Enter the air velocity and the air duct cross-sectional area (or any two fields) into the Duct Noise Calculator. The calculator estimates a simplified Duct Noise Index (DN) using the empirical relation shown below (this is a rough, relative estimate and is not a substitute for a full HVAC acoustics/sound power analysis).

Duct Noise (Index) Calculator

Basic
CFM & Size

Enter any 2 values to calculate the missing variable. Note: DN is a simplified, relative noise index (not a measured sound pressure level).

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).