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Engineering ToolBox > Fans and Sound Power Generation
The empiric expressions below can be used to estimate the Sound Power Levels from fans. Note! Exact Sound Power Levels must be obtained from the manufacturers 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
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:
| 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 a fans should in general be kept within certain limits to avoid a noisy operation. As a guidance the following values 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 |