Low frequency sound absorption of resonators with flexible tubes

Abstract

Classically, passive acoustic liners, used in aeronautic engine nacelles to reduce radiated fan noise, have a quarter-wavelength behavior, thanks to perforated sheets backed to honeycombs (SDOF, DDOF). So, their acoustic absorption ability is naturally limited to medium and high frequencies because of constraints in thickness. To drastically improve their capabilities to the lowest frequencies, the combination with active control systems or the using of foam architecture have shown an interest, but the industrial application is tricky (i.e., problems of fouling, robustness). A possible approach is to carry out a perforated panel resonator with flexible tube bundles to shift the resonance frequency to a lower frequency by a prolongation of air column length (Yadong Lu et al., Internoise 2007). This paper describes theoretically this concept that allows a significant change in the acoustic impedance due to the large thickness of the resistive and reactive material and the coupling with the surrounding cavity. Applied to aeronautical configurations, the resonance frequency decreases considerably compared to a conventional resonator (factor of about 1/5) but with a reduction of the maximum absorption when the tubes fill the cavity. Experiments in impedance tube validate the theoretical approach.