SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
www.swegon.comAcoustics - Planning Tips
Duct connection to fan outlet
The connection of the duct to the fan is the first place where excess pressure drops can occur, resulting in excessively high sound levels. When the air is deflected by a bend, consideration must be given to the velocity distribution in the duct before the bend.
An incorrect design, with a 90°-bend directly to the fan, increases the total sound power level by 4 dB. In addition, if the fan is turned "upside down" the sound power level will be 6 dB higher than with a proper layout.
Some examples of a correct and incorrect design are given below:
Figure 72.Examples of correct and incorrect duct connections to a fan.
A = Correct design
B = Incorrect design
C = The bend should be chamfered 45° on the outside
D = Correct design - The duct bend should be aimed in the same direction as the fan rotation
E = Incorrect design
Sound attenuator in combination with an exhaust air unit
One way of counteracting transferred sound through a common ventilation duct is to install a sound attenuator between the main duct and the exhaust air unit. By placing the attenuator as shown in the diagram below, resonance effects can be removed and acoustic insulation is considerably improved.
Figure 73.Counteracting sound transfer.
a) | low sound insulation | b) | high sound insulation.
with duct lengths of
1-3 m especially high
attenuation can be obtained. |
The same procedure can be applied to supply air terminals to prevent cross-talk and to attenuate duct noise.
Choice of sound level from unit:
Selecting an air terminal should be done so that the units sound generation is 5 dB lower than the requirement for the room in question.
Distance between the duct and the terminal
Values for pressure drop and sound generation in this catalogue apply when there is an even velocity distribution in the terminal's inlet connection.
A common mistake is to place the unit too close to a branch duct, which creates sound problems.
One recommendation is that the ubnit is placed at a distance of at least 3 times the duct diameter from the branch. See example in Figure 74.
Duct design for displacement air terminals
The structure of the duct greatly affects sound generation.
Bends directly before the terminal can cause considerable increases in sound. See examples in the figure 75.
Figure 74. The branch length for the duct should be at least 3 x D.
Table duct connections
|
| Duct connections | | | |
v m/s | A | B | C | D |
4-5 m/s | + 2 | + 6 | + 3 | + 3 |
6-8 m/s | + 4 | + 10 | + 6 | + 6 |
|
Table 16. Sound increases (dB) for different duct connections and equivalent sound absorption areas, | | | | |
Figure 75.Examples of different connections affect sound generation.
Calculation model for dB(A) to dB(C)
The model applied to a PMT 250 diffuser that, at an air flow of 170 l/s and with an open damper, produces 30 dB(A) according to the design graph.
Sound power level
Correction factor KOK
|
| Mid-frequency (octave band) Hz | | | | | | | | |
| 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | |
dB(A) | 30 | 30 | 30 | 30 | 30 | 30 | 30 | 30 | |
KOK | 12 | 8 | 4 | 2 | 0 | -10 | -22 | -23 | |
LW | 42 | 38 | 34 | 32 | 30 | 20 | 8 | 7 | |
Plus C-filter | -0.8 | -0.2 | 0 | 0 | 0 | -0.2 | -0.8 | -3 | |
Result, dB(C) | 41.2 | 37.8 | 34 | 32 | 30 | 19.8 | 7.2 | 4 | 44 |
| Room attenuation at 10 m2 Sabine | | | | | | | | 4 |
| Result dB(C) | | | | | | | | 40 |
|
Result:
At 170 l/s with open damper the PMTc 250, produces 40 dB(C).