Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

The current trend for multipurpose rooms requires enhanced acoustic treatments capable to meet ever more demanding specifications in terms of performance, compactness and versatility. The reason is the variety of activities to be hosted and the corresponding requirements in terms of acoustic quality which may be very different and even conflicting. In any process to improve listening comfort, the treatment of low-frequency sound is a major concern. The problem stems from the proven ineffectiveness of passive soundproofing solutions of the state of the art, or from their bulkiness that may be prohibitive. This thesis focuses on the analysis, design, realization and characterization of tunable electroacoustic resonators intended to specifically address this issue. This concept deals with loudspeakers, the acoustic impedance of which can be easily adjusted in a controlled fashion. Creating an electroacoustic resonator out of a loudspeaker is the result of an interdisciplinary effort. Such a challenging task combines conceptual tools, models, and applied solutions, drawing from the fields of audio engineering, control theory, and electrical engineering, both in the analog and digital domains. A unifying theory is introduced, covering different strategies from passive electrical shunt to active control of acoustic impedance in a single formalism. This research shows that achieving a desired acoustic impedance at the transducer diaphragm is equivalent to the implementation of a specific functional relationship between the electrical current and voltage across the transducer terminals, and vice versa. From a design perspective, the specific electrical load is tailored by using an internal model of the transducer. The result is an innovative model-based synthesis methodology where the active control of acoustic impedance is reformulated as an electrical impedance synthesis, thus removing the use of sensor. This concept opens new opportunities to improve listening spaces by providing efficient acoustic absorption at low frequencies. Experiments clearly show the benefits of the proposed methodology in a field where there is currently no competitive solution. It is believed that the technological advances resulting from the coupling of a loudspeaker with a synthetic load should pave the way to innovative techniques in noise control and, hopefully, stimulate research in related areas

Acoustic impedance synthesis at the diaphragm of moving coil loudspeakers using output feedback control

This paper discusses a time-domain technique for synthesizing acoustic impedance at the diaphragm of a loudspeaker using a proportional-plus-derivative output feedback. The dynamics of electroacoustic transducers such as moving-coil loudspeakers can be readily controlled either by direct feedback principle on acoustic quantities, or by plugging a shunt network at the electrical terminals. Any conventional loudspeaker first intended to be a sound transmitter may then become a versatile electroacoustic resonator capable of absorbing (or of reflecting as much) the incident sound energy in a frequency-dependent way by simple electronic controls. Instead of counteracting some unwanted sound by using superposition principle, as is the case for conventional active noise control, such actuator-based strategy aims at monitoring the reaction of a loudspeaker embedded into walls so as to control the proportion of reflected sound waves on this boundary. After a short description of the dynamics of moving-coil loudspeakers giving emphasis on the advantage of electromechanical coupling reversibility, a proportional plus derivative output feedback combined to a feed-forward action is proposed for synthesizing of desired acoustic impedance. As a conclusion, the overall performance of the proposed method is presented along with computed results and general discussions on practical implementation

Stratégies de contrôle actif de l’acoustique des salles aux basses fréquences – une étude expérimentale

Le comportement modal des salles est une des principales causes d’inconfort acoustique, dans l’habitat et dans les salles destinées à accueillir du public, dans le domaine des basses fréquences. Les solutions techniques de l’état de l’art ne permettent par ailleurs pas de traiter efficacement ces spécificités acoustiques des espaces clos. Afin de surmonter ces limitations pratiques, des solutions de contrôle actif peuvent être efficacement déployées, avec des gains acoustiques significatifs. Ces performances sont cependant limitées à des bandes fréquentielles relativement étroites, un contrôle large-bande nécessitant des systèmes complexes en termes de transducteurs et de contrôle. Dans le même temps, des techniques alternatives de contrôle dits « semi-actif », tels que des absorbeurs électroacoustiques (transducteurs shuntés), ont montré une efficacité significative sur une base fréquentielle plus large. Ces systèmes sont particulièrement avantageux dans la mesure où ils ne dépendent pas de la nature des sources acoustiques et des positions relatives des capteurs et actionneurs, mais directement de la dynamique des transducteurs et de leur faculté d’absorber de l’énergie acoustique. Par ailleurs, les énergies requises pour leur fonctionnement sont extrêmement faibles, voire nulles (systèmes passifs). Cependant, certaines limitations doivent être prises en considération, en particulier pour ce qui concerne l’intégration de ces systèmes dans des salles réelles. Ce papier présente les différentes stratégies assimilées au contrôle actif modal de l’acoustique des salles sur la base d’une approche expérimentale, en comparant les performances et les limitations de chacune, afin d’en tirer des conclusions quant à l’implémentation pratique de tels systèmes dans des salles réelles

Conception de matériaux electroacoustiques intelligents par contrôle d'un haut-parleur à l'aide de filtres numériques

Ce poster présente les "matériaux acoustiques intelligents", qui sont des haut-parleurs actifs dont l'impédance électrique peut-être changée de façon contrôlée, soit de manière passive, ou semi-active

Electroacoustic absorbers II: implementation of a digital synthetic admittance for controlling the dynamics of electroacoustic absorbers

This paper discusses the implementation of a synthetic electrical admittance for controlling the dynamics of electroacoustic absorbers. Indeed, different basic control techniques are capable of varying the acoustic impedance of an electroacoustic transducer diaphragm, in view of achieving sound absorption. Among these techniques, one must consider the direct acoustic impedance control, based on acoustic feedbacks (pressure and velocity), and the straightforward shunt loudspeaker approach. Shunting the electric terminals of a loudspeaker with a resistive load will slightly modify the resonance quality factor of the electroacoustic absorber, by merely adding Joule losses, yielding enhanced narrow-band sound absorption. The extension of the resonance bandwidth requires then active circuits, as in acoustic feedback control. Lately, it has been demonstrated that feedback-based principles reveal formal analogies with electrical shunt approaches. Based on this observation, the design of electroacoustic absorbers can be performed through the design of active electric networks shunting the loudspeaker terminals, mimicking the behavior of acoustic feedbacks used in a direct acoustic impedance control. In this paper we present the design of equivalent electrical network in the digital domain (FPGA-based) as well as the practical implementation of this synthetic admittance with an actual electroacoustic transducer, so that the whole device behaves as a broadband sound absorber. Numerical simulations are given to illustrate the dynamic behavior of the transducer once shunted with the designed synthetic admittance. An experimental assessment using a conventional moving-coil loudspeaker in a one-dimensional duct is also presented, thus showing the effectiveness of the synthetic admittance for making it a broadband sound absorber. As a conclusion, general remarks on the overall acoustic performances of such a shunted transducer are discussed, along with practical considerations about stability issues

Advanced control for modifying the acoustic impedance at the diaphragm of a loudspeaker

The control of low-frequency sound fields can be addressed efficiently through acoustic impedance matching. Basically, the diaphragm of electroacoustic transducers are used as refracting surface that controls the reaction of the boundaries in any surrounding sound fields. The general idea is to absorb the incident sound energy or to contain it, simply by altering the transducer dynamics in a controlled fashion. Usual techniques operate either by feedback control of acoustic variables (sound pressure or velocity) or by connecting some electrical load at the transducer terminals. The paper focuses on how to transform an electrodynamic loudspeaker in an active electroacoustic resonator through the use of sensor and controller. It is discussed how to achieve broadband sound absorption at the transducer diaphragm. Phase compensation technique are also introduced as a convenient way to overcome a practical issue that may arise in some cases, taking the form of an over-reflective behavior of the diaphragm. For illustrative purposes, computed results and measurements obtained in impedance tube are provided to show the performance of a controlled loudspeaker in terms of acoustic absorption capability and stability

Design of a built-in electroacoustic resonator for active noise reduction

The paper focuses on the design of a built-in electroacoustic device for active noise reduction purposes. The device basically encompasses a loudspeaker connected to a synthetic electrical impedance that enhances its ability to dissipate part of the incoming acoustic energy. The strategy is therefore to control some boundaries of enclosed sound spaces (such as room, cavity, etc.) rather than targeting a global control that requires significant input of additional acoustic energy. The main attraction of the proposed methodology is to achieve broadband sound absorption while bypassing the use of sensors. It is discussed how the relevant information on the sound field which is usually provided by sensors can be encapsulated into a synthetic electrical load. Computed and experimental results are provided to illustrate the benefits and potential of a built-in electroacoustic device compared to other options. Concluding remarks are made to discuss the foreseen future developments

Design of Remote Quiet Zones Using Spot-Type Sound Reducers

This paper presents a local control approach to generate remote quiet zones. To deal with situations where global control can hardly be achieved, it is proposed to use an arrangement of spot-type sound reducers as originally suggested by Olson and May. Assuming that cross-coupling between control units is weak, each can be controlled independently and a decentralised feedback controller is implemented without the need for direct monitoring of the primary source. Active noise attenuation in the remote target region is achieved using a linear quadratic optimization based on prior knowledge of the transfer path of the system. The performance of a particular configuration comprising three control units is examined by numerical simulation and experimentally evaluated for a tonal noise source in a free-field environment. An average noise reduction of about 6 dB was measured in a target region of volume 0.25 ×0.25 ×0.25 m3 for a 160 Hz tonal primary source distant more than one wavelength from the secondary sources. The performance of the control system in relation to changes in the primary field is also considered with a view to extending the concept to more realistic enclosed sound field conditions in future work

Optimisation d'un absorbeur électroacoustique par plans d'expériences : approche expérimentale et numérique

Dans cet article le processus d'optimisation d'absorbeurs électroacoustiques est étudié à l'aide de la méthode des surfaces de réponses. Un absorbeur électroacoustique est un système haut-parleur dont l'impédance acoustique peut être ajustée électriquement, de manière passive ou bien active. Un modèle mathématique est établi par plans d'expériences pour analyser l'effet de certains paramètres de conception sur les performances acoustiques de l'absorbeur. Pour quantifier la sensibilité d'un tel système soumis à différentes contraintes physiques, la méthode de surface de réponse a été choisie pour développer un modèle multivariable. Ces contraintes ont été choisies de manière à représenter les différents mécanismes dissipatifs mis en jeu au sein de l'absorbeur électroacoustique : dissipation par effet résonateur mécanique, dissipation par viscosité de l'air sollicitée par la pénétration des ondes sonores dans un milieu poreux, et dissipation par effet Joule induite par la charge résistive branchée aux bornes du haut-parleur. En vue d'évaluer la contribution de ces différents mécanismes sur les performances d'un absorbeur électroacoustique, notre étude à retenu les quatre paramètres d'entrée suivants : la masse de l'équipage mobile, la compliance induite par le volume de l'enceinte close, le taux de remplissage en matériaux poreux et la résistance électrique de shunt. Une étude préliminaire, réalisée par une double approche expérimentale et numérique, illustre les possibilités du processus d'optimisation développé. Le résultat obtenu est une mise en équation du facteur d'absorption permettant de piloter des réglages et de trouver un optimum en fonction de la fréquence