Silence of the Fans
The physics that decides whether the plant room is heard three floors away, or not at all.
ACOUSTICS
9/28/20263 min read
A rooftop air handling unit is not a quiet neighbour. Left unmanaged, it is one of the most predictable sources of occupant complaint in any building with mechanical plant above or beside occupied space — and unlike most acoustic problems, the physics of exactly how loud it will be, and exactly how much quieter it needs to become, can be calculated to the decibel before a single duct is hung.
The gap that has to be engineered shut
A typical air handling unit fan radiates a sound power level of roughly 85 to 95 dB(A) at the source. A private office beneath or beside that plant room needs a background noise level of roughly 35 to 40 dB(A). That is a required reduction of 45 to 55 decibels between source and occupied space — and on a logarithmic scale, that is not a modest engineering margin. It is the difference between a jet engine at close range and a library, and it does not happen from a few extra ceiling tiles.
Why duct velocity is the real lever
Air flowing through ductwork generates its own noise at every transition, elbow, branch take-off, and damper — and the relationship between velocity and that regenerated noise is steep: doubling air velocity increases regenerated noise by approximately 18 decibels. That single relationship is why duct sizing and velocity limits, not insulation wrapped around the outside of a duct after the fact, are the primary tool for controlling air-side HVAC noise. A duct silencer of standard 1200 mm length typically adds a further 15 to 25 decibels of insertion loss above 250 Hz, which is why silencers are specified in addition to, not instead of, correct duct sizing.
The path noise takes through the structure, not the air
The ASHRAE HVAC Applications Handbook, in its chapter on noise and vibration control, is specific about the structure-borne side of the problem: chiller and condenser noise concentrates in the 63 to 4000 Hz octave bands and depends heavily on compressor type, and rotating equipment bolted directly to a concrete slab sends vibration through the building structure the way a tuning fork transmits a strike. This is precisely why chiller plant rooms adjacent to occupied or residential space routinely require concrete inertia bases and seismic-rated spring isolators — the goal is not to quiet the compressor, which is often impossible, but to physically disconnect it from the structure that would otherwise carry that vibration into a bedroom or office three floors away.
Where the standard rating system stops looking
Noise Criteria, or NC, curves have rated background HVAC noise in buildings for roughly seventy years, and they remain in wide use because they are simple and well understood. But NC's lowest octave band starts at 63 Hz, which means it is structurally incapable of assessing the low-frequency rumble that large fans and cooling towers actually generate. A room can measure a compliant NC rating and still leave occupants reporting an unexplained, unpleasant hum. Room Criteria, or RC, curves were developed specifically to close that gap, extending assessment down to 16 Hz and adding a spectral quality descriptor that flags rumble-dominated noise an NC rating would miss entirely.
The AcouBIM approach
This is exactly the calculation AcouBIM's Mechanical Noise Control service performs on every project: in-duct, breakout, and radiated noise calculated for chillers, AHUs, pumps, and generators, with attenuators, lagging, plant room construction, and equipment layout specified to bring every affected space within its criteria — assessed against both NC and RC, not NC alone.
If your project has mechanical plant above, beside, or near occupied space, AcouBIM Engineering can calculate the reduction it actually needs before the equipment is even ordered.
Email: info@acoubim.com | Call or WhatsApp: +971 58 563 0037
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REFERENCES
• ASHRAE. ASHRAE Handbook — HVAC Applications, Chapter 49: Noise and Vibration Control. American Society of Heating, Refrigerating and Air-Conditioning Engineers.
• Air Movement and Control Association International (AMCA). Publication 300, "Reverberant Room Method for Sound Testing of Fans" — the industry test standard for fan sound power ratings.
• Blazier, W.E. "RC Mark II: A Refined Procedure for Rating the Noise of Heating, Ventilating and Air-Conditioning (HVAC) Systems in Buildings." Noise Control Engineering Journal, 1997 — origin of the Room Criteria (RC) rating system.
• Beranek, L.L. "Revised Criteria for Noise in Buildings." Noise Control, 1957 — origin of the Noise Criteria (NC) curve system still referenced today.