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Noise - Occupational - Noise Control Design 2

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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