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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.
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)

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 |
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)
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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