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
LN = 10 + 50 log (v) + 10 log (A) (1)
LN = 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
LN = 10 + 50 log (vi / 197) + 10 log (Ai / 1550) (1b)
vi = air velocity (ft/min)
Ai = cross sectional area (in2)
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
LN = 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
LN = 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.
Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
Logarithmic unit used to describe ratios of signal levels - like power or intensity - to a reference level.
The velocity reduction method can be used when sizing air ducts.
Ductwork air leakage classes.
The sound generated by a fan depends on the motor power, the volume capacity, the static pressure increase and the discharged volume.
Sound attenuation vs. frequency in rotating heat exchangers.
The logarithmic decibel scale is convenient when adding signal values like sound power, pressure and others from two or more sources.
Recommended air velocities in ventilation ducts