Controlling compressed air 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.

Compressed air use

Compressed air is widely used in industry. Its uses include:

• operating pneumatic tools, machines and valves
• cleaning surfaces
• blowing metal swarf off machined parts
• cooling, drying and moving parts in production processes

Sources of noise

There are two main sources of compressed air noise. One is the compressor itself. The other is noise caused by air turbulence or shearing noise as compressed air comes out of a nozzle at high speed or is suddenly released through a valve. The noise may be continuous, intermittent or impulsive.

The level of turbulence and therefore noise relates to the speed of the air jet and the speed of the ambient air around it. Introducing an extra lower-speed airstream alongside the main jet can reduce air speed difference between them which causes less turbulence and hence less noise.

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.

Controlling compressor noise

Common types of compressors include piston and screw types. Screw type compressors are generally quieter and register about 70 dB(A). When buying a compressor, request noise level data from the supplier or manufacturer.

Suitable risk controls for controlling compressor noise may include:

• buying a quieter compressor, for example, screw type
• modifying the existing compressor, if reasonably practicable to do so. Consult with the manufacturer or an engineer to determine if this is possible or reasonably practicable
• moving the compressor outside or into another room not normally occupied by employees
• moving the compressor further away from employees but away from hard, reflective, untreated surfaces such as brick or concrete walls. Doubling the distance from the source can decrease noise levels by up to 6 dB(A) in an open field
• installing the compressor in an enclosure lined with high-density sound absorbent material such as foam, glass wool or rock-wool
• using an acoustic barrier lined with sound-absorbing material where a noise reduction of about 5 dB is required.
• installing isolation mounts on the compressor or attached parts to reduce vibration

Controlling continuous compressed air noise

Suitable risk controls for continuous air jet noise include:

• eliminating or reducing the use of compressed air for cleaning. Use a vacuum cleaner or rubber squeegee instead
• using quiet compressed air guns. to achieve 8 to 20 dB(A) reduction, depending on the design and use, see figure 2
• fitting quiet compressed air nozzles which can reduce noise by about 10 to 15 dB(A), see figure 3
• lowering the nozzle air speed. Halving the speed can reduce noise by up to 20 dB(A)
• using expansion chambers in a duct or tubing, see figure 4. This is particularly useful for low-frequency noise

Controlling pneumatic impulse noise

Pneumatic impulse noise can reach levels of 100 dB(A) or more. Various types of silencers are available which can reduce noise by about 10 to 30 dB(A), see figures 3 and 5. Noise reduction data for silencers should be available from suppliers or manufacturers.

If back pressure is an issue, fit a larger coupling and silencer/nozzle.

If clogging is an issue, use a straight-through silencer/nozzle that cannot clog and does not create back-pressure. Where there are multiple sources of impulse air, direct the exhaust air into one manifold or box and fit the manifold or box with one silencer instead of multiple silencers. This approach can reduce noise by about 25 dB(A).

Impulse air exhausts can also be re-directed outside through large-diameter tubing.