Sound Attenuation in Ducts due to End Reflection
Low frequency noise transferred from main duct to end terminals is reflected back to main duct.
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Sound attenuation from ducts to end terminals due to end reflection for typical duct dimensions with sharp ends and different frequencies are indicated below:
| Attenuation (dB) | ||||||
|---|---|---|---|---|---|---|
| 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
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