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Sound Attenuation in Ducts due to End Reflection

Some 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)

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Octave Band Center Frequency (Hz)
63.512525050010002000
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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