Attenuation of plane and high order modes in a circular and annular lined duct

An experimental and theoretical programme was undertaken to measure and predict the attenuation of plane and spiral modes within a cylindrical and annular duct. The duct was lined with a partitioned absorber designed to act as a locally reacting surface. A detailed duct modal theory was evolved for the cylindrical duct and used to compare with the measured results. A thin annulus theory was adopted for the annular duct which made use of an existing computer programme originally written for the rectangular duct problem. The experimental work was conducted using a siren rig and also, in order to obtain greater detail and reliability, a loudspeaker rig was built and used extensively for the m = 0 and m = 1 modes. These results confirm, within the limits of experimental accuracy, that the theoretical approach used is a valid one at least under the prevailing laboratory conditions of zero mean air flow and low sound pressure levels. Excellent agreement was obtained between theory and experiment for the cylindrical duct. In the case of the annular duct the comparison was less satisfactory but provided at least qualitative agreement. The principle observed effects are the increase of attenuation rate with increasing mode number and decreasing cut-off frequency ratio. The thesis is written with a bias towards the problems of the aero-engine industry and includes a brief account of present day absorption technology in this field.