The acoustic impedance at the diaphragm of an electroacoustic transducer can be varied using a range of basic electrical control strategies, amongst which are electrical shunt circuits. These passive shunt techniques are compared to active acoustic feedback techniques for controlling the acoustic impedance of an electroacoustic transducer. The formulation of feedback-based acoustic impedance control reveals formal analogies with shunt strategies, and highlights an original method for synthesizing electric networks ("shunts") with positive or negative components, bridging the gap between passive and active acoustic impedance control. This paper describes the theory unifying all these passive and active acoustic impedance control strategies, introducing the concept of electroacoustic absorbers. The equivalence between shunts and active control is first formalized through the introduction of a one-degree-of-freedom acoustic resonator accounting for both electric shunts and acoustic feedbacks. Conversely, electric networks mimicking the performances of active feedback techniques are introduced, identifying shunts with active impedance control. Simulated acoustic performances are presented, with an emphasis on formal analogies between the different control techniques. Examples of electric shunts are proposed for active sound absorption. Experimental assessments are then presented, and the paper concludes with a general discussion on the concept and potential improvements.

At echnique to setup as imply supported rectangular plane panel for laboratory vibroacoustic tests is described and validated. Forag iven panel fixed to thin vertical supports, ad imensionless parameter is proposed to size these supports following adesired frequencyprecision compared to theoretical eigenfrequencies of apanel with such boundary conditions. An umerical study confirms the potential of this design parameter.D etailed instructions for assembling apanel with adequate thin vertical supports on arigid frame are then given. Finally,three laboratory cases are described which illustrate possible experimental vibroacoustic applications using ap anel assembled following previous guidelines. The design parameter viability is experimentally confirmed, and all obtained results depicted good agreement with analytical solutions and numerical predictions.

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This paper deals with active sound attenuation in lined ducts with flow and its application to duct modes damping in aircraft engine nacelles. It presents an active lining concept based on an arrangement of electroacoustic absorbers flush mounted in the duct wall. Such feedback-controlled loudspeaker membranes are used to achieve locally reacting impedances with adjustable resistance and reactance. A broadband impedance model is formulated from the loudspeaker parameters and a design procedure is proposed to achieve specified acoustic resistances and reactances. The performance is studied for multimodal excitation by simulation using the finite element method and the results are compared to measurements made in a flow duct facility. This electroacoustic liner has an attenuation potential comparable to that of a conventional passive liner, but also offers greater flexibility to achieve the target acoustic impedance in the low frequencies. In addition, it is adaptive in real time to track variable engine speeds. It is shown with the liner prototype that the duct modes can be attenuated over a bandwidth of two octaves around the resonance frequency of the loudspeakers.

a b s t r a c tIn multivariate systems, when it comes to identifying actual operating conditions ranges, or optimal settings, the use of constrained optimization is often required. Among the different tools for the engineer to perform such optimization, designed experiments offer accurate performances. In this paper, the optimization process of ''electroacoustic absorbers" is investigated by means of response surface methodology. A multivariate linear model is established by a series of designed experiments in order to analyze the modification of electroacoustic absorber performances due to the variation of several constitutive parameters (such as the moving mass of the loudspeaker, the enclosure volume, the filling density of mineral fiber within the enclosure, and the electrical load value to which the loudspeaker is connected), that influence their whole absorbing mechanisms. A simple case study is then provided to illustrate the capabilities of the developed optimization procedure, from which general conclusions on such design methodology, as well as on electroacoustic absorbers sensitivity, are drawn.