Sound Attenuation in Duct Elbows
Noise attenuation in a duct elbow is caused by reflected sound. It is only in larger ducts and at higher frequencies the attenuation is considerable
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Sound power level attenuation in an elbow is based on sound reflection. It is only in larger ducts and at higher frequencies the attenuation is considerable.
Attenuation in ducts at different sizes and at different frequencies for some common configurations are indicated in the table below.
| Attenuation (dB) | ||||||||
| Type | 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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