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 - L_N

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 - L_p - are converted to sound power level - L_w - 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

• acoustic calculation of a ventilation system - an example

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Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm