Fans and Noise Power Generation
Sound Power Level from a fan depends on motor power, capacity, static pressure and discharged volume
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The empiric expressions below can be used to indicate the Sound Power Levels from fans. Note! Exact Sound Power Level should be obtained from manufacturer specifications.
Sound Power Level - SI-units
Lw = 67 + 10 log( S ) + 10 log( p ) (1a)
Lw = 40 + 10 log( Q )+ 20 log( p ) (1b)
Lw = 94 + 20 log( S ) - 10 log( Q ) (1c)where
Lw = sound power level (dB)
S = rated motor power (kW)
p = fan static pressure (Pa, N/m2)
Q = volume discharged (m3/s)
Sound Power Frequencies
The sound power calculated in the expressions and diagram above can be determined for each octave by adding:
| dB added | ||||||||
| Fan Type | Octave | |||||||
| 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | |
| 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
Lw = 90 + 10 log( s ) + 10 log( h ) (2a)
Lw = 55 + 10 log( q ) + 20 log( h ) (2b)
Lw = 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)
Discharge Velocities from Fans for Quiet Operations
The discharge velocity from fans should be kept within certain limits to avoid noisy operations. As an indication the values below can be used:
| Application | Maximum Discharge Velocity (m/s) | |
| Supply System | Exhaust System |
|
| Sound studios, churches, libraries | 4 - 5 | 5 - 7 |
| Cinemas, theatres, ballrooms | 5 - 7 | 6 - 8 |
| Restaurants, offices, hotels, shops | 6 - 8 | 7 - 9 |
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