Sound pressure level criterion
Determine the sound pressure level criteria. This table scan be used but should be checked against domestic rules.2. Room and terminal effect
In the example below Noise Rating - NR30 - is used as the criterion where the NR-values are entered in line 1.
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 aprox. 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 sabin. 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 indB/m.5. Elbow attenuation, terminal to checkpoint
Enter the manufacturer's sound power data or calculate the fan sound power.9. Safety factors
Enter safety factors - recommended to -3 dB.10. Silencer requirements
The silencer must be selected to provide the necessary attenuation. For this the manufacturer's data should be used.Example -Acoustic Calculation of HVAC Application
ProcedureOctave band center frequency, Hz6312525050010002000400080001. Sound pressure level criterion59484034302725232. Room and terminal effect555555553. Allowance for end reflection1284100004. Ductwork attenuation, terminal to checkpoint17179555555. Elbow attenuation, terminal to checkpoint00715252519156. Power-level split, branch to terminals666666667. Power-level split, main duct to branch duct1010101010101010Totals109917873787567618. Sound power level of fan - Lw102999898979586819. Safety factors33333333Total sound power level of fan - Lw10510210110110098898410. Silencer requirements08202519201920