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# Unlined Sheet-Metal Ducts - Sound Attenuation

Attenuation of sound in unlined sheet metal ducts at different frequencies are indicated below:

Unlined Sheet-Metal Ducts - Sound Attenuation
DuctAttenuation (dB/m duct)
Octave band center frequency (Hz)
63.512525050010002000
and higher
Round 100 - 300 mm 0.1 0.1 0.1 0 0 0
Round > 300 mm 0 0 0 0 0 0
Rectangular 150 x 150 mm 0.65 0.65 0.5 0.35 0.35 0.35
Rectangular 600 x 600 mm 0.65 0.65 0.35 0.15 0.15 0.15
Rectangular 1800 x 1800 mm 0.35 0.35 0.15 0.03 0.03 0.03

### Example - Sound Attenuation in an Unlined Duct

The sound attenuation in a rectangular 600 x 600 mm unlined duct is 0.35 dB/m for frequency 250 Hz. The sound attenuation in 10 m duct is

(10 m) (0.35 dB/m) = 3.5 dB

## 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

### Lined Sheet-Metal Ducts - Sound Attenuation

Noise attenuation in rectangular straight, lined, sheet metal ducts.

### Sound Attenuation - Duct split to Terminals

Estimating sound attenuation where ducts splits to terminals.

### Sound Attenuation in Duct Elbows

Noise attenuation in duct elbows by reflected sound.

### Sound Attenuation in Ducts due to End Reflection

Low frequency noise transferred from main duct to end terminals is reflected back to main duct.

### Sound Transmission through Duct Walls

Sound transmission from ducts to surrounding rooms.

### Sound Transmission through Flanking Paths

Sound transmitted through duct walls, floors and ceilings.

### Ventilation Systems - Acoustic Calculation Procedure

Acoustic noise calculation procedure HVAC systems.

### Ventilation Systems - Classification of Noise Reduction Silencers

Noise in a ventilation system can be reduced by reactive, diffuser type, active and/or dissipative silencers.

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