Study on multi-degree-of-freedom septum liners for broadband noise reduction

Silent aeroengine nacelle highlights the demand for advanced acoustic liners providing broadband absorption. This paper studies the multi-degree-of-freedom (MDOF) septum liner that is a promising candidate. The steady-flow resistance of the septum as the necessary input parameter is measured on a steady-flow resistance tube herein, and an impedance prediction model is introduced and validated through the experimental measurement and the numerical simulation. Then, three liners with embedded septa of varied types, numbers and depths are designed, and the acoustic characteristics are analyzed in a wide frequency range under different incident sound pressure levels (SPL). It indicates that such liners can perform well in an ultra-broadband range up to 10000 Hz, and the impedance is relatively insensitive to the incident SPL. It is worth noting that, achieving broadband absorption for such liners is influenced by the coupling effect between zeros of the reflection coefficient, which correspond to the resonance frequencies of the liner. Moreover, the simulated results illustrate that the significant absorption for MDOF septum liners stems mainly from the sound energy dissipation at specific spatial positions depending on the frequency.