| 📋 Article Summary — Engineering Noise Controls | |
|---|---|
| Key Fact 1 | Engineering controls must be considered before relying on hearing protection — PPE is the last resort in the hierarchy of controls for noise |
| Key Fact 2 | Doubling the distance from a noise source (in a free field) reduces sound level by approximately 6 dB — simple but often overlooked |
| Key Fact 3 | A 3 dB noise reduction (achievable by halving the number of identical noise sources running simultaneously) halves the noise dose |
| Read Time | 10 min |
Why Engineering Controls Are the Gold Standard for Noise
Hearing protection devices are a necessary fallback when noise cannot be engineered to acceptable levels, but they are the least reliable control in the hierarchy. Engineering controls — modifications to the noise source, the propagation path, or the receiver environment — provide permanent noise reduction that does not depend on worker behaviour, fitting technique, or consistent compliance. Once engineering controls are installed and verified, noise reduction occurs for every worker, every shift, automatically.
Australian WHS Regulations require that PCBUs first eliminate and then minimise noise through engineering and administrative controls before relying on HPD. This section explains the practical toolkit of engineering noise controls available to Australian employers and facilities engineers.
Source Controls: Reducing Noise at Its Origin
Substitution with Quieter Equipment
The most effective source control is purchasing quieter equipment in the first place. Many modern machines produce substantially less noise than older models through improvements in design, balancing, and acoustic treatment. When specifying new plant and equipment, noise emission levels (typically specified as sound power level, LW, in dB(A)) should be part of the procurement specification.
Under AS/NZS 1269.2, PCBUs should consider noise emission data when purchasing equipment. Some jurisdictions’ WHS Regulations specifically require consideration of noise in procurement decisions. Requiring suppliers to provide third-party verified noise emission data, tested to ISO 3744 or equivalent, is a practical step.
Maintenance and Lubrication
Many machines become noisier as they age and wear. Worn bearings, loose panels, unbalanced rotating components, and inadequate lubrication all increase radiated noise. Regular maintenance schedules with specific attention to noise-generating failure modes can maintain noise levels close to the original specification.
Process Modification
Changing the process to reduce impact energy, drop height, or machine speed can reduce noise significantly. Examples:
- Reducing the height from which parts are dropped into bins (impact noise reduction)
- Reducing conveyor speed to reduce impact noise from product collisions
- Using rubber-lined chutes and bins instead of steel
- Replacing pneumatic ejectors with mechanical part ejection systems
- Replacing impact operations (riveting, hammering) with press operations
Path Controls: Interrupting Sound Propagation
Distance
In a free field (outdoors, away from reflective surfaces), sound level decreases by 6 dB for every doubling of distance from a point source. Moving a worker from 1 m to 4 m from a noise source achieves approximately 12 dB reduction. While often not practically achievable through relocation alone, distance is a useful tool in facility layout planning — separating noisy processes from office or other occupied areas.
Acoustic Enclosures
Enclosing a noise source in an acoustically lined cabinet reduces the radiated noise level in the surrounding space. The effectiveness of an enclosure depends on:
- Transmission loss (TL): The mass, stiffness, and construction of the enclosure panels. For low frequencies, mass law applies: TL increases by 6 dB for each doubling of panel mass. Heavy concrete or steel enclosures achieve high TL; lightweight panels achieve much less.
- Internal absorption: Without acoustic lining on internal surfaces, sound builds up inside the enclosure and leaks out through any gaps at increased level. Mineral wool or foam lining converts acoustic energy to heat through friction.
- Air gaps and penetrations: Any gap, pipe penetration, or ventilation opening significantly reduces the effective TL of an enclosure. “Acoustic leaks” can dominate enclosure performance — a gap of 1% of the total panel area can reduce TL by up to 20 dB.
Barriers and Screens
Acoustic barriers placed between the noise source and the receiver can reduce direct sound by 5–15 dB (the insertion loss depends on the barrier geometry, frequency, and diffraction). Barriers are more effective at high frequencies and for workers in the shadow zone of the barrier. They provide no benefit for reflected (reverberant) sound in enclosed spaces.
Room Acoustics and Absorption
In reverberant environments (hard floors, concrete walls, metal ceilings), sound energy accumulates in the room and raises the ambient noise level for all workers regardless of their distance from the source. Adding acoustic absorption (acoustic ceiling tiles, wall panels, hanging absorbers) reduces the reverberant field and can reduce ambient noise by 3–6 dB in well-designed applications — modest but meaningful.
Vibration Isolation and Damping
Many machines radiate noise through structural vibration transmitted to floors, walls, and pipework. Two distinct control strategies address vibration-radiated noise:
- Vibration isolation: Mounting machines on anti-vibration mounts (rubber, spring, air mounts) reduces vibration transmission to the support structure. Effective isolation requires that the isolation mount natural frequency is well below the dominant vibration frequency of the machine — typically requiring soft, compliant mounts for low-speed machinery.
- Vibration damping: Applying constrained-layer damping (bitumen-backed material, proprietary damping compounds) to panels that vibrate and radiate noise. Particularly effective for thin metal panels, machine guards, and conveyor surfaces that act as radiating surfaces.
Silencers for Airborne Noise Sources
Pneumatic exhausts, fans, air jets, and HVAC duct noise are commonly addressed with silencers:
- Reactive silencers: Use expansion chambers and flow restrictions to reduce noise through interference — suited to low-frequency tonal noise from exhaust pulses
- Absorptive silencers: Lined ducts or splitter silencers that attenuate sound through absorption material — suited to broadband high-frequency fan and airflow noise
- Duct lagging: Wrapping ventilation ducts in mineral wool and dense loaded vinyl reduces breakout noise from the duct walls
Frequently Asked Questions
We installed a noise enclosure but workers keep leaving the access panels open because the machine overheats — how do we fix this?
Machine heat rejection is one of the most common reasons enclosure performance fails in practice. The enclosure design must include a ventilation strategy that removes heat without creating significant acoustic leaks — typically through attenuated ventilation paths (acoustic baffles in the inlet and exhaust, or fan-driven cooling through lined ducts). A noise enclosure without adequate heat management will be defeated by operators who open it for ventilation. Involving production and maintenance staff in the enclosure design process helps ensure practical obstacles are addressed at the design stage.