An acoustic calculation of a ventilation system can follow a procedure similar to:
1. 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.
2. 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 the 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 below.
3. Allowance for end reflection
In the example the duct dimension is 250 mm. Attenuation due to end reflection is entered below.
4. Ductwork attenuation, terminal to checkpoint
Calculate the attenuation in unlined and lined ducts. Note that the table and diagrams list the attenuation in dB/m.
5. Elbow attenuation, terminal to checkpoint
Calculate the attenuation in the elbows.
Determine the allowance for split - branch to terminals.
6. Power-level split, main duct to branch duct
Determine the allowance for split - main duct to branch.
7. Sound power level of fan - Lw
Enter the manufacturer's sound power data or calculate the fan sound power.
8. Safety factors
Enter safety factors - recommended to -3 dB.
9. Silencer requirements
The silencer must be selected to provide the necessary attenuation. Manufacturer's data are preferred.
Example -Acoustic Calculation of HVAC Application
| Procedure | Octave band center frequency, Hz | |||||||
| 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | |
| 1. Sound pressure level criterion | 59 | 48 | 40 | 34 | 30 | 27 | 25 | 23 |
| 2. Room and terminal effect | 5 | 5 | 5 | 5 | 5 | 5 | 5 | 5 |
| 3. Allowance for end reflection | 12 | 8 | 4 | 1 | 0 | 0 | 0 | 0 |
| 4. Ductwork attenuation, terminal to checkpoint | 17 | 17 | 9 | 5 | 5 | 5 | 5 | 5 |
| 5. Elbow attenuation, terminal to checkpoint | 0 | 0 | 7 | 15 | 25 | 25 | 19 | 15 |
| 6. Power-level split, branch to terminals | 6 | 6 | 6 | 6 | 6 | 6 | 6 | 6 |
| 7. Power-level split, main duct to branch duct | 10 | 10 | 10 | 10 | 10 | 10 | 10 | 10 |
| Totals | 109 | 91 | 78 | 73 | 78 | 75 | 67 | 61 |
| 8. Sound power level of fan - Lw | 102 | 99 | 98 | 98 | 97 | 95 | 86 | 81 |
| 9. Safety factors | 3 | 3 | 3 | 3 | 3 | 3 | 3 | 3 |
| Total sound power level of fan - Lw | 105 | 102 | 101 | 101 | 100 | 98 | 89 | 84 |
| 10. Silencer requirements | 0 | 8 | 20 | 25 | 19 | 20 | 19 | 20 |
