Three questions come up on every quiet-fan project: what do six fans together sound like, what will a fan rated "65 dB(A) sound power" read on a meter at 3 m, and how much quieter does it get if I slow it down 20 %? This tool answers all three with the standard acoustic formulas, and compares the result to room noise limits.
No signup · Works on mobile · Formulas shown below · Updated 2026-08-23
Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.
The formulas
Adding sources Ltotal = 10·log₁₀( Σ 10Li/10 ) // n identical: +10·log n → 2 fans +3 dB, 4 fans +6 dB, 10 fans +10 dB Power → pressure Lp = Lw + 10·log₁₀( Q ÷ 4πr² ) // Q = 1 free field, 2 on a surface, 4 at an edge, 8 in a corner Distance −6 dB per doubling of distance (point source, outdoors) Speed change ΔL ≈ 50·log₁₀( N₂ ÷ N₁ ) // −20 % speed ≈ −4.8 dB; −50 % ≈ −15 dB Fan size (same duty) ΔL ≈ 70·log(D₂/D₁) at equal rpm — but a bigger, slower fan for the same airflow is usually quieter
Sound power (Lw) is a property of the fan — how much acoustic energy it emits — and is what serious datasheets quote (ISO 13347, AMCA 300). Sound pressure (Lp) is what a meter reads at a place, and depends on distance and surroundings. Comparing an "Lp at 1 m" fan with an "Lw" fan directly is the most common mistake in fan selection; at 1 m on a wall, Lp ≈ Lw − 8 dB.
Typical noise limits and fan levels
Situation
Typical limit dB(A)
Bedroom, night (WHO)
30
Living room, hotel room, library
35
Office, classroom (NC 30–35)
40
Residential site boundary, night
40–45
Residential site boundary, day
50–55
Restaurant, retail, gym
45–55
Plant room, workshop
70–80
Hearing protection required (8 h)
85
Fan
Typical LwA
120 mm DC fan, 2,000 rpm
40–50
Bathroom exhaust fan 100 mm
45–55
250 mm EC axial, 1,800 rpm
60–68
450 mm EC axial condenser fan, 1,000 rpm
68–75
500 mm EC plug fan at 800 Pa
78–85
630 mm axial, 1,400 rpm
82–90
Industrial centrifugal 15 kW
90–100
Ten ways to make a fan installation quieter, ranked
Slow it down — 50·log rule; the biggest lever, free with EC fans.
Bigger fan, slower for the same airflow.
Lower system pressure (bigger ducts, fewer bends) so the fan can run slower.
Clean inflow: inlet cone, 1 D clearance, no struts upstream.
Backward-curved or airfoil impeller instead of forward-curved/propeller at the same duty.
Distance and orientation: −6 dB per doubling; point the discharge away from receivers.
Silencers / lined duct: 10–25 dB in mid/high frequencies, little at < 125 Hz.
Acoustic enclosure / louvre: 10–20 dB, watch the pressure drop.
Night setback: run at 60 % after 22:00 — −11 dB, and 78 % less energy.
Frequently Asked Questions
How do I add the noise of two fans?
Two equal sources add 3 dB: 60 dB + 60 dB = 63 dB. In general L = 10·log(10^(L1/10) + 10^(L2/10)). Four identical fans add 6 dB, ten add 10 dB. A source more than 10 dB quieter than another adds almost nothing.
What is the difference between sound power and sound pressure?
Sound power (LwA, dB) is the total acoustic energy the fan emits and does not depend on where you stand. Sound pressure (LpA, dB(A)) is what is heard or measured at a location; it falls with distance and rises in reflective surroundings. Outdoors on a wall at 1 m, LpA ≈ LwA − 8 dB; at 3 m ≈ LwA − 17.5 dB.
How much quieter is a fan at lower speed?
Roughly 50·log(speed ratio): 20 % slower ≈ −5 dB, 30 % slower ≈ −8 dB, half speed ≈ −15 dB. Airflow drops in proportion to speed, so this works when the fan is oversized or at part load.
How much does distance reduce fan noise?
Outdoors from a compact source, 6 dB for every doubling of distance: 70 dB(A) at 1 m becomes 64 at 2 m, 58 at 4 m, 52 at 8 m. Indoors the reverberant field limits the reduction after a few metres.
How many dB(A) is a quiet fan?
For a room: under 35 dB(A) sound pressure at 1.5–3 m is "quiet", under 30 is near-inaudible at night. In fan datasheets this corresponds to a sound power of about 45–50 dB(A) for a small exhaust fan and 60–65 for a 300 mm EC axial at moderate speed.
Link to or Cite This Tool
This calculator is free to reference in articles, forums, specifications and course material. Please credit it with a link:
<a href="https://www.longwellfans.com/resources/fan-noise-calculator/">Fan Noise & Decibel (dB) Calculator</a> by LONGWELL Fans
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