Noise generated in Air Ducts

Air flow in ducts generates noise. The noise in the duct is determined by

• air velocity
• duct size (cross sectional area)

Generated noise can be calculated with the empirical equation

L_N = 10 + 50 log(v) + 10 log(A)                           (1)

where

L_N = sound power level in the duct (dB)

v = air velocity (m/s)

A = air duct cross sectional area (m²)

The equation modified for imperial units

L_N = 10 + 50 log(v_i / 197) + 10 log(A_i / 1550)                   (1b)

where

v_i = air velocity (ft/min)

A_i = cross sectional area (in²)

• Air Ducts - Cross Sectional Areas

Example - Generated Noise in Duct by Air Flow

The cross section area of a 200 mm duct can be calculated as

A = π ((0.2 m) / 2)²)

   = 0.0314  m²

The noise generated in the duct with an air flow velocity 10 m/s can be calculated as

 L_N = 10 + 50 log(10 m/s) + 10 log(0.0314 m²)

    = 45 db

The noise generated in the same 200 mm circular duct as above with air flow velocity 20 m/s can be calculated as

 L_N = 10 + 50 log(20 m/s) + 10 log(π ((0.2 m) / 2)²)

    = 60 db

Note! - due to the noise generated by fans - noise generated inside ducts by air flow can in general be neglected.