Sound pressure estimation at an electroacoustic transducer's voicing face by way of all-electrical sensing

When it comes to designing active control disposals, sound pressure sensing is usually required. In the framework of active control of acoustic impedance, this work aims at conceiving a digital estimator that processes voltage and current measurement to compute an estimation of the pressure in the vicinity of the diaphragm of an electro-dynamic speaker. After dealing with modeling concerns of the actuator, the bilinear transform is used to obtain equations of two IIR filters. An adaptive identification using the LMS algorithm and a SVD-based identification are tested to compute filters coefficient, both leading to poor experimental results. Accordingly, the actual transfer functions of the speaker are measured and optimal IIR filters are designed using the simulated annealing algorithm. It is then shown that good results can be achieved experimentally.

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.

2011

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

Romain Boulandet, Hervé Lissek, Etienne Thierry Jean-Luc Rivet

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.

2011

Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

Romain Boulandet

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.

EPFL2012

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

Romain Boulandet, Hervé Lissek, Etienne Thierry Jean-Luc Rivet

2011

Tunable Electroacoustic Resonators through Active Impedance Control of Loudspeakers

Romain Boulandet

EPFL2012