Publication
Sound attenuation along a waveguide is a highly demanded research field, for applications ranging from heating and air-conditioning ventilation systems, to aircraft turbofan engines. Electroacoustic devices and digital control have provided the tools for crafting innovative liners where the boundary condition can be programmed. Hence, the question about “optimal” boundary conditions for noise transmission attenuation becomes more and more urgent. The most straightforward idea is to program classical local impedance operators, as these are the ones generally employed for modelling the current state-of-art of acoustic liners, especially for aeronautic applications. A strategy which has been proved to be at the same time simple and sufficiently robust, is to pilot the vibration of each speaker diaphragm based upon the sensed pressure on it (obtained by quasi-collocated microphones), in order to modify the resonator dynamics (varying its quality factor, or resonance frequency). Nevertheless, it might be worthy to navigate off the beaten track, and try to exploit the progammability of our electro-active systems, in order to target boundary operators which could never be physically produced by purely passive treatments. In this contribution, we focus the attention on a particular boundary law, called “advective”, as it possesses a convective character achieved thanks to the introduction of the first spatial derivative. We implement such boundary condition on our electroacoustic liner and demonstrate its potentialities in reducing the noise transmission and radiation in a circular waveguide. Numerical simulations and experimental implementation on a scaled turbofan mock-up show promising results.