2.6.2. Noise exposure standard
The main noise exposure standard is GOST 12.003-86 SSBT “Noise safety regulations”. The one, which has been found to commonly describe the damaging effect of noise, is the A-weighting network. Zones having sound level exceeding 85 dB need isolation and setting signal labels. Everyone working in such zone should use personal protective equipment.
If sound level in any octave band exceeds 135 dB such area is restricted.
Machinery or another equipment is referred as noisy one if sound level in the workplace exceeds exposure standard by 10 dB.
Equivalent sound pressure level is applied to calculate average level for period T taking into account every interval t1 during which level Li was specified:
,
where i – interval number (i=1.2..n).
Summary sound level estimates common action of a number equal noise sources with levels L1=L2..=Ln:
,
where n – number of noise sources.
2.6.3. Noise control
Noise control at source is the most effective. Once the cause of noise is identified, some modifications can be introduced to address it.
For excessive noise generated by mechanical impacts, control options may include reducing the driving force, reducing the distance between components, balancing rotating equipment or installing vibration isolation fittings.
Full enclosure of noisy machines.
Total enclosures are normally used when a high noise reduction is required. It is possible to achieve a reduction of 10-25 dB for single-shell enclosures with absorbent lining and 20-50 dB for double-shell enclosures with absorbent lining.
To design an enclosure that will perform well and reduce noise levels significantly the following points should be observed:
-
outer shell should be made from stiff and heavy materials reflecting sound waves. Usually used materials are sheet metal, plasterboard, masonry, timber, glass and loaded vinyl;
-
inner shell needs to be lined with a sound absorbing material to reduce sound inside the enclosure. The preferred materials are mineral wool, glass wool, foam or polyurethane. Sound absorbing materials often need to have a protective facing to prevent any damage or dirt build up. The protective facing can be perforated sheet metal, perforated foil or vinyl;
-
any gaps or openings should be properly sealed or blocked by flexible flaps if openings are necessary. A 10% opening will limit the reduction to 10 dB;
-
any windows can be double-glazed for better reduction;
-
doors, windows, hatches or removable panels should be installed with full seals on every edge;
-
cooling air intakes and extracts should be fitted with noise attenuators;
-
no enclosure parts should touch any vibrating parts of the enclosed machine. In the case of pipework, which goes through the enclosure, all gaps need to be sealed with a soft sealant.
Isolation means separating noise sources from people involved in the work or others standing near by. It could mean relocating the noise source or relocating the operators or others to positions away from the noise source. For example, by doubling the distance from the source inside a workshop the noise level decreases by about 2-4 dB (A), or in the open space the noise level decreases by 6 dB (A).
Barriers and shields are used in situations where only a small reduction in noise levels of about 5 dB is needed.
Barriers and shields work in the same way, redirecting the sound away from a receiver. Shields are usually small transparent barriers such as 5mm thick perspex placed between a worker and a noise source. They can also be mounted on a machine and used as a safety shield.
An acoustic barrier is a larger piece of solid material, usually free-standing on the floor. Barriers and shields are most effective when both workers and a source are close to them, and the ceiling and other nearby reflecting surfaces are lined with sound absorbing material.
Materials used to construct barriers are sheet metal, plywood, clear plastic or safety glass. Best results are achieved when at least some parts are covered with absorbing material to minimize sound reflection.
Transmission of sound to a nearby room or from outside can be minimized by increasing the sound insulation of walls, ceiling, doors and windows. The amount of sound that will pass through depends on the characteristics of the material used for partitions and most importantly on the mass per unit area. The higher the mass the higher is the sound reduction. The frequency of the passing sound is also important, as the higher frequencies are easier to stop from penetrating through. In general, an increase in airborne sound insulation of about 5 dB is obtained when the mass per unit area is doubled.
Current developments in some industries are directed towards automated machines and processes, thus allowing operators to operate from a control or monitoring room and so minimizing the amount of time spent near noisy machinery to starting, repairing and maintenance work.
A properly designed control room can achieve a reduction of up to 30 dB. The room has to be properly ventilated, with ventilation openings fitted with attenuators, acoustic louvers or a silenced air conditioner unit. Care should be exercised to ensure cables, pipes and fittings are not rigidly connected and doors and windows are properly sealed.
Sound absorbing material on surfaces.
In general terms the noise received by the operator consists of noise coming directly from noise sources and noise reflected from walls, floor, ceiling and other equipment. After treating all the direct noise sources, all the reflecting surfaces should be treated to reduce the noise received by the operator. This is achieved by applying sound absorbing material, which helps to reduce the reflected sound.
A convenient method of employing sound absorption is the installation of acoustical baffles. Where large surface areas are to be treated, a spray-on treatment may be more economical.