Acoustic Calculation of Ventilation Systems

A procedure for acoustic calculation of noise in ventilation systems

1. Sound pressure level criterion

Determine the sound pressure level criteria. This table scan be used but should be checked against domestic rules.

In the example below Noise Rating - NR30 - is used as the criterion where the NR-values are entered in line 1.

2. 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 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

Elbow attenuation, terminal to checkpoint

Calculate the attenuation in the elbows.

6. Power-level split, branch to terminals

Sound Power Level Split, branch to terminals

Determine the allowance for split - branch to terminals.

7. Power-level split, main duct to branch duct

Determine the allowance for split - main duct to branch.

8. Sound power level of fan - L_w

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

Procedure	Octave band center frequency, Hz
Procedure	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 - L_w	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 - L_w	105	102	101	101	100	98	89	84
10. Silencer requirements	0	8	20	25	19	20	19	20

Noise Rating Curves NR

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