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Sound absorption by lined plenums, such as fan discharges, can provide considerable attenuation. For effective low-frequency absorption, use thick sound-absorbing materials. To isolate high frequencies, divide the plenum into small subsections so that the line of sight between duct openings is blocked. The surface area of a lined plenum should be at least 10 times greater than the inlet area. Attenuation performance increases as the ratio of cross-sectional area of plenum to cross-sectional area of inlet (or outlet) duct increases. Lined plenums can provide attenuation of more than 10 dB at low frequencies.
Note: Corridors with sound-absorbing surfaces can reduce room-to-room noise trans mission by acting as lined plenums. Stagger doors on opposite sides of double-loaded corridors so openings don't allow direct cross talk between rooms.
Reference
R. J. Wells, “Acoustical Plenum Chambers,” Noise Control, July 1958.
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