Compact 2DOF liner based on a long elastic open-neck acoustic resonator for low frequency absorption

Abstract

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.

Design and optimization of acoustic liners with a shear grazing flow: OPAL software platform description

“…In the limit where the pore radius tends to +1 (or, a bit more realistically, to 1m), the honeycomb behaves like the open ambient fluid. • A LEONAR [10,13] consists in the combination of tubes and a cavity [14,15]. The tubes are modeled similarly to a perforated plate of increased thickness, while the cavity is taken as a honeycomb layer of adequate radius.…”

Low-Frequency-Noise Attenuation through Extended-Neck Double-Degree-of-Freedom Helmholtz Resonators

“…Past research has shown neck extensions leading to lower-frequency sound attenuation [19][20][21][22][23], due to the introduction of multiple resonance frequencies, when the neck extensions become significantly large [24][25][26][27]. Following the recent surge in studies such as Simon et al [28] and Jones et al [29] investigating the use of extended-neck resonators for low-frequencysound absorption, this paper presents the underlying mechanism of sound attenuation from double-degree-of-freedom (2-DoF) resonators with varying neck extensions.…”