Breaking the limitations of local impedance noise control: passivity, and scattering performances of the Advection Boundary Law

In the aim of attenuating noise transmission through air-ducts, research is prompted for overcoming the limitations of classical acoustic liners, especially in the aero-engines applications. The new generation of Ultra- High-By-Pass-Ratio (UHBR) turbofans while considerably reducing fuel consumption, increases noise pollution especially at lower frequencies because of their larger diameter, lower number of blades and rotational speed. Moreover, they present a shorter nacelle, leaving less available space for acoustic treatments. In case of simplified one-dimensional propagation, integral constraints exist which analytically define the limits of the scattering performances of reciprocal systems, such they are the local impedance liners, for a fixed length of the acoustic treatment along the duct. In this contribution, we analyse a special boundary condition breaking the reciprocity principle, and overcoming the limitations of locally reacting liners. We call it Advection Boundary Law as it introduces a convection on the boundary, responsible of non-reciprocal behaviour at grazing incidence, and of the enhancement of transmission loss with respect to pure locally-reacting resonators. Performances and passivity of such boundary law are numerically analysed first in grazing-incidence problems. The grazing-incidence problem is experimentally studied in a plane-wave acoustic waveguide lined by electroacoustic resonators which can be programmed to reproduce such advection boundary law.