HVAC Noise Calculator
Estimate HVAC duct noise levels in a room from fan data, duct geometry, and fittings. Calculates sound power, duct attenuation, breakout noise, and NC rating.
About this calculator
Fan Static Pressure has the largest effect on this calculator's headline output, Fan Sound Power, well ahead of Airflow -- across the full 0.1-10 in. w.g. range the pressure term can swing sound power by roughly 40 dB, versus about 30 dB for airflow across its own full 100-100,000 CFM range, and that gap holds up close to the calculator's defaults too. Duct Width and Duct Height, by contrast, never touch Fan Sound Power at all: the fan's estimated output is a property of the fan curve (pressure and flow), not the ductwork carrying it, so widening or narrowing the duct changes nothing about how loud the fan itself is -- it only changes how much of that noise survives the trip to the room, through the End Reflection term inside Duct Attenuation. Duct Length is the dominant driver of Duct Attenuation, the noise lost along the run: every added foot of duct subtracts a small fixed increment (a broadband 0.05 dB/ft for unlined rectangular duct), so a long commercial run still attenuates more than a handful of 90-degree elbows can, even though each elbow adds its own fixed 2 dB.
That attenuation is gradual enough, though, that at this calculator's other default values, Duct Length's own full 1-500 ft range only pulls the Estimated NC Rating down from the low 50s to the mid 20s -- it never reaches this calculator's NC-15 floor (the quietest rating this calculator reports) within the declared range; reaching that floor takes a duct roughly twice the maximum declared length, or a shorter long run combined with a quieter fan (low Fan Static Pressure and Airflow) and a large Room Volume. Duct Breakout Noise moves in the opposite direction from Duct Attenuation on Duct Length: a longer run means more radiating duct surface working against the same fixed internal cross-section, so Duct Breakout Noise rises monotonically with Duct Length across the full declared range, and it falls as Duct Width (or Height) grows, since a bigger cross-section shrinks the ratio of radiating surface to airflow-carrying area for the same run. Room Volume works in the opposite direction from the duct-side inputs on Room Sound Level: it doesn't touch the sound power leaving the duct at all, it just spreads that same power over a bigger or smaller room, so a larger room reads quieter than a smaller one for otherwise identical ductwork across its entire declared range.
Inputs
Results
Fan sound power (dB)
74
Figures current as of 2007. Source: AHRI Standard 885-2008, Table D6, "Rectangular Sheet Metal Duct, Lined & Unlined" -- reprinted in the ASHRAE Handbook--HVAC Applications, Ch. 47, "Sound and Vibration Control" (2007 ed.; renumbered in later editions)
How to Use This Calculator
- Enter Fan static pressure (in. w.g.), Airflow (CFM), and Duct width (in).
- Set Duct height (in), Duct length (ft), and Number of elbows.
- Adjust Room volume (ft³) as needed.
- Review the Fan sound power (dB) result.
- Use Duct attenuation (dB) and Room sound level (dB) to inform your decision.
How the result changes with Fan static pressure (in. w.g.)
| Fan static pressure (in. w.g.) | Fan sound power (dB) |
|---|---|
| 1 | 68 |
| 1.5 | 71.5 |
| 3 | 77.6 |
| 5 | 82 |
What each input means
- Fan static pressure (in. w.g.)
- Fan total static pressure in inches of water gauge. Higher pressure = more noise.
- Airflow (CFM)
- Fan airflow in cubic feet per minute.
- Duct width (in)
- Internal width of the rectangular duct in inches.
- Duct height (in)
- Internal height of the rectangular duct in inches.
- Duct length (ft)
- Total length of ductwork from fan to room diffuser in feet.
- Number of elbows
- Number of 90-degree elbows in the duct run.
- Room volume (ft³)
- Volume of the receiving room in cubic feet (L × W × H).
What each result means
- Fan sound power (dB)
- Estimated broadband fan sound power level at the fan discharge.
- Duct attenuation (dB)
- Total noise reduction from duct length, elbows, and end reflection.
- Room sound level (dB)
- Estimated sound pressure level at the receiver location in the room.
- Duct breakout noise (dB)
- Sound radiating through duct walls into adjacent spaces.
- Estimated NC rating
- Noise Criteria rating estimate. NC-25 = very quiet, NC-35 = office, NC-45+ = loud.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersFan static pressure (in. w.g.) = 2, Airflow (CFM) = 2000, Duct width (in) = 12, Duct height (in) = 12 = 7 input(s) provided
- Calculate Fan sound powerFan sound power = 10 +74 = 74
- Calculate Duct attenuationDuct attenuation = ductAttenuation + elbowAttenuation + endReflection7 = 7
- Calculate Room sound levelRoom sound level = LwAtRoom - 10 * log10(roomVolume) + 2557.3 = 57.3
Figures and sources
- Rectangular sheet-metal duct breakout noise relationship (Lw,out = Lw,in - TLout + 10*log10(Ar/Ai)) (2007) — AHRI Standard 885-2008, Table D6, "Rectangular Sheet Metal Duct, Lined & Unlined" -- reprinted in the ASHRAE Handbook--HVAC Applications, Ch. 47, "Sound and Vibration Control" (2007 ed.; renumbered in later editions)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Does making the duct bigger reduce fan noise?
No -- Duct Width and Duct Height never change the Fan Sound Power result; that output only responds to Fan Static Pressure and Airflow. Duct dimensions affect noise later in the calculation, through the End Reflection term inside Duct Attenuation, not the fan's own generated sound power.
What has the biggest effect on how loud the fan sounds?
Fan Static Pressure. Across its full 0.1-10 in. w.g. range it can move Fan Sound Power by roughly 40 dB, more than Airflow's roughly 30 dB swing across its own full 100-100,000 CFM range, and Fan Static Pressure stays the larger factor even near the calculator's default values.
Does a longer duct run mean more breakout noise, and does the estimated NC rating keep falling forever?
Breakout: yes, more. Duct Breakout Noise rises monotonically with Duct Length across the full declared range: a longer run has more duct wall surface radiating sound into the surrounding space, even though the sound power flowing through the duct's fixed cross-section hasn't changed. The relationship uses the ratio of that radiating surface area (duct perimeter times length) to the duct's internal cross-sectional area, per the AHRI Standard 885-2008 rectangular-duct breakout relationship reprinted in the ASHRAE Handbook--HVAC Applications' Sound and Vibration Control chapter -- not duct length by itself. NC rating: not indefinitely, but not within this calculator's declared 1-500 ft range at its other default values either -- Duct Length alone only pulls the Estimated NC Rating down from the low 50s (at 1 ft) to the mid 20s (at 500 ft), never reaching the NC-15 floor this calculator reports for its quietest ratings. Reaching that floor takes either a duct roughly twice the maximum declared length, or a shorter long run combined with other quiet-fan inputs (low Fan Static Pressure, low Airflow) and a large Room Volume to spread the sound over.
Why does the number of elbows matter less than duct length for total attenuation?
Duct Length is the dominant term in Duct Attenuation across its full 1-500 ft range. Even a handful of 90-degree elbows -- each adding a fixed 2 dB -- can't match the cumulative effect of a long run, since attenuation accrues per foot of ductwork rather than per fitting.
Does a bigger room always sound quieter for the same ductwork?
Yes, across the whole declared range. Room Volume doesn't change how much sound power leaves the duct -- it only changes how that fixed amount of power is spread across the receiving space, so raising Room Volume steadily lowers the estimated Room Sound Level with no reversal.
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