Helmholtz resonators with integrated vibrating cantilevers for low-frequency noise control in aircraft sidewalls

Abstract

For more efficient aircraft engines, the trend is toward larger bypass ratios and gear-driven fans or counter-rotating open-rotor engines. As a result, engine noise emissions will change to include not only broadband noise, but also discrete frequencies with high sound pressure levels in the low-frequency range. The current aircraft sidewall can be considered acoustically as a double wall consisting of the outer fuselage skin and the inner cabin lining with insulation material sandwiched in between these layers. This is a sufficient sound barrier in the high-frequency range i.e. above 1000 Hz, but in the low-frequency range, where a number of tones from the future aircraft engines are expected, the sound insulation is less effective in reducing noise. Due to lightweight aircraft design principles, better sound insulation cannot be achieved by increasing the double wall mass or spacing. Therefore, there is a need for a new concept of an aircraft sidewall with improved low-frequency sound insulation that does not notably increase the weight or space constraints of the conventional wall. To address this challenge, this work presents a novel Helmholtz resonator type with two resonance frequencies integrated into an aircraft-like double wall to improve the sound transmission loss over a broader frequency spectrum, especially in the low-frequency range. The resonator incorporates a U-shaped slit that forms a cantilever. This cantilever provides a second resonance frequency, effectively broadening the frequency range with significant noise reduction. Constructed of a lightweight closed-cell foam, the resonator maintains its weight and volume compared to conventional Helmholtz resonators, offering a distinct advantage over the Helmholtz resonators with two resonance frequencies presented in the current literature. In addition, an analytical model for the Helmholtz resonator with integrated cantilever is developed in this thesis. This model allows an accurate determination of resonance frequencies and an evaluation of the absorption and transmission behavior. Furthermore, the model is verified and validated by finite element simulations and experiments, respectively, and extended to analyze multi-layer structures using the transfer matrix method. Parameter studies based on these analytical models are used to identify tuning mechanisms of the resonance frequencies and geometry parameters that influence the noise reduction behavior of the resonator. Optimization calculations were carried out to design a resonator panel that, when integrated into the aircraft sidewall, expands the frequency range of increased transmission loss in the low-frequency domain. This sound insulation improvement was successfully demonstrated in a laboratory test setup.