Absorption Characteristics of Membrane-Embedded Acoustic Liners

Acoustic liners are used extensively to reduce noise emitted by aircraft engines. However, conventional acoustic liners are largely ineffective for low-frequency noise content (~1000 Hz) due to impractical mass and volume requirements. In this study, membrane-embedded liners are investigated as a potential solution to attenuate low-frequency noise in aerospace applications. In its basic form, such a liner is akin to a double degree-of-freedom liner implementing a membrane as septum in lieu of a typical wire mesh. Introducing the membrane provides a lightweight, compact structural element whose resonance can be tuned to interact with acoustic resonances to enhance absorption and bandwidth for low-frequencies. Material, geometry, tension and location of membrane are used as tunable parameters. Tests on several membrane-embedded and baseline double degree-of-freedom liner configurations were run using the normal incidence tube at NASA Langley Research Center with broadband excitation at sound pressure levels of 120 and 140 dB. Results indicate low-frequency broadband absorption is significantly enhanced for membrane-embedded liners without any appreciable increase in overall mass or volume vis-à-vis the baseline. Correlations with simulations are utilized to understand underlying interactive mechanisms involved. Realizing such liners using aerospace materials is expected to provide an effective means to attenuate low-frequency airborne noise.