Sound attenuation vs. change in duct size.
Sound attenuation when changing duct areas or dimensions:
dL = 10 log( ( 1 + A1 / A2 )2 / 4 (A1 / A2) ) (1)
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