Air handling unit noise can significantly impact building occupants and neighbours. Excessive noise reduces comfort, impairs concentration, and may violate regulatory limits. Understanding noise sources and reduction techniques enables creation of quiet, comfortable environments.
Understanding AHU Noise
Noise Sources
Multiple components within AHUs generate noise. Fans produce both broadband noise from turbulence and tonal noise at blade passing frequencies. Motors contribute mechanical noise. Airflow through dampers, coils, and filters creates turbulence noise.
Different components dominate at different frequencies. Fan noise typically dominates at low frequencies whilst airflow turbulence contributes higher frequency noise. Comprehensive noise control addresses the full frequency range.
Noise Transmission Paths
Noise reaches occupied spaces through various paths. Ductwork carries noise from AHUs directly to room outlets. Structure-borne vibration transmits through building elements. Breakout through duct walls radiates noise into adjacent spaces.
Effective noise control addresses all significant transmission paths. Treating only obvious paths may provide limited improvement if other paths remain uncontrolled.
Noise Criteria
Various metrics quantify acceptable noise levels. Noise Rating (NR) curves specify limits across frequency bands, commonly NR35-40 for offices. NC (Noise Criteria) curves provide similar frequency-weighted limits common in North American standards.
Understanding applicable criteria guides noise control measures. Different building uses have different noise sensitivities as recording studios demand much lower noise than warehouses.
Fan Selection and Operation
Fan Type Selection
Different fan types have different noise characteristics. Forward-curved fans generate lower noise but operate less efficiently. Backward-curved fans are more efficient but potentially noisier. Plenum fans within insulated housings reduce noise transmission.
Selecting appropriate fan types balances efficiency, noise, and cost requirements. Sometimes accepting slightly lower efficiency provides worthwhile noise reduction.
Speed Reduction
Fan noise increases dramatically with speed, typically varying with the fifth power of speed. Reducing fan speed, when system design permits, provides substantial noise reduction with the bonus of energy savings.
Variable speed drives enable operation at minimum necessary speed rather than constant high-speed operation. Properly designed systems with low pressure drop allow reduced fan speeds whilst meeting airflow requirements.
Attenuators
Silencer Types
Duct-mounted attenuators absorb sound energy as air passes through. Splitter silencers divide airflow through parallel passages lined with absorptive material. Tubular silencers suit round ducts with absorptive lining or central pod.
Attenuator selection considers required attenuation across frequency bands, available length, and acceptable pressure drop. Different configurations provide varying performance characteristics.
Attenuator Positioning
Attenuator positioning affects both effectiveness and system efficiency. Attenuators immediately downstream of fans address noise before it enters ductwork. Terminal attenuators near room outlets control noise that has bypassed upstream treatment.
Sufficient straight duct upstream and downstream of attenuators ensures proper performance. Turbulent conditions from elbows or branches compromise attenuator effectiveness.
Cabinet Design
Panel Construction
AHU cabinet panels should prevent noise breakout. Double-skin construction with internal insulation provides both thermal and acoustic performance. Panel mass and stiffness affect transmission loss at different frequencies.
Quality cabinet construction includes sealed joints, gaskets at access doors, and attention to potential noise leakage paths. Poor cabinet integrity compromises internal acoustic treatment.
Internal Lining
Absorptive lining within AHU cabinets reduces internal noise levels and prevents reverberant buildup. Mineral wool or similar materials provide cost-effective absorption. Facing materials protect insulation from airflow erosion.
Vibration Isolation
Mounting Systems
Flexible mounts isolate AHU vibration from building structure, preventing structure-borne noise transmission. Spring or rubber mounts suit different applications depending on vibration frequencies and loads.
Proper mount selection considers both static load support and dynamic isolation performance. Inappropriate mounts may transmit rather than isolate vibration.
Flexible Connections
Flexible connections between AHUs and ductwork prevent vibration transmission into duct systems. Canvas or rubber connectors accommodate movement whilst breaking the vibration path.
Duct Design
Velocity Control
Lower duct velocities produce less aerodynamic noise. Appropriately sized ductwork maintains low velocities throughout the system. Localised high velocities at restrictions create noise problems.
System design should minimise sharp transitions, abrupt area changes, and poorly designed fittings that create turbulence and noise.
Working with Acoustic Specialists
Complex noise control projects benefit from acoustic expertise. Specialists measure existing conditions, predict noise levels, and design effective treatments. Their involvement ensures noise targets are achieved efficiently.
i-Flow Technologies incorporates acoustic design into our air handling units, providing quiet operation suitable for noise-sensitive applications. We address noise control during design rather than attempting retrofit corrections to noisy equipment.
Conclusion
AHU noise control requires attention to multiple sources and transmission paths. Fan selection, attenuators, cabinet design, vibration isolation, and ductwork all contribute to overall noise performance. Understanding noise mechanisms enables effective treatment creating comfortable, productive environments.
