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Previous research shows that open-celled metal foam liners have the potential to provide substantial aircraft noise reduction benefit. Our recent results show that their normal incidence sound absorption performance can be further improved by compressing them uniformly; this compression reduces their effective pore size and increases the thermoviscous losses. Further, their performance may be optimized by stacking foams with varying degree of compression and creating a stepwise effective relative density gradient. Here, we present the results from our recent experiments, evaluating the attenuation performance of such a stepwise gradient metal foam liner using the Advanced Noise Control Fan (ANCF). Liner spools with two different stepwise configurations were fabricated and tested using the ANCF test rig at the Notre Dame Turbomachinery Laboratory. Farfield data was gathered with the liner spool mounted at the inlet and aft locations. In this paper, we present our results comparing the attenuation performance under both test conditions as compared to measurements obtained using a hard wall baseline configuration. Our results show that stepwise metal foam configurations can provide robust acoustical performance for aircraft noise reduction applications. [ABSTRACT FROM AUTHOR]

Additive manufacturing allows the fabrication of acoustical materials with previously unrealizable micro- and macrostructural complexities. However, the still nascent understanding of various geometrical defects occurring during the additive process remains a barrier to accurately predicting the acoustical behavior of such complex absorbers. In this study, we present the results from our efforts on numerically modeling the absorption behavior of periodic porous absorbers fabricated using the stereolithography (SLA) technique using the hybrid micro-macro multiphysics approach. Specifically, we focus on understanding the role played by the expansion or shrinkage of the solid ligaments during the SLA process on the acoustical properties of the final printed samples. First, the periodic absorbers are modeled using COMSOL multiphysics, where the transport properties are derived using the micromodeling method, and sound absorption behavior using the Johnson-Champoux-Allard-Lafarge-Pride (JCALP) semiempirical model. Then, results from the expansion study, guide the changes in the ligament sizes in the unit cell modeling. Finally, the fabricated samples are tested using an impedance tube, and the measured absorption properties are compared to the a priori numerical predictions. Results indicate that accounting for fabrication defects within the numerical modeling schema can provide reliable sound absorption predictions for additively manufactured porous absorbers. [ABSTRACT FROM AUTHOR]