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

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)
Octave Band Center Frequency (Hz)
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

Duct attenuation due to end reflection

  • 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

Related Topics

  • Acoustics - Room acoustics and acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
  • Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

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