  Sound attenuation along a waveguide is intensively studied for applications
ranging from heating and air-conditioning ventilation systems, to aircraft
turbofan engines. In particular, the new generation of Ultra-High-By-Pass-Ratio
turbofan requires higher attenuation at low frequencies, in less space for
liner treatment. This demands to go beyond the classical acoustic liner
concepts and overcome their limitations. In this paper, we discuss an
unconventional boundary operator, called Advection Boundary Law, which can be
artificially synthesized by electroactive means, such as Electroacoustic
Resonators. This boundary condition entails nonreciprocal propagation,
meanwhile enhancing noise transmission attenuation with respect to purely
locally-reacting boundaries, along one sense of propagation. Because of its
artificial nature though, its acoustical passivity limits are yet to be
defined. In this paper, we provide a thorough numerical study to assess the
performances of the Advection Boundary Law, in absence of mean flow. An
experimental test-bench validates the numerical outcomes in terms of passivity
limits, non-reciprocal propagation and enhanced isolation with respect to local
impedance operators. This work provides the guidelines to properly implement
the Advection Boundary Law for optimal noise transmission attenuation.
Moreover, the tools and criteria provided here can also be employed for the
design and characterization of other innovative liners.
