# Fans - Noise Power Generated

## Noise or sound generated by a fan depends on the motor power, volume capacity, static pressure and discharged volume

The empiric expressions below can be used to indicate generated Sound Power Level from fans.

Note! - Exact Sound Power Levels should be obtained from manufacturers data.

### Sound Power Level - SI-units

LN = 67 + 10 log(S) + 10 log(p)                              (1a)
LN = 40 + 10 log(Q)+ 20 log(p)
(1b)
LN = 94 + 20 log(S) - 10 log(Q)
(1c)

where

LN = sound power level from fan (dB)

S = rated motor power  (kW)

p = fan static pressure  (Pa, N/m2)

Q = volume discharged (m3/s)

### Sound Power Frequencies

Sound power level in the octaves can be calculated by adding the values below to the sound power level calculated in the expressions or diagram above.

Fan Type

Octave (Hz)

631252505001000200040008000
Centrifugal fan, backward-curved blades -4 -6 -9 -11 -13 -16 -19 -22
Centrifugal fan, forward-curved blades -2 -6 -13 -18 -19 -22 -25 -30
Centrifugal fan, straight radial blades -3 -5 -7 -7 -8 -11 -16 -18
Axial fan -7 -9 -7 -7 -8 -11 -16 -18

### Sound Power Level - Imperial units

LN = 90 + 10 log(s) + 10 log(h)                         (2a)
LN = 55 + 10 log(q) + 20 log(h)
(2b)
LN = 125 + 20 log(s) - 10 log(q)
(2c)

where

s = rated motor power (hp)
h = fan static head (inch water gauge)
q = volume discharged (ft3/min)

### Maximum Discharge Velocity from Fans for Quiet Operations

The discharge velocity from fans should be kept within certain limits to avoid noisy operations.

ApplicationMaximum Discharge Velocity (m/s)
Supply SystemExhaust
System
Sound studios, churches, libraries 4 - 5 5 - 7
Cinemas, theaters, ballrooms 5 - 7 6 - 8
Restaurants, offices, hotels, shops 6 - 8 7 - 9
• 1 m/s = 3.6 km/h = 196.85 ft/min = 2.237 mph

### ACMA Certified Sound Levels

Certified Sound-Power Level (dB)
Octave Band (Hz)Fan 1Fan 2Fan 3Fan 4
63 117 113 120 101
125 118 116 113 98
250 109 111 111 107
500 106 106 107 120
1000 103 103 103 116
2000 97 98 98 114
4000 94 95 93 109
8000 92 92 88 105

## Related Topics

• Acoustics - Room acoustics and acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
• Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

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