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), composed of a perforated sheet backed to a honeycomb, whose absorption ability is limited to one frequency range around the Ilelmholtz frequency. Thus, to widen the frequency range of absorption, manufacturers double the concept, called 2DOF (Double Degree Of Freedom), with an internal layer upper another honeycomb. But, one constraint is the limited thickness of global system which reduces the space dedicated to each honeycomb. A possible approach, inspired by a previous concept called LEONAR ("Long Elastic Open Neck Acoustic Resonator), could be to link each perforated layer with hollow tubes introduced in honeycombs, to shift resonance frequencies to lower frequencies by a prolongation of air column lengths. The presence of honeycomb on both sides of the internal perforated layer allows also, by tubes crossing the two cavities, to increase the tube length by keeping the liner thickness. The aim of this paper is to describe mathematically the principle of a 2DOF LEONAR, to validate by FEM simulations and experiments its behavior, and, finally, to lead a parametric analysis to explore the potentialities.
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