noise exposure
"insulating the noise sensitive location" amounts to
Hearing
Protection (ear plugs or
muffs).
As with most sections on this site, the
directory
can be queried for a suitable consultant.
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