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# Sound Attenuation in Ducts due to End Reflection

## 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:

Sound Attenuation in Ducts due to End Reflection (dB)
Duct Dimension1)
(mm)
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

## Related Topics

• ### Acoustics

Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate 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.

## Related Documents

• ### Sound Attenuation - Duct split to Terminals

Estimating sound attenuation where ducts splits to terminals.
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Noise attenuation in unlined sheet-metal ducts.
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Acoustic noise calculation procedure HVAC systems.
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Noise in a ventilation system can be reduced by reactive, diffuser type, active and/or dissipative silencers.

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## Citation

• The Engineering ToolBox (2003). Sound Attenuation in Ducts due to End Reflection. [online] Available at: https://www.engineeringtoolbox.com/sound-attenuation-end-reflection-d_73.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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