About Helmholtz Resonators with Flexible Walls for Acoustic Damping

Kurzfassung

Aircraft engines are a major source of noise, affecting health and quality of life near airports. To reduce fuel consumption, future engines are expected to be larger and produce more low-frequency, broadband noise, challenging conventional liners. This thesis investigates a new liner concept combining a Helmholtz resonator with flexible walls to enhance damping via structural losses and modify resonance behaviour. An analytical and numerical model were developed to predict the acoustic performance of the coupled system, consisting of a perforated face sheet and a cavity divided by a flexible wall. The analytical model enables physical insight into the damping mechanisms, whereas the finite element model resolves detailed coupling effects and visualises sound fields and vibrations. A modular Helmholtz resonator with a flexible wall was designed and tested in a normal incidence tube, enabling a systematic study of face sheet, wall material, shape, and cavity depth to validate the models. In-situ measurements used a laser vibrometer and cavity microphones. An additional multi-cell liner tested under grazing flow with high sound pressure levels confirmed additional dissipation in realistic conditions. Results show the concept effectively enhances damping under both normal and grazing incidence, with and without flow. The validated models provide a foundation for future optimisation to efficiently reduce aircraft noise.