Various configurations of materials suitable for use on surfaces of engine ducts, guide vanes, and splitters were investigated for sound absorption. The development and screening process began with basic tests in standing-wave-tube and dc-flow-resistance facilities. Promising configurations were then progressively studied in traveling-wave tubes, rectangular and annular ducts with air flow, and, ultimately, in rotating pressure fields with model compressors. Theoretical and empirical relationships were developed for the resistive and reactive acoustic absorption, including the effects of air flow, sound intensity, and sound source (electromagnetic, aerodynamic, or spinning modes). The effect of air flow on the reduction of sound was studied with a two-dimensional choked-flow facility containing vanes simulating engine inlet ducts and guide vanes. Theoretical and empirical relationships were developed for the noise reduction as a function of velocity and guide-vane setting angle, with and without acoustic-absorption treatment on the surfaces of the vanes and duct. [Work supported by the Federal Aviation Agency.]