NONLINEAR SOUND ABSORPTION OF HELMHOLTZ RESONATORS WITH EXTENDED NECKS AT HIGH SOUND PRESSURE LEVELS

The Helmholtz resonator with an extended neck (HREN) has shown effective sound absorption with a small thickness. It is therefore a promising candidate for noise mitigation in aeroengines where the installation space for the acoustic treatment is strictly constrained. In aeroengine applications, the noise intensity is extremely high, resulting in nonlinear acoustic properties of HRENs, which receive rare attentions. In this work, the sound absorption of a HREN under high-amplitude sound excitations is systematically studied. An analytical model to characterize the nonlinear absorption performance of the HREN is proposed based on the equivalent medium theory and the transfer matrix method. The nonlinear effects are considered by a velocity-dependent equivalent density of the air in the neck. Numerical simulations are also conducted to investigate the acoustic and flow characteristics in the HREN at high sound pressure levels (SPLs). Moreover, experiments are carried out in an advanced flow tube facility at Aerodynamics, Acoustics & Noise Control Technology Centre in HKUST (aantc.ust.hk) to measure the absorption coefficient of the HREN at the incident SPL up to 140 dB. Results show that the analytical prediction, numerical simulation, and experimental measurement of the absorption spectrum of the HREN are in good agreement. The vorticity of the flow field near the opening increases notably with SPL, transforming extra acoustic energy into vortex kinetic energy. Therefore, the acoustic resistance tends to increase with SPL, yielding an improved absorption performance of the HREN at high SPLs. The HREN liner demonstrates good absorption with the absorption coefficient greater than 0.9 at high SPLs (120 dB-140 dB) in a thin thickness around 1/20 wavelength of the operating frequency, which holds great potential in aeroengine noise problems.