# Noise generated in Air Ducts

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

*L _{N} = 10 + 50 log (v) + 10 log (A) (1)*

*where *

*L _{N} = sound power level in the duct (dB)*

*v = air velocity (m/s)*

*A = air **duct cross sectional area (m ^{2})*

The equation modified for imperial units

*L _{N} = 10 + 50 log (v_{i} / 197) + 10 log (A_{i} / 1550) (1b)*

*where *

*v _{i} = air velocity (ft/min)*

*A _{i} = cross sectional area (in^{2})*

### 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 m ^{2}*

The noise generated in the duct with an air flow velocity *10 m/s* can be calculated as

* **L _{N} = 10 + 50 log (10 m/s) + 10 log (0.0314 m^{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 fans - noise generated inside ducts by air flow can in general be neglected.

## Related Topics

### • Acoustics

Room acoustics and acoustic properties, decibel A, B and C, Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation.

### • Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.

## Related Documents

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### Fans - Noise Power Generation

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### Noise Attenuation in Rotary Heat Exchanges

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### Signals - Adding Decibels

The logarithmic decibel scale is convenient when adding signal values like sound power, pressure and others from two or more sources.

### Velocity Classification of Ventilation Ducts

Recommended air velocities in ventilation ducts