Asymmetric or peripherally discontinuous acoustic linings for absorbing sound radiating within acoustic ducts, when properly located in the peripheral or circumferential direction, alter the directivity of sound emitted from an end of the duct to provide preferential enhanced suppression in a predetermined general direction. In a jet engine fan duct, treating the upper half produces approximately the same noise suppression on the ground as a fully treated duct. Another application is side lobe suppression. Two different treatments can be used providing the asymmetric lining is the better sound absorber.