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Accession Number N20120010290
Title Evaluation of Parallel-Element, Variable-Impedance, Broadband Acoustic Liner Concepts.
Publication Date Jun 2012
Media Count 17p
Personal Author B. M. Howerton E. Ayle M. G. Jones
Abstract Recent trends in aircraft engine design have highlighted the need for acoustic liners that provide broadband sound absorption with reduced liner thickness. Three such liner concepts are evaluated using the NASA normal incidence tube. Two concepts employ additive manufacturing techniques to fabricate liners with variable chamber depths. The first relies on scrubbing losses within narrow chambers to provide acoustic resistance necessary for sound absorption. The second employs wide chambers that provide minimal resistance, and relies on a perforated sheet to provide acoustic resistance. The variable-depth chambers used in both concepts result in reactance spectra near zero. The third liner concept employs mesh-caps (resistive sheets) embedded at variable depths within adjacent honeycomb chambers to achieve a desired impedance spectrum. Each of these liner concepts is suitable for use as a broadband sound absorber design, and a transmission line model is presented that provides good comparison with their respective acoustic impedance spectra. This model can therefore be used to design acoustic liners to accurately achieve selected impedance spectra. Finally, the effects of increasing the perforated facesheet thickness are demonstrated, and the validity of prediction models based on lumped element and wave propagation approaches is investigated. The lumped element model compares favorably with measured results for liners with thin facesheets, but the wave propagation model provides good comparisons for a wide range of facesheet thicknesses.
Keywords Acoustic attenuation
Acoustic impedance
Honeycomb structures
Mathematical models
Transmission lines
Wave propagation

Source Agency National Aeronautics and Space Administration
NTIS Subject Category 46A - Acoustics
Corporate Author National Aeronautics and Space Administration, Hampton, VA. Langley Research Center.
Document Type Conference proceedings
Title Note N/A
NTIS Issue Number 1226
Contract Number N/A

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