Controlling fan and ventilation 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.

Types of fans

Fans and ventilation systems in workplaces perform a variety of tasks, including:

• general ventilation
• extraction of airborne contaminants
• blowers
• dryers

The two most common types of fans are axial fans and centrifugal fans as shown in figures 1 and 2.

Axial fans have blades like a propeller and draw air straight through the blades.

Centrifugal fans draw air into the center of the fan and exhaust it at a 90-degree angle.

There are also mixed-type fans that incorporate centrifugal and axial design features.

Axial fans are for use in low-pressure high-volume applications. Centrifugal fans are for high-pressure low-volume applications.

Fans can be installed:

• inline, for example, ducting at both ends
• as inlet or outlet fans on a wall, roof or acoustic enclosure

The noise from a fan is due to the turbulence of the air around the fan's blades and housing. There is also mechanical vibration noise from parts of the fan, such as the motor, bearings and belts. This noise can be transmitted through ductwork.

Air turbulence can also occur because of:

• abrupt changes within ductwork and associated fittings such as dampers, obstacles, louvres etc.
• poor maintenance, which can lead to increased turbulence and mechanical vibration noise

Controlling fan and ventilation noise

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.

Reduce turbulence and vibration

Reducing air turbulence and mechanical vibration can lower noise levels. To reduce air turbulence:

• use quieter fans
• reduce fan speed
• improve the ductwork design and layout of the system
• use silencers
• use acoustic treatments on ductwork
• installing isolation mounts and connectors
• use damping
• ensure regular maintenance

Quieter fans

There are various axial and centrifugal fan designs. Each has different performance characteristics for different applications. Noise-level data for fans should be available from suppliers or manufacturers.

Suitable controls for fan noise include:

• using a quieter fan, for example, a backward-curved blade or different profiles/shapes
• modifying the existing fan, for example, blade, air inlet smoothing
• replacing radial/straight fan blades with backward-curved blades which produce lower pressure

Backward-curved fan blades usually produce lower pressure and emit lower noise levels. Replacing straight fan blades with backward-curved blades can cut noise by approximately 8 to 10 dB.

Fan speed

The speed, profile and shape of the blade affects fan noise. A small reduction in fan speed can cause a large reduction in fan noise. Fan speed can be reduced by:

• changing control systems, for example, variable-speed drive
• changing pulley sizes
• re-setting dampers

Reducing fan speed also reduces the volume of air. It may be necessary to use more blades on the fan to compensate for the reduced volume of air.

Ductwork design and system layout

Smooth changes in ductwork can reduce noise from air turbulence. To reduce air turbulence noise:

• avoid sharp and sudden bends in ducts
• use turning vanes in bends to smooth air flow
• do not place fans too close to sharp bends or objects such as dampers, louvres, attenuators, vanes, etc

Placing a fan close to turbulent air also increases turbulence and the noise produced by the fan. Examples of good and bad ventilation design and layout appear below.

Example 1 - Smooth airflow transition

Example 2 - Dampers further away from splitters

Example 3 - Attenuator splitters

Example 4 - Attenuators and fans

Example 5 - Silencers and fans

Example 6 - Fan inlet

Example 7 - Bends and takeoffs

Lined ductwork or silencers

Absorptive and dissipative silencers are used in heating, ventilation and air-conditioning duct systems to deal with a wide range of noise frequencies. The silencers can be cylindrical, rectangular or curved. They are lined with sound-absorbing material such as, foam, glass wool or rock wool.

Absorptive liners are often protected with perforated metal or clot and can reduce noise levels by about 10 to 20 dB(A), see figure 4.

Silencers can be in the form of:

• lined ductwork
• fabricated silencers, either inline, outlet or inlet
• acoustic louvres
• unused absorbent lined cavities, for example, in walls or roofs

As a general guide, the length of the absorbent duct or silencer liner should be at least 3 times the diameter of the duct.

Isolation mounts

Isolation mounts can reduce fan and motor vibrations that travel through supporting structures such as walls, the roof and ceiling and re-radiate as noise.

Similarly, flexible connections should be used to connect the fan casing to the ductwork.

Damping

Damping, also known as sound deadening, can reduce noise in ventilation systems. Damping involves coating or sticking a viscoelastic or bituminous type of material to thin, vibrating surfaces. The coating reduces the surface’s tendency to amplify vibrations and radiate noise.

Damping vibrations in the ductwork and fan casing can reduce noise, as can a thicker fan casing.

Maintenance

Maintaining ventilation systems is necessary to ensure efficient and quieter performance. Inspection and maintenance should include:

• the motor
• shaft
• bearings
• belts
• tightening loose parts or connections