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