Abstract
Hybrid (active/ passive) absorbers are developed at the LMFA mainly as wall treatments of flow ducts. Active control is used to increase the absorption of a porous layer at low frequencies by cancelling the imaginary part of the surface impedance presented by the absorber. The resulting surface impedance is purely real and constant. For the focused application, however, the optimal surface impedance in terms of absorption is neither purely real nor constant. This paper deals with the enhancement of the hybrid absorber in a way that complex and frequency dependent surface impedances can be realized. Normal velocity is measured by the use of a microphone on either side of a resistive layer, which allows the estimation of the surface impedance. This estimation is then substracted from a target impedance to form a new error signal. First experiments carried out in a standing wave tube show that a typical optimal impedance with positive increasing real part and negative decreasing imaginary part can be obtained. Measurements in a grazing incidence tube confirm that transmission loss is significantly increased compared to the former absorber.
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