Design and Assessment of an Electrodynamic Loudspeaker Used in a Variable Acoustic Lining Concept

The acoustic impedance at the diaphragm of an electroacoustic transducer can be modified using a range of active control strategies, amongst which is “direct impedance control”. This technique employs a combination of feedbacks, one on the acoustic pressure and the other on the diaphragm velocity, to the loudspeaker electric terminals, allowing significant modifications of the acoustic resonator properties of the diaphragm within a specified range. This concept has been specifically adapted to a novel concept of active acoustic liners for aircraft engines within the European project OPENAIR. One of the objectives of active acoustic liners is the achievement of real-time modifications of the acoustic impedance of a 1 degree of freedom resonator. These properties should then match variable target impedances, specified by phases of flight. The paper presents the development of a specific electrodynamic loudspeaker for use as active liners, with an emphasis on integration issues, as well as acoustic performances. The general concept of direct impedance control is first introduced, followed by design considerations on the loudspeakers side, and then experimental assessments of performances are presented.