Air handling units generate noise that can disturb building occupants or neighbours. Understanding noise sources and reduction techniques enables quieter installations.
Noise Sources in AHUs
Fan Noise
The primary noise source in most AHUs. Fans generate noise through blade passing, turbulence, and motor noise. Larger fans running slowly are quieter than smaller fans running fast.
Airflow Noise
Air moving through ducts, dampers, and fittings generates turbulence noise. Higher velocities mean more noise. Poorly designed transitions create additional turbulence.
Component Noise
Dampers, coils, filters, and other components create noise as air passes through. This is usually lower than fan noise but contributes to total levels.
Breakout Noise
Noise transmitting through ductwork walls into surrounding spaces. Thin duct walls and unlined ductwork worsen breakout.
Vibration Transmission
Fan vibration transmitted through structure-borne paths. Inadequate isolation allows vibration to travel through buildings, causing noise remote from the AHU.
Reduction Strategies
Fan Selection
Select fans for quiet operation at the required duty point. Avoid running fans near surge or in inefficient regions of their performance curve. Consider plug fans with inherently lower noise than housed centrifugal alternatives.
Speed Reduction
Running fans slower dramatically reduces noise. Slightly larger fans running at lower speed are often quieter than smaller fans at higher speed for the same airflow.
Attenuators
Silencers in ductwork reduce airborne noise. Various types suit different situations:
- Splitter silencers for high attenuation
- Lined duct sections for moderate attenuation
- Acoustic pods/bends for compact installations
Attenuators create pressure drop. Factor this into fan selection.
Duct Design
Smooth transitions, appropriate velocities, and radius bends reduce airflow-generated noise. Avoid abrupt changes, high velocities, and sharp turns.
Vibration Isolation
Mount AHUs on anti-vibration isolators. Use flexible connections between AHU and ductwork. Isolate supports for connected ductwork near the unit.
Acoustic Enclosures
Where plant room breakout is a problem, acoustic enclosures contain noise. Ensure adequate ventilation cooling for equipment within enclosures.
Duct Lagging
Acoustic lagging on ductwork reduces breakout. Particularly important where ducts pass through sensitive areas.
Location
Locating AHUs away from sensitive areas provides distance attenuation. Plant room position in the building matters for overall noise impact.
Specification and Design
Address noise during design, not after installation:
Establish Criteria
Define acceptable noise levels for occupied spaces and any external limits. Work backwards to determine allowable plant room and ductwork noise.
Calculate Noise Paths
Trace noise paths from source to receiver. Identify where attenuation is needed and how much.
Specify Equipment
Specify maximum noise levels for AHUs. Manufacturers can select components to meet noise requirements.
Detail Attenuation
Include attenuators, lagging, and isolation in design. Don’t assume it can be added later.
Troubleshooting Existing Noise Problems
For AHUs that are already too noisy:
Identify the Source
Is it fan noise, airflow noise, or vibration? Different sources need different solutions.
Check Operation
Is the fan running at designed speed? Is airflow as designed? Operating away from design conditions may increase noise.
Add Attenuation
Retrofit attenuators may be possible if space allows. Lined duct sections can be added relatively easily.
Improve Isolation
Upgrading mounts and flexible connections reduces structure-borne noise.
Speed Reduction
If the system is oversized, reducing fan speed with a VSD may be the simplest solution.
Our Approach
i-Flow considers acoustics in every AHU design. We select components for quiet operation and can advise on attenuation requirements. Contact us to discuss noise-sensitive applications.
