From Sound Power to Sound at the Fence: Axial Fan Noise Over Distance

By the NextAir Systems engineering team. Last reviewed October 2026.

A fan datasheet gives a sound power level in decibels. A planning condition gives a limit in decibels at the property boundary. Between the two sits a simple calculation that is often skipped, and the skipped calculation is behind a great many noise complaints.

Sound power is a property of the source. Sound pressure is what a person hears, and it depends on distance and surroundings. Here is how to convert one to the other for a fan in the open, what happens when you add more fans, and why there is no easy WHO number to compare it with.

The short version

• Sound pressure level at distance r from a source on the ground, radiating into a half-space, is Lp = Lw − 20 log₁₀(r) − 8 dB, before any attenuation from barriers, air or ground.
• For a fan with a sound power level of 105 dB at a boundary 100 m away, that gives 57.0 dB. To reach 45 dB in free field, the boundary would have to be about 398 m away.
• Each doubling of the number of identical fans adds 3 dB.
• WHO’s 2018 noise guidelines cover transport, wind turbines and leisure noise, and make no recommendations for industrial sources, so the limit you design to comes from your local planning conditions.^[1]

Power versus pressure

The sound power level, Lw, is the total acoustic energy the fan radiates, in decibels relative to a reference. It does not depend on where you stand. The sound pressure level, Lp, is what a microphone or an ear picks up at a given point, and it falls with distance because the same energy spreads over a larger area. This is why a datasheet that says “85 dB(A)” without a distance is not comparable with one that gives sound power. Our guide to axial fan noise explains the sources and the reduction methods. This article is about distance.

The free-field calculation

For a source on the ground radiating into a half-space, the sound energy spreads over a hemisphere of area 2πr². The result is Lp = Lw − 20 log₁₀(r) − 8. For a source in the open air, radiating in all directions, the constant is 11 instead of 8. Each tenfold increase in distance cuts the level by 20 dB, and each doubling by 6 dB. The formula ignores barriers, ground absorption, atmospheric absorption, wind and temperature gradients. In real conditions those usually reduce the level, though wind and temperature inversions can raise it.

Fan sound power Lw (dB)	10 m	25 m	50 m	100 m	200 m	400 m
95	67.0	59.0	53.0	47.0	41.0	35.0
100	72.0	64.0	58.0	52.0	46.0	40.0
105	77.0	69.0	63.0	57.0	51.0	45.0
110	82.0	74.0	68.0	62.0	56.0	50.0

All figures are in dB and are calculated from the formula, assuming a single point source on the ground with no other attenuation. They are not A-weighted unless the sound power input is.

Worked example. A fan with Lw = 105 dB sits 50 m from a boundary. Lp = 105 − 20 log₁₀(50) − 8 = 105 − 34.0 − 8 = 63.0 dB at the fence. If the planning condition is 55 dB, you are 8 dB over and need that much attenuation, from a silencer, a barrier, a lower speed or a quieter fan. At 45 dB the free-field distance would be about 398 m, which is why fans near houses almost always need treatment.

Adding fans

Identical uncorrelated sources add on an energy basis, so the level rises by 10 log₁₀ N for N fans:

Number of identical fans	Increase over one fan
1	+0 dB
2	+3 dB
4	+6 dB
8	+9 dB

Eight small fans in a row are 9 dB louder than one of them. If you are choosing between one large fan and several small ones, the sound power of each, and the count, both matter, as discussed in one large fan or several small ones. A fan running slower is also quieter, as shown in how speed changes noise and power.

What to compare it with

People often reach for WHO guidelines. The 2018 WHO Environmental Noise Guidelines for the European Region give recommended levels for road traffic (below 53 dB Lden and 45 dB Lnight), railways, aircraft, wind turbines and leisure noise.^[1][2] They make no recommendations for other sources such as occupational noise,^[1] and a factory fan is not covered. Use them as a sense of what communities consider disturbing, not as a limit for your fan.

The limit that applies to you is almost always in your planning permission or local environmental regulation, usually as a level at the nearest dwelling at night, often with an allowance for tonal character. Find it before you specify the fan.

Practical steps

1. Ask the supplier for the sound power level, in octave bands, at the duty point, not only a sound pressure at 1 m.
2. Calculate the level at the boundary using the formula and your distance.
3. Add the penalty for the number of fans, and for tonal character if the fan has a distinct pitch.
4. If you are over the limit, compare the options: a slower fan, a larger diameter, an inlet or outlet silencer, a barrier or an acoustic enclosure.
5. Verify with a measurement after commissioning, at the boundary, under the operating conditions named in the planning condition.

Where this fits with NextAir

NextAir provides sound power data for its axial fans and can add silencers and acoustic treatment where the boundary calls for it. Send the distance to the nearest receptor and the applicable limit to our axial fan manufacturer team. Our axial fan guide covers the range, and cooling towers and heat exchangers are a typical noise-sensitive application.

Sources

1. WBM (summary of WHO Environmental Noise Guidelines for the European Region, 2018). Summary of the WHO Environmental Noise Guidelines for the European Region (2018).
2. Queen Mary University of London (on the WHO 2018 noise guidelines). Reduce traffic noise to prevent heart disease.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

Frequently Asked Questions