Axial Fan Noise: Causes, Sound Power Ratings and How to Reduce It

Axial fans tend to be noisier than centrifugal fans of similar duty, because their blades run at higher tip speed and produce strong tones at the blade-pass frequency. Noise is manageable, but it has to be planned at selection stage, since cures after installation are expensive and often add resistance. This guide explains what makes a fan noisy, how noise is rated, and what actually reduces it.

Where Fan Noise Comes From

• Blade-pass tones. Each blade passing a fixed point produces a pressure pulse, so the sound has a strong tone at the blade-pass frequency and its harmonics. The main sounds from a fan are tonal and tied to this frequency.
• Turbulence. Air leaving the blades, separating from surfaces, and flowing through obstructions creates broadband noise.
• Operating point. Running near stall or far from the best-efficiency point raises noise sharply.
• Inlet and outlet conditions. Distorted inflow from bends, guards, or obstructions close to the fan increases noise, as do sharp transitions at the outlet.
• Mechanical noise. Motor, bearings, belt drives, and imbalance add to the total, and structure-borne vibration can radiate from ducts and supports.

Blade-pass frequency equals the number of blades multiplied by the rotational speed in revolutions per second. An eight-bladed fan at 1,450 rpm has a blade-pass frequency of 8 × 1,450 ÷ 60 = about 193 Hz. Knowing it helps you predict which octave band will dominate and whether resonances in ducts or casings could amplify it.

Sound Power and Sound Pressure Are Not the Same

Sound power level (Lw) is the acoustic energy the fan radiates, a property of the fan and its operating point. It does not depend on the room or the distance.

Sound pressure level (Lp) is what you measure at a position, and it depends on distance, room absorption, reflections, and how many sources are present.

Manufacturers rate fans by sound power, because it is independent of the environment. You then calculate sound pressure for your site. In free field, for a source on a hard surface, a simple estimate is:

Lp = Lw − 20 × log₁₀(distance in metres) − 8

Example. A fan with a sound power level of 95 dB, listened to at 10 m, gives 95 − 20 − 8 = 67 dB. At 20 m it gives about 61 dB. Every doubling of distance lowers the level by about 6 dB in free field. Real sites with reflections, absorption, and multiple fans differ, so treat this as an early estimate.

How Fan Sound Is Rated

Fan sound power is determined in laboratory conditions. The ISO 13347 series describes methods using a reverberant room, an enveloping surface, or sound intensity, and AMCA 300 covers the reverberant room method for fans. In-duct sound power is determined under ISO 5136. Because the inlet, outlet, and casing radiate different levels, one fan can have several sound power figures, depending on how it is installed. These use the same installation categories as air performance tests: free or ducted inlet and outlet. See fan performance testing.

Ask which installation category and which speeds the sound data represents, and whether the rating is measured or calculated for your size and speed. Rules exist for converting ratings to sizes and speeds that were not tested, and they assume geometric similarity, similar tip speeds, and matching system conditions.

Speed Is the Strongest Lever

A common rule of thumb is that sound power changes by roughly 50 times the base-10 logarithm of the speed ratio, for the same fan on the same system.

Speed change	Approximate sound power change
Speed × 0.9 (10% slower)	−2.3 dB
Speed × 0.8 (20% slower)	−4.8 dB
Speed × 0.7 (30% slower)	−7.7 dB

Conversion rules for certified ratings are more accurate than this rule of thumb, but it shows why slowing a fan, or choosing a larger fan at lower speed, is the most effective way to reduce noise. Speed reduction also cuts power sharply, as covered in the fan laws. The trade-off is size, cost, and stall margin at the new operating point.

Ways to Reduce Noise

1. Select a larger fan at lower speed. Lower tip speed reduces noise and energy.
2. Choose the operating point carefully. Stay near the best-efficiency region and clear of stall.
3. Improve inlet conditions. Use a bellmouth, straight approach, and avoid guards or obstructions close to the blades.
4. Use silencers or attenuators on inlet and outlet where needed. Add their pressure loss to the system curve.
5. Enclose or shield the fan, or add acoustic casing where the source cannot be moved.
6. Isolate vibration. Use flexible connections and anti-vibration mounts to stop structure-borne noise.
7. Control speed so the fan runs slower when full duty is not needed. See adjustable pitch versus VFD control.
8. Position the fan away from sensitive areas, and consider the direction of the discharge.
9. Balance and maintain the impeller. Unbalance and deposits raise vibration and noise.

Silencers: Benefits and Costs

Splitter silencers and lined ducts absorb sound, but they add resistance, take space, and can collect dust. A silencer’s insertion loss varies with frequency, so match it to the fan’s dominant tones. Include the pressure drop when sizing the fan, since a silencer that cuts noise but moves the operating point toward stall solves one problem by creating another.

Noise Limits

Limits come from occupational exposure rules, environmental permits, or client specifications. State whether the limit applies at the fan, at a work position, or at a site boundary, and at what distance. Without that, a limit cannot be checked.

Where Our Range Fits

We manufacture axial fans with impeller diameters up to 1,600 mm, static pressure up to 2,400 Pa, and air volume up to 185,000 m³/h. Sound data and attenuation options are confirmed for your duty and installation, so give us your noise limit and the distance and location it applies to.

Give Us Your Noise Limit

Send your noise limit, its location, and your duty, and we’ll advise on selection and attenuation. Contact us for a free quote.

Frequently Asked Questions