Wave-conversion active liner based on generalized Snell-Descartes law

The Cremer’s impedance theoretical framework has been used for years to optimally design acousticliners for the aeronautic industry, such as honeycomb liners. This passive mean of sound absorption,which consists in rather thin (a few centimeters) honeycomb-shaped cavities glued between a perfo-rated sheet and a rigid back-wall, has several downsides: an optimal acoustic absorption can only beachieved over a limited bandwidth, centered around one (eventually two) prescribed central frequency.Their performance at low frequencies is also limited by their size, owing to the "quarter-wavelengthrule". In this paper, we propose a fully tunable active liner based on electrodynamic resonators,employing loudspeaker membranes as sound absorbers, and an active control framework allowingmodifying their acoustic impedance over a wide frequency band. An optimization of these activeabsorbers is achieved by considering the theoretical framework of the generalized Snell-Descarteslaw, which accounts for impedance gratings over an interface. Through this formalism, it is possibleto devise a wave conversion strategy within an active acoustic liner in order to absorb multimodalsound propagation inside a rigid duct, improving the noise attenuation performance of the liner overan extended bandwidth.