Ventilation Systems - Acoustic Calculation Procedure
An acoustic noise calculation of a ventilation system can follow the procedure indicated in the example below:
Sound and Noise Sources
Estimate all sound power sources.
1. Estimate the Sound Power Level from the Fan - LN
Enter the manufacturer's sound power data or calculate the fan sound power.
2. Add Safety factors
Add safety factors - 3 dB is recommended.
Attenuation
Estimate the attenuation in the system.
10. Room and terminal effect
The sound pressure levels - Lp - are converted to sound power level - Lw - at the terminals. The acoustic characteristics of the room as well as the number and location of the terminals must be considered.
a) Determine the acoustic characteristics of the room. In this example an average sound absorption for the room is used.
b) Determine whether the receiver are in the direct or reverberant field. In the example the listener is approximately 1.5 m from the terminal.
c) Find the rooms absorption characteristics. In the example the absorption for the walls, ceiling, floor, persons, curtains and their areas, is calculated to 30 m2 Sabine. According a) and c) the attenuation is 8 dB.
d) Determine how many terminals the listener are influenced by. Note! Remember to include supply and return fans. In the example the listener are influenced by two terminals. From c) we subtract 3 dB.
The attenuation from the terminal to the room is 5 dB. The values are entered in the example.
11. Allowance for end reflection
In the example the duct dimension is 250 mm. Attenuation due to end reflection is entered below.
12. Duct work attenuation
Calculate the attenuation in unlined and lined ducts. Note that the table and diagrams list the attenuation in dB/m.
13. Elbow attenuation
Calculate the attenuation in the elbows.
14. Power-level split, branch to terminals
Determine the allowance for split - branch to terminals.
15. Power-level split, main duct to branch duct
Determine the allowance for split - main duct to branch.
16. Other attenuation
Add attenuation from other components.
Calculate Resulting Sound Power and Required additional Attenuation
20. Resulting Sound Power Fan
Subtract summarized attenuation from the fan sound power - inclusive safety factors
21. Sound pressure level criterion
Determine the sound pressure level criteria. This table indicates acceptable levels at different locations. Check against domestic rules.
In the example below Noise Rating - NR30 - is used as a criterion. NR-values are entered in line 1.
22. Silencer requirements
The silencer must be selected to provide the necessary attenuation. Manufacturer's data are preferred.
Example - Acoustic Calculation of a Ventilation System
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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.
• HVAC Systems
Heating, ventilation and air conditioning systems - design and dimensions.
• Noise and Attenuation
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
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