Fiber metal acoustic sheet materials are used in lined duct sound absorbers for gas turbine engine noise. Duct treatment utilizes a fiber metal face sheet backed by a cavity of controlled depth. The fiber metal facing and cavity depth provide a tuned system that is broadly effective in a desired frequency band. Fiber metal lined ducts are used in engine applications such as fan ducts, inlet cowls, auxiliary power units and environmental control systems. This approach is used in engine treatments to provide effective absorption within engine limits of weight and space.
This paper discusses design methodology for fiber metal ducts. Topics include frequency tuning the absorber, treated area versus noise reduction, matching the acoustical impedance of the fiber metal to the air in the duct, the effects of flow and sound pressure level in the duct on noise reduction, and an example of noise reduction achieved in an engine application.