Sound Attenuation in Ducts due to End Reflection
Low frequency noise transferred from main duct to end terminals is reflected back to main duct.
Sound attenuation from ducts to end terminals due to end reflection for typical duct dimensions with sharp ends and different frequencies are indicated below:
| Duct Dimension1) (mm) | Octave Band Center Frequency (Hz) | |||||
|---|---|---|---|---|---|---|
| 63.5 | 125 | 250 | 500 | 1000 | 2000 and higher | |
| 125 | 17 | 12 | 8 | 4 | 1 | 0 |
| 250 | 12 | 8 | 4 | 1 | 0 | 0 |
| 500 | 8 | 4 | 1 | 0 | 0 | 0 |
| 1000 | 4 | 1 | 0 | 0 | 0 | 0 |
| 2000 | 1 | 0 | 0 | 0 | 0 | 0 |
1)The duct dimension is the diameter of round ducts - or square root of area of rectangular duct.
- 1 mm = 0.03937 in
- Noise with lower frequencies are reflected more than sound with higher frequencies
- Rounded terminal ends reflects sound less than sharper ends
- Reflection in funnel shaped ends like trumpets or similar can be nearly neglected
Example - Sound Attenuation from Duct to Terminal due to End Reflection
From the diagram above - the sound attenuation for noise with frequency 250 Hz and duct diameter 250 mm is approximately
4 dB
For the same duct with frequency 500 Hz - the sound attenuation due to end reflection is approximately
1 dB
