• Chris Bibby Acoustics and Noise Research Group, University of British Columbia
  • Murray Hodgson


This paper investigates theoretically how duct geometry and liner thickness affect the attenuation of fundamental-mode
sound propagation in a lined duct. The study was done to satisfy the need for a greater understanding of interior natural-
ventilation openings and of silencers implemented to improve the acoustical performance (‘ventilators’), and to provide
engineers and architects with optimal-design guidelines. It assumed ventilators of the simplest form – straight, acoustically-
lined ducts of rectangular cross-section. An analytical solution is presented for the attenuation of the fundamental mode in
such a duct. Duct-liner thickness does not affect high-frequency performance; however, it limits low-frequency performance.
A 25-mm liner is likely not thick enough to be effective, but a 100-mm liner may be excessive. Increasing the duct height
reduces the attenuation at all frequencies; in order to provide effective attenuation through the 4000-Hz band, the height
should not exceed 100 mm. If the cross-sectional aspect ratio of a duct is greater than 10, or the duct is only lined on two
opposing surfaces, the attenuation of its fundamental mode is in effect identical to that of a 2D lined duct. Provided that the
duct liner and height are such that the silencer is effective at absorbing sound at a given frequency, reducing the aspect ratio
towards unity will result in large attenuation gains.



How to Cite

Bibby C, Hodgson M. THEORETICAL ANALYSIS OF LINED -DUCT SOUND ATTENUATION. Canadian Acoustics [Internet]. 2014Jan.30 [cited 2021May12];41(3):15-22. Available from:



Article - Engineering Acoustics / Noise Control

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