THEORETICAL ANALYSIS OF LINED -DUCT SOUND ATTENUATION

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.