Noise attenuation in a chamber:
dL = 10 log( 1 / Ao (cosθ / 2 π s2 + (1 - α) / (α A)) ) (1)
dL = sound attenuation (dB)
Ao = outlet area (m2)
A = chamber inside surface area (m2)
θ = angle between inlet and outlet (degrees)
s = distance between inlet and outlet (m)
α = inside surface sound absorption coefficient
Example - Attenuation in a Chamber
The attenuation in a chamber lined with mineral wool with a sound absorption coefficient α = 0.65, inside surface area A = 16 m2, outlet area Ao = 0.25 m2, distance between inlet and outlet s = 2 m, angle between inlet and outlet α = 45o, can be calculated as
dL = 10 log( 1 / (0.3 m2) (cos45o / 2 π (2 m)2 + (1 - 0.65)/(0.65 (16 m2))) )
= 17.3 dB
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
Sound attenuation vs. change in duct size.
The speed and attenuation of sound in moist air varies with sound frequency and air humidity.