# Ducts and Air Flow Noise

## Air flow generated noise in air ducts

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

LN = 10 + 50 log (v) + 10 log (A)                                                       (1)

where

LN = sound power level (dB)

v = air velocity (m/s)

A = air duct cross sectional area (m2)

### 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

LN = 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

LN = 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.

## 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

• Adding Decibels - The logarithmic decibel scale is convenient calculating sound power levels for two or more sound sources
• Decibel - Decibel is a logarithmic unit used to describe the ratio of a signal level - like power, sound pressure, voltage, intensity etc. to a reference level
• Fans - Noise Power Generated - Noise or sound generated by a fan depends on the motor power, volume capacity, static pressure and discharged volume
• Air Ducts Sizing - The ductwork of ventilation systems are often sized with either the Velocity, the Constant Pressure Loss (or Equal Friction Loss) - or the Static Pressure Recovery Methods
• Rotary Heat Exchanger - Noise Attenuation - Noise attenuation in rotating heat exchangers

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