Overview of Low Frequency Resonators Based on LEONAR Design

Passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a resonator behavior. The simplest systems are SDOF-type (Single Degree Of Freedom), composed of a perforated sheet backed to a honeycomb, whose absorption is limited to a medium or high frequency range due to a small thickness. To widen the frequency range of absorption, manufacturers stack two SDOF to create a DDOF (Double Degree Of Freedom). The main constraint is the limited thickness of the global system which reduces the space dedicated to each honeycomb. The LEONAR concept ("Long Elastic Open Neck Acoustic Resonator) proposed by authors was shown to overcome the problem of available space in case of "low frequency" range requirements. It consists in linking the perforated layer with hollow tubes introduced in the honeycomb, to shift resonance frequencies to lower frequencies by a prolongation of air column lengths. This paper highlights the characteristics related to different LEONAR designs, as SDOF, DDOF and meta-surface, compared with classical resonators. Moreover, the influence of tube cross-section shape, the material of which it is made and the elasticity are addressed. This overview is led through a theoretical approach, simulations and experimentations in impedance tube and aero-acoustic benches.