Impedance assessment of a dual-resonance acoustic liner

Acoustic liners are commonly used to reduce noise from commercial aircraft engines. Engine liners are placed in the nacelle inlet and aft bypass duct to attenuate the noise radiated from the engine. Traditional engine liners are constructed of a perforated facesheet over a honeycomb structure to create a quarter-wave absorber. With this design, the low frequency performance of the liner is limited by the depth of the honeycomb. However, with advances in engine design, lower frequency sound absorption is becoming more critical while liner depth must be minimized. Acoustic metamaterials can exhibit unique acoustic behavior using periodically arranged sub-wavelength resonators. Researchers have shown that acoustic metamaterials can effectively block the propagation of low-frequency acoustic waves. Therefore, acoustic metamaterial-inspired concepts are being investigated to improve the low frequency performance of engine liners. A proposed dual-resonance liner is presented here that combines the idea of a Helmholtz resonator metamaterial with a traditional quarter-wave acoustic liner. The low frequency acoustic absorption of a traditional liner can be significantly increased by adding a second, low frequency resonance to the system. The normal incidence absorption coefficient of the proposed liner is more than 10 times larger than a conventional honeycomb liner at the designed Helmholtz resonance frequency while retaining similar performance at higher frequencies.