Sound Attenuation due to End Reflection
Some of the low frequency noise transferred to the end terminals in a ventilation system, reflects back to the duct.
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The attenuation due to the end reflection at some typical duct dimensions with sharp ends and different frequencies are indicated in the table and diagram 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
Sound at lower frequencies reflects more than sound with higher frequencies. Rounded ends reflects sound less than sharp ends. Reflection in funnel shaped ends like trumpets or similar can be nearly neglected.
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