# Ducts and Air Flow Noise

## Air flow generated noise in air ducts

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The air flow through ducts generates noise. The noise generated depends on the air velocity and the duct size (cross sectional area).

Generated noise can be calculated with the empirical equation

L_{N}= 10 + 50 log (v) + 10 log (A) (1)

where

L_{N}= sound power level (dB)

v = air velocity (m/s)

A = airduct cross sectional area (m^{2})

### Example - Noise generated by Air Flow in Duct

The noise generated in a *200 mm* circular duct with air flow velocity *10 m/s* can be calculated as* *

L_{N}= 10 + 50 log (10 m/s) + 10 log (π ((0.2 m) / 2)^{2})

= 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* *

L_{N}= 10 + 50 log (20 m/s) + 10 log (π ((0.2 m) / 2)^{2})

= 60 db

**Note! **- due to the noise generated by the fans - noise generated inside ducts by the air flow can in general be neglected.

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