Estimate HVAC duct outlet noise from straight-run and elbow losses, combine background sound correctly, or reverse-solve the attenuation rate needed for a target.

Choose a solve target, then complete the visible required fields marked with *.

Use a level with the same weighting, frequency scope, and receiver reference as background.
Enter dB per selected length unit.
Combined logarithmically with transmitted duct noise.

Duct Noise (Index) Formula

The following two example problems outline the steps and information needed to calculate the Duct Noise Index (DN). This DN value is a simplified, relative index derived from velocity and area; it is not a full prediction of HVAC sound pressure level (SPL) or sound power in real installations.

DN = 10 + 50*LOG10(V / 1 m / s) + 10*LOG10(A / 1 m²)

Variables (use base-10 logarithms):

  • DN is the Duct Noise Index (dB, relative)
  • V is the air velocity (m/s)
  • A is the air duct cross-sectional area (m²)

How to Calculate Duct Noise (Index)?

The following steps outline how to calculate the Duct Noise Index (DN).

  1. First, determine the air velocity (m/s).
  2. Next, determine the air duct cross-sectional area (m²).
  3. Next, gather the formula from above: DN = 10 + 50*LOG10(V / 1 m/s) + 10*LOG10(A / 1 m²).
  4. Finally, calculate the Duct Noise Index (DN).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

air velocity (m/s) = 2.35

air duct cross-sectional area (m²) = 1.25

DN = 10 + 50*LOG10(V / 1 m/s) + 10*LOG10(A / 1 m²) = ?

Result: DN = 10 + 50·LOG10(2.35) + 10·LOG10(1.25) ≈ 29.52 dB (relative).