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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 and different frequencies, can be found in the table 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.
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