Abstract: An acoustic duct such as a jet engine fan duct is lined with a plurality of circumferentially spaced strips of sound-absorbing materials which extend helically about the duct axis and which are effective to scatter spinning mode acoustic pressure fields to higher order attenuating modes for improved noise suppression. To maximize noise reduction, the pitch angle of the helix may be aligned to the direction of the wave front generated by the first stage rotor blades of a fan or compressor disposed in the duct when operating at a supersonic velocity relative to the incoming airstream. Alternate strips may be tuned to different frequencies and/or be separated by untreated strips.