Measurement and prediction of nonlinear acoustic liners in the presence of high level multiple tones

Aircraft noise is a problem near airports all around the world. As a result, aircraft certification requirements are becoming more stringent over time. The engine fan is one of the most prominent noise sources that produces high amplitude blade passing frequency tones, their harmonics, and engine order buzz tones. Acoustic panels, consisting of resistive layers backed by honeycomb cells and a reflective hard wall, are used to line the engine nacelle in order to attenuate both broadband and annoying tonal noise. The acoustic impedance of single layer perforate liners (SDOF) show greater sensitivity to high sound pressure levels and grazing flow in comparison with wire mesh liners. The focus of the work reported here is to understand the physical loss mechanisms of SDOF perforate liners under high SPL multiple tones. This has been realised through measurements, modelling, and semi-empirical prediction of perforate liner impedance. Two harmonically related tones with varying combinations of amplitude and frequency were used as excitation signals. Punched Aluminium perforates, along with rapid 3D printing prototyped samples composed of ABS and Stainless Steel, were tested. A sample holder was designed to allow simultaneous in situ and traditional TMM measurements. A grazing flow test rig developed and commissioned by the author and others at the LVA/UFSC (Laboratory of Acoustics and Vibration at the Federal University of Santa Catarina), in Brazil, used TPM, MMM, and SFM impedance eduction techniques and the in situ method, which were cross validated. The semi-empirical 1D impedance models of Rice, Cummings, Boden, and Maa, were implemented numerically using MATLAB, and a proposed 1D impedance model with a frequency-dependent discharge coefficient was developed. Also, 2D Multiphysics numerical models in time domain, based on FEM, were developed and validated using the MATLAB & COMSOL Multiphysics 5.2a livelink. The COMSOL code reproduces high SPL experimental conditions, allowing direct comparisons with measurements and published data. These studies have aided the development of an improved semi-empirical 1D impedance model for pure tone excitation, and a resistance correction for multiple tone excitation