This paper deals with the complete design procedure of a new type of hybrid acoustic liner, combining passive absorbent properties of a porous layer and active control. The final application concerns noise reduction in aircraft engine nacelles. The objective is to achieve, over a wide frequency range, a targeted impedance of the porous layer. This impedance is theoretically predetermined to produce the best noise reduction when applied to a flow duct. The impedance control is achieved by minimising the pressure at the rear face of the porous layer having well-suited characteristics. Using high resistive materials and a piezoelectric transducer as a secondary source, a thin active liner is achieved. Standing wave tube measurements have validated the concept and promising results have been obtained in a flow duct with feedforward control. Significant noise reduction has been measured with 4 active cells, with and without flow. However, performances with a feedback controller were limited due to relevant contributions of the secondary source response in the high frequency range, which lead to spillover effects.
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