Passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a quarter-wavelength behavior. The simplest systems are SDOF-type (single degree of freedom), consisting of a perforated sheet backed with a honeycomb, whose absorption ability is limited
to frequencies near the Helmholtz frequency. Thus, to widen the absorption frequency range, manufacturers use a 2DOF (double degree of freedom) system, with an internal layer over another honeycomb (stack of two resonators). However, one constraint is the limited thickness of the overall system,
which reduces the space allotted to each honeycomb. A possible approach, based on a previous concept called LEONAR (long elastic open-neck acoustic resonator), could be to link each perforated layer to hollow tubes inserted in each honeycomb layer, in order to shift resonance frequencies to
lower frequencies by extending the air column lengths. The presence of an empty chamber on both sides of the internal perforated layer also allows the tube length to be increased through tubes crossing both cavities, preserving the liner thickness. The main aim of this article is to mathematically
describe the principle of a 2DOF LEONAR and to show the relevance of the mathematical model through FEM simulations and experiments performed in an impedance tube. Moreover, its behavior is analyzed through a parametric study, in order to explore its potential for an aeronautic application.
A remarkable feature of 2DOF LEONAR-type materials with insertion of bottom tubes in the higher cavity is the possibility of maintaining the low frequency band provided by the original LEONAR concept, while adding a second absorption peak at a higher frequency, by the second layer and the
accompanying tubes. There is a fundamental difference from classical SDOF/2DOF resonators, for which the thicknesses are obviously different.
Th e North American Association for the Study of Religion (NAASR) was formed in 1985 to encourage theoretical refl ection on the study of religion among North American scholars; to represent North American scholars of religion internationally; and to sustain communication between North American scholars and their international colleagues.NAASR is committed to examining any intellectual practices in the fi eld regardless of geographical location or disciplinary boundaries. Our central concerns are with the intellectual and theoretical foundations of the study of religion, with the historical, social, and political conditions that shape the intellectual orientations and practices of the fi eld, and with the place of criticism, experimentation, and theorizing of religion.We invite proposals for individual papers and for panels that engage important theoretical issues in the academic study of religion (both historical and contemporary). We are particularly interested in the following topics:1. Why do certain works, authors, and frameworks concerning religion and its study command more authority and attention than others? What
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