Using enclosures, barriers and screens to control noise

Protecting employees from exposure to noise

Employers have a duty to protect the health and safety of their employees. This duty includes protecting employees from exposure to noise. The Occupational Health and Safety Regulations 2017 (OHS Regulations) set a noise exposure standard measured in units called decibels (dB). The noise exposure standard is an 8-hour average of 85 dB(A) and a peak noise level of 140 dB(C) at the employee's ear position.

Exposure to noise that exceeds the standard is considered dangerous to employees' hearing. Employers must ensure employees' exposure to noise does not exceed the noise exposure standard.

If there is uncertainty about whether noise exposure exceeds or may exceed the standard, employers must determine an employee's exposure to noise in the workplace. When determining noise exposure, employers must not take into account the effect of any hearing protectors employees may be using.

Employers must take into account:

• the level of noise to which employees are exposed
• the duration of the exposure
• plant and other sources of noise at the workplace
• systems of work at the workplace
• any other relevant factors

Information about employers' duties is available on the WorkSafe website, including the Noise compliance code. The Noise compliance code provides practical guidance on how to comply with obligations under Victoria’s occupational health and safety legislation to manage risks associated with workplace noise exposure.

Noise controls

Use the hierarchy of control

The hierarchy of control is a step-by-step approach to eliminating or reducing risks. It ranks risk controls from the highest level of protection and reliability through to the lowest.

Employers must control noise in line with the following hierarchy of control measures:

• eliminate the source of noise
• substitute noisy plant with quieter plant or processes, isolate the plant or use engineering controls
• use administrative controls
• provide hearing protection

Employers must apply each level of the hierarchy so far as reasonably practicable before moving down to the next control measure. This means employers cannot go straight to hearing protection to control the noise without applying the higher-level control measures, so far as reasonably practicable.

It is often necessary to use a combination of control measures to effectively control noise.

Review processes and noise sources

Before considering enclosures as a noise control measure, employers should review the workplace layout, the processes and noise sources.

It is important to determine what is causing the noise and whether it is reasonably practicable to control noise risks by:

• eliminating noisy processes or equipment. For example, buying pre-cut or pre-fabricated materials
• substituting noisy plant, processes or methods of work with quieter alternatives. For example, laser cutting to avoid grinding
• using engineering controls such as acoustic enclosures, isolation mounts, damping of vibrating panels, quieter gears and cutters and silencers

These noise control measures may prove more cost-effective and less restrictive than enclosures. However, if they do not control the risk, a full or partial noise enclosure, control room or employee refuge, acoustic barrier or screen may be needed.

Noise-controlling enclosures

The following information describes different types of noise-controlling enclosures.

Full acoustic enclosures for plant

Full acoustic enclosure of plant can be an effective way to control employee exposure to noise. A well-designed full acoustic enclosure will:

• be relatively airtight
• be lined with high-density sound-absorbing material
• reduce plant noise by as much as 30dB to 40dB

Number 1 points to resilient flanges to seal the entry point of equipment. Number 2 indicates a silencer-design air vent, 3 points to a steel outer casing and 4 shows an air vent for cooling air. Number 5 indicates an acoustic absorbent lining and item 6 points to the use of neoprene or similar resilient material to seal an air gap between the enclosure and the floor. Number 7 points to a resilient vibration mounting. Ref: ACC, NZ.

Design guidelines for enclosures appear later in this guidance.

Control rooms or employee refuges

Control rooms or employee refuges are enclosures designed to keep noise out. Employees can control machines and monitor or view machinery and processes from within the enclosure. Typically, a control room or employee refuge can reduce noise by 15dB to 30dB.

Control rooms or employee refuges may be an effective risk control measure when:

• it is not reasonably practicable to enclose plant. For example, where there are large machines or a large number of machines
• there is a relatively small number of employees to accommodate
• the process is or can be largely automated or operated remotely

Portable or demountable sound-insulating cabins are an option for employers to consider. The cabins are easy to assemble, dismantle or move.

Partial acoustic enclosures

A partial acoustic enclosure is an option if a full acoustic enclosure is not practicable. Generally, partial enclosures do not achieve the same noise reduction as full enclosures.

Partial enclosures should be built in a similar way to full enclosures. Keep the number and size of openings to a minimum. Direct openings away from employees where possible. Extend product access chutes and line them with sound-absorbing materials, so as far as is reasonably practicable.

The noise reduction achieved by partial enclosures depends on:

• the shape and structure of the enclosure
• the number and size of openings
• the materials used to build the enclosure

Partial enclosures can reduce noise by about 10dB.

Acoustic barriers and screens

Acoustic barriers and screens are other noise-control options. Barriers and screens may be appropriate where a full or partial enclosure is not reasonably practicable or a noise reduction of about 5dB to 7dB is sufficient.

Fixed, portable or demountable barriers or screens should be:

• between the source of noise and employees
• as tall and wide as possible
• positioned close to employees or the noise source

Barriers and screens are less effective in highly reflective environments. This includes workplaces with concrete floors or walls. To improve the effectiveness of barriers and screens:

• line reflective surfaces such as walls and ceilings with sound-absorbing material such as foam to reduce noise reflection
• minimise gaps at floor level
• seal gaps at floor level with flexible materials

Noise enclosure design guidelines

The following guidelines can help when designing or building an enclosure that will reduce employee exposure to noise.

Wall materials

Suitable materials for constructing an acoustic enclosure include bricks, concrete, metal, plywood, MDF, plaster, glass and Perspex.

The level of noise reduction depends on:

• the type of wall material used
• how well the enclosure is sealed
• the main frequencies of the noise being controlled — low, medium or high pitch
• the weight per unit surface area of the wall material. Compact, dense and heavy materials are typically more effective at reducing noise

Table 1 shows the noise reduction data for common materials.

As a general rule, select materials that provide about 10dB higher noise reduction than required. This will compensate for any weaknesses in the acoustic enclosure.

Using 2 sheets of a given material rather than one sheet of the same material of the same thickness can improve noise reduction up to about 10dB.

Single and double walls

Double walls can reduce noise 10dB to 20dB more than single-shell walls for the same weight per unit area. Improve the sound insulation by:

• increasing the distance between the shells in the walls up to 15cm
• filling the cavity with sound-absorbing material
• avoiding rigid connections between the shell. For example, staggered wall systems or using resilient furring channels, as shown in Figure 6 and Figure 7

In diagram 1, left, a double wall made from 2 sheets of 13mm plasterboard with a 5cm cavity will reduce noise by an average 37db. In diagram 2, centre, a double wall made from 2 sheets of 13mm plasterboard with a 15cm cavity will reduce noise by an average 47dB. Diagram 3, right, shows a double wall made from 2 sheets of 13mm plasterboard with a 15cm cavity filled with 30mm of mineral wool sound-absorbing material. Sound reduction in diagram 3 is an average 55dB.

Number 1 shows the direction of airborne sound and 2 shows how the resilient furring channel and mounting brackets reduce structure-borne sound. Number 3 shows how dense plasterboard provides increased sound insulation and 4 shows reduced transmitted sound. Number 5 shows insulation to absorb low, medium and high-frequency sound.

Size of enclosure

Because there is a greater build-up of noise in close-fitting enclosures, build the acoustic enclosure as large as possible. If there is not enough space in the workplace for a large acoustic enclosure, compensate for the extra noise build-up by:

• using materials that provide higher noise reduction
• lining the inside of the enclosure with sound-absorbing material

Isolation

Avoid rigid connections between plant and the acoustic enclosure. This will minimise mechanical vibration transmitting and radiating as sound.

Ensure that service ducts, pipes or electrical equipment do not come in contact with the acoustic enclosure. Alternatively, use flexible pipe sections or flexible sealants around services. This will ensure service ducts, pipes and electrical equipment are mechanically isolated.

If floor vibration is an issue:

• install vibration-isolation mountings to the machine
• isolate the acoustic enclosure

Gaps

Minimise gaps or openings. Without absorption material, a 10% gap or opening can limit the effectiveness of an acoustic enclosure to about 9dB. A 5% gap or opening can limit the effectiveness to about 19dB.

In some cases, gaps or openings are necessary but a high level of noise reduction is still required. Examples include gaps required for product access. In these cases, minimise the amount of noise that can escape by using:

• product chutes and tunnels lined with sound-absorbing material
• self-closing flaps
• brushes

Sound-absorbent material

Acoustic enclosures are most effective when their internal surfaces are lined with sound-absorbent material. Suitable absorbent materials include mineral wool, glass wool and polyurethane foam. The absorption efficiency of a material depends on its density, porosity and thickness. Table 2 shows the absorption data for common materials.

Table 2: Noise absorption data for common materials. The larger the number, the better the noise absorption.

Tips on using absorbent material

Line the inside of the acoustic enclosure with at least 50mm of a dense and appropriate sound-absorbing material. At least 50% of the enclosure should be lined with absorbent material to prevent a build-up of noise within the acoustic enclosure.

Sound-absorbing materials often need a protective facing to prevent damage or dirt build-up. Protective facings include perforated sheet metal, perforated foil wire mesh and thin plastic sheet. At least 25% of the protective cover needs to be open for the sound-absorbing material to be effective.

Windows, doors and access hatches

Ensure windows, doors and access hatches are tightly sealed.

Use double-glazing or laminated glass where higher noise reduction is required.

Line walkways or tunnels with absorbent material. This will minimise noise escaping when doors are opened or where doors cannot be used.

In control rooms, avoid a direct path to the noise source where possible. Alternatively, insulate or shield doorways and openings from the direct path of the noise, as shown in Figure 13.

Product flow and employee access

Keep openings to a minimum where objects or products have to pass through the acoustic enclosure. Line chutes or tunnels with sound-absorbing material to minimise noise escaping.

Relocate controls outside the enclosure where practicable. This will ensure the enclosure is not frequently accessed.

Ventilation of enclosures

Ventilation of plant in an enclosure may be necessary to prevent overheating. Use sound-absorbing material to line natural ventilation inlets, outlets or vents.

To reduce noise by 15dB to 30dB, the length of the absorbent lining in a silencer or duct should be at least 3 times the maximum duct diameter. Preferably, the lining should be up to 6 times the maximum duct diameter.

Insert absorptive splitters inside ducts or silencers where higher noise reduction is required. See Figures 14 and 15.

Use a quiet fan or quiet air conditioning unit where mechanical ventilation is required. Alternatively, fit a silencer outside the fan, such as absorbent lined ducts, chutes, vents or mufflers.

Use large slowly rotating fans when high-volume airflow is necessary. Large slowly rotating fans are generally quieter than small high-speed fans.