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Coupling of Helmholtz resonators to improve acoustic liners for turbofan engines at low frequencyAn analytical and test program was conducted to evaluate means for increasing the effectiveness of low frequency sound absorbing liners for aircraft turbine engines. Three schemes for coupling low frequency absorber elements were considered. These schemes were analytically modeled and their impedance was predicted over a frequency range of 50 to 1,000 Hz. An optimum and two off-optimum designs of the most promising, a parallel coupled scheme, were fabricated and tested in a flow duct facility. Impedance measurements were in good agreement with predicted values and validated the procedure used to transform modeled parameters to hardware designs. Measurements of attenuation for panels of coupled resonators were consistent with predictions based on measured impedance. All coupled resonator panels tested showed an increase in peak attenuation of about 50% and an increase in attenuation bandwidth of one one-third octave band over that measured for an uncoupled panel. These attenuation characteristics equate to about 35% greater reduction in source perceived noise level (PNL), relative to the uncoupled panel, or a reduction in treatment length of about 24% for constant PNL reduction. The increased effectiveness of the coupled resonator concept for attenuation of low frequency broad spectrum noise is demonstrated.
Document ID
19760014122
Document Type
Contractor Report (CR)
Authors
Dean, L. W.
(Pratt and Whitney Aircraft East Hartford, CT, United States)
Date Acquired
September 3, 2013
Publication Date
August 1, 1975
Subject Category
Aircraft Propulsion And Power
Report/Patent Number
PWA-5311
NASA-CR-134912
Funding Number(s)
CONTRACT_GRANT: NAS3-18552
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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