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Abstract
Acoustically absorptive duct linings for jet engine nacelles usually consists of a single porous layer backed by air-filled cavities, suitably partitioned, and an impervious backing sheet, Two common types of porous surfaces are perforated sheets and resin-impregnated woven-fiberglass sheets. The depth of the backing cavities and the size of the core-support structures are chosen to give the desired acoustical reactance yielding maximum attenuation at a frequency corresponding to maximum perceived noisiness at a given operating condition. The presumed advantage of the distributed-resistance design is wider attenuation bandwidth. Tests were conducted with four different porous surface designs successively installed in the fan-discharge ducts of a full-scale turbofan engine. There were two different perforated aluminum surfaces (nominal open areas of 8.6% and 15%) and two different fiberglass surfaces (nominal flow resistances of 70 and 150 mks rayls). The results of the tests indicated that the distributed-resistance linings had a wider attenuation bandwidth than the discrete-resistance (perforated) linings. Maximum attenuation values were nearly the same.
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