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Attenuation in duct elbows is caused by reflected sound. It is only in big ducts and at high frequencies the attenuation is considerable.
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Sound power level attenuation in elbows is based on sound reflection. Only in big ducts and at high frequencies is the attenuation considerable.
Attenuation in ducts at different sizes and at different frequencies for some common configurations can be taken from the table below.
| Attenuation (dB) |
Width
B (mm) |
Octave band center frequency Hz | ||||||
| 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | ||
| Duct without insulation | 125 | 0 | 0 | 0 | 6 | 8 | 4 | 3 |
| 250 | 0 | 0 | 6 | 8 | 4 | 3 | 3 | |
| 500 | 0 | 6 | 8 | 4 | 3 | 3 | 3 | |
| 1000 | 6 | 8 | 4 | 3 | 3 | 3 | 3 | |
| Insulation in front of elbow | 125 | 0 | 0 | 0 | 6 | 8 | 6 | 8 |
| 250 | 0 | 0 | 6 | 8 | 6 | 8 | 11 | |
| 500 | 0 | 6 | 8 | 6 | 8 | 11 | 11 | |
| 1000 | 6 | 8 | 6 | 8 | 11 | 11 | 11 | |
| Insulation after the elbow | 125 | 0 | 0 | 0 | 7 | 11 | 10 | 10 |
| 250 | 0 | 0 | 7 | 11 | 10 | 10 | 10 | |
| 500 | 0 | 7 | 11 | 10 | 10 | 10 | 10 | |
| 1000 | 7 | 11 | 10 | 10 | 10 | 10 | 10 | |
| Insulation in front and after the elbow | 125 | 0 | 0 | 0 | 7 | 12 | 14 | 16 |
| 250 | 0 | 0 | 7 | 12 | 14 | 16 | 18 | |
| 500 | 0 | 7 | 12 | 14 | 16 | 18 | 18 | |
| 1000 | 7 | 12 | 14 | 16 | 18 | 18 | 18 | |
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