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Basic noise control measures
How you reduce sound depends on where it comes from. Sound produced inside a room is dealt with by absorbing it. Airborne sound that comes from outside is kept out by insulating the space. Sound transmitted through the structure of a building needs the structure to be isolated from the source of vibration. Cellular and porous solids absorb sound well and can help with isolation, but they are poor at insulating against sound.
The basic principles of noise control are as follows.
Isolation. Reduces sound that travels as vibration. The equipment is isolated from radiating surfaces with springs, elastomeric materials, cork or foam rubber. Heavy vibrating machinery, for example, can stand on isolating springs and rubber inserts, and vibrating pipes can be carried on brackets padded with rubber isolators or springs.
Absorption. Absorbent material on surfaces cushions the impact of objects falling on walls, ceilings and floors and reduces noise from reverberation.
Insulation. Stops noise passing through by putting a mass barrier in its way. Dense materials such as brick, concrete and metal do this well.
Damping. Surfaces that vibrate under mechanical forces, such as chutes and hoppers, are covered with viscoelastic materials such as bitumastic, plastic, silicone, hard rubber and other elastomeric polymers.
Silencing. Silencers let the fluid pass but restrict the passage of sound by reflecting or dissipating it. Absorbing materials inside a silencer dissipate sound. In a reflective silencer, such as a car muffler, sound is reduced by expansion chambers or openings.
Hearing protection. Ear plugs and ear muffs are the last choice in the hierarchy of control. They are generally less effective than the controls above because they rely on human action. Used together with other controls, hearing protection can protect workers when engineering controls cannot adequately remove the noise hazard.
There are two main types of hearing protector.
- Earplugs fit into or seal the ear canal. Some have cords or bands. Disposable and reusable types have manufacturer-specified cleaning, inspection and replacement requirements; reusable does not mean indefinite use.
- Earmuffs are normally hard plastic cups that fit over and surround the ears. Cushion seals filled with plastic foam or a viscous liquid seal them to the head, and the inside of each cup is lined with sound-absorbing material, usually soft plastic foam. They can be mounted on a headband or a helmet, and some have communication equipment built in. Earmuffs are also called ear defenders.
Hearing protection is used effectively by following these points.
| Do | Do not |
|---|---|
| Make sure the protectors give enough protection | Provide protectors that cut out too much noise, which can cause isolation or an unwillingness to wear them |
| Target use to assessed exposure and required protection zones | Substitute a blanket PPE rule for assessment and practicable noise reduction |
| Select protectors suited to the working environment, considering comfort and hygiene | Take a blanket approach: it is better to target use and encourage people to wear protection only when they need it |
| Think about how they will be worn with other protective equipment such as hard hats, dust masks and eye protection | |
| Provide a range of protectors so employees can choose ones that suit them |
To make sure hearing protection keeps working, check that:
- it stays in good, clean condition
- earmuff seals are undamaged
- the tension of the headbands has not dropped
- there are no unofficial modifications
- compressible earplugs are soft, pliable and clean
Summary notes for Element 5 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.