Hybrid passive-active multilayer absorbers

This paper deals with the design and optimization of a hybrid passive/active liner for broadband noise reduction in a standing wave tube. The thickness of the multilayer absorbent is reduced in two steps: firstly a porous layer backed by an air gap is substituted by a microperforated panel followed by a reduced air gap; and secondly, the secondary loudspeaker is replaced by a thin plate driven by a piezoceramic patch. Experimental results show that high absorption may be achieved in a wide frequency range (100-900 Hz) with a thin hybrid passive/active absorber.