From the electrical shunting of a loudspeaker to active impedance control

Hervé Lissek, Florian Sandoz
2008
Discussion par affiche

Variable acoustic properties can be obtained on an electroacoustic transducer’s voicing face by very basic control strategies, among which are the shunting of the loudspeaker (shortcut, variable electric load, or negative resistance disposals). It is proven that enhanced controllability and better performances can be obtained by way of hybrid feedback control, consisting in a double feedback loop, one on acoustic pressure at the diaphragm and the other on its velocity, leading to a global acoustic impedance control. The present work will describe the theory of the hybrid feedback control, by way of block diagrams aiming at visualizing the control principle, starting from the shunted loudspeaker. Simulations of performances obtained on Matlab/Simulink will then be presented, and compared to experimental results obtained on analog prototypes mounted at the end of a dedicated impedance tube. A discussion on stability will then follow, leading to concluding remarks on the disposal behavior and possible means of enhancements.

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Hervé Lissek, Florian Sandoz
2008
Discussion par affiche

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https://infoscience.epfl.ch/record/118544?ln=fr

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Les Matériaux Actifs à Propriétés Acoustiques Variables

Hervé Lissek

This work deals with the concept of active materials with variable acoustic properties, dedicated to room acoustics applications. This concept is based on an acoustic impedance active control principle by hybrid motionnal feedback, applied on an electroacoustic transducer. This hybrid feedback associates a motionnal feedback, obtained by an electric impedance bridge, and a pressure feedback, obtained by a microphone placed in the neighbourhood of the actuator. After the concept of active material has been modelized and tested experimentally with single moving-coil loudspeaker, this principle is extended to the concept of active wall. A prototype of active wall with active movingcoil loudspeakers is modelized and tested, in order to find specifications for a specific electroacoustic transducer dedicated to the « active materials » application. This work leads to the implementation of the isodynamic transducer, for which a numeric model is elaborated. The study of the electroacoustic behaviour of this transducer underlines several problems, likely to limit its performances as active material with variable acoustic properties, and lead us to develop a numeric tool for sizing the transducer for this application. On the basis of this study, many prototypes are build in order to, in one hand, validate the numeric model, and in the other hand, point out the limitations of the transducer to find practical solutions to improve its performances. The results obtained with such transducers as active materials, obtained for plane waves and normal incidence, help us estimate future works for the achievement of active materials with variable acoustic properties; the isodynamic principle seems to be the most suitable for this concept, in terms of results as well as in terms of product costs.

Université du Maine2002

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Design of Shunt Electric Networks in View of Sound Absorption with Loudspeakers

Romain Boulandet, Hervé Lissek

Variable acoustic properties can be obtained at the diaphragm of a loudspeaker, with the help of very basic control strategies, among which is the simple electrical shunting of the transducer. These shunt techniques have been demonstrated to present singular similarities with active acoustic feedback strategies, dedicated to the control of the acoustic impedance of the loudspeaker. Based on this observation, an interesting strategy has been developped, intending at designing electric networks which, when connected to the loudspeaker, can make the latter reach a desired acoustic impedance over a certain frequency badnwidth. This paper presents a methodology for designing electric networks, that can be either passive or active, capable of achieving variable sound absorption at the loudspeaker diaphragm. In a first part, the theory underlying the concept of "electroacoustic absorber" is provided, highlighting formal equivalences between shunt and active feedback control, especially through the introduction of equivalent electric networks that mimic the performances of acoustic feedbacks. Simulated acoustic performances are presented, followed by discussions on the design of active electric shunts in view of active sound absorption. At last, experimental assessments of the studied configurations are presented, with general discussions on the potential improvements and applications.

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Design and assessment of an electrodynamic loudspeaker used in a variable acoustic lining concept

Hervé Lissek, Anne-Sophie Moreau

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 developed for 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.

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