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Unlined Sheet-Metal Ducts - Sound Attenuation

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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)
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

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