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# 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

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

where

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)

where

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.

## Related Topics

### • Acoustics

Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate 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

### Decibel

Logarithmic unit used to describe ratios of signal levels - like power or intensity - to a reference level.

### Ducts Sizing - the Velocity Reduction Method

The velocity reduction method can be used when sizing air ducts.

### Ductwork - Seal Classes

Ductwork air leakage classes.

### Fans - Noise Power Generation

The sound generated by a fan depends on the motor power, the volume capacity, the static pressure increase and the discharged volume.

### Noise Attenuation in Rotary Heat Exchanges

Sound attenuation vs. frequency in rotating heat exchangers.

### 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

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