Change in Duct Area and Noise Attenuation

Sound attenuation when changing duct areas or dimensions:

dL = 10 log( ( 1 + A₁ / A₂ )² / 4 (A₁ / A₂) )                                                                (1)

where

dL = noise attenuation (dB)

A₁ = inlet area (m²)

A₂ = outlet area (m²)

Note! - valid only for sharp changes.

Example - Attenuation when changing the size of a duct

The attenuation of noise when the ratio between area before and after A₁ / A₂ = 5 - can be calculated as

dL = 10 log( ( 1 + 5 )² / 4 (5))

    = 2.5 dB

Temperature ^oC
K
^oF

Length m
km
in
ft
yards
miles
naut miles

Area m²
km²
in²
ft²
miles²
acres

Volume m³
liters
in³
ft³
us gal

Weight kg_f
N
lbf

Velocity m/s
km/h
ft/min
ft/s
mph
knots

Pressure Pa
bar
mm H₂O
kg/cm²
psi
inches H₂O

Flow m³/s
m³/h
US gpm
cfm