Electroacoustic metamaterials: achieving negative acoustic properties with shunt loudspeakers

Acoustic metamaterials constitute a new class of structures that exhibit acoustic properties not readily available in nature. These properties can be a negative mass density, expressing the opposition of the acceleration of a particle to the application of pressure, or a negative bulk modulus, signifying the rarefaction of the particle in reaction to a compression (resp. a condensation in reaction to a depression). However, these artificial behaviors result from a periodic arrangement of passive unit-cells (such as membranes and side holes), and not from individual "meta-properties" of each unit-cell. It is however possible to observe such intrinsic metamaterial properties out of a passive electroacoustic resonator. This concept encompasses a loudspeaker, connected to a specific electric load, thus altering the acoustic dynamics of the loudspeaker diaphragm when subject to an exogenous sound source. It is especially possible to achieve negative acoustic impedance at its diaphragm thanks to the connection of passive electric shunt circuits, such as simple RLC series resonators. This paper aims at highlighting the metamaterial nature of such electroacoustic resonators through computational and experimental results, followed by discussions on ongoing developments

Shunt loudspeaker techniques for use as liners for engine nacelles

In this paper, the liner concept is an electroacoustic transducer which acoustic impedance can be changed by electrical means, be it passive or active. Among the different ways to obtain variable acoustic properties on an electroacoustic transducer's voicing face is the shunting of the transducer's electrical input. With such shunt devices, the acoustic impedance that the transducer's membrane presents to the acoustic field takes account of an acoustic equivalent of the electrical load that can take many values within a specified range. Shunt strategies can either be passive, with a resistor, or active, with a negative resistance circuit including at least one operational amplifier, or with more complex control systems (“hybrid control”). These active strategies allow very versatile acoustic impedances, assuming the whole device remains linear. Though, the use of the operational amplifier in active devices presents many drawbacks such as operational instability, non-linear distortion, high sensitivity to settings. In order to counteract this, alternative designs have been assessed. This presentation will give an overview of the above-mentioned techniques for use as liners in the engine nacelle, and discuss on their performances and limitations

Absorbeurs électroacoustiques actifs: application à l’égalisation des salles aux basses fréquences

Les absorbeurs électroacoustiques sont des résonateurs à membrane actionnés par un moteur, par exemple un transducteur électrodynamique, et contrôlés à travers une boucle d’asservissement, de manière à modifier l’absorption acoustique de la membrane, et la rendre optimale sur une large bande de fréquence autour de sa résonance. Tirant profit du fait que les haut-parleurs conventionnels présentent une fréquence de résonance dans les basses fréquences, cette technique présente des avantages certains pour l’amortissement des modes propres aux basses-fréquences dans les salles. Ce type de dispositif a des applications directes pour de nombreux problèmes acoustiques, comme l’égalisation de salles pour la diffusion de musique dans le registre grave, ou la réduction de l’exposition aux bruits basses fréquences venant de sources extérieures dans les habitations, entre autres exemples. Dans ce papier, nous présentons le principe de dimensionnement d’un prototype d’absorbeur électroacoustique en vue de l’égalisation modale de la salle réverbérante de l’EPFL. Nous présentons ensuite les performances d’un ensemble de 4 prototypes conçus sur ce design, et mesurés dans la salle réverbérante de l’EPFL, avec différentes excitations sonores (bruit large bande stationnaire, sons purs de durées limitées, musique). Il est ainsi mis en évidence que ces 4 prototypes d’absorbeurs électroacoustiques, qui représentent à peine 0.1% de la surface totale des parois de la salle, permettent une atténuation globale du niveau sonore SPL d’environ 8 dB dans la gamme [20 – 100 Hz], avec des atténuations modales individuelles pouvant atteindre 12 dB. Par ailleurs, la mesure des temps de décroissances modales, obtenues avec des excitations aux différentes fréquences de résonances de la salle, montre des réductions significatives pour tous les modes, pouvant atteindre 84% de réduction du temps d’extinction des modes. Enfin, l’effet bénéfique des absorbeurs sur des sons fortement modulés, comme de la musique, est mis en évidence de manière qualitative

Robust direct acoustic impedance control using two microphones for mixed feedforward-feedback controller

This paper presents an acoustic impedance control architecture for an electroacoustic absorber combining both a feedforward and a feedback microphone-based strategies on a current-driven loudspeaker. Feedforward systems enable good performance for direct impedance control. However, inaccuracies in the required actuator model can lead to a loss of passivity, which can cause unstable behaviors. The feedback contribution allows the absorber to better handle model errors and still achieve an accurate impedance, preserving passivity. Numerical and experimental studies were conducted to compare this new architecture against a state-of-the-art feedforward control method

Constant-pressure sound waves in non-Hermitian disordered media

When waves impinge on a disordered material they are back-scattered and form a highly complex interference pattern. Suppressing any such distortions in the free propagation of a wave is a challenging task with many applications in a number of different disciplines. In a recent theoretical proposal, it was pointed out that both perfect transmission through disorder as well as a complete suppression of any variation in a wave intensity can be achieved by adding a continuous gain-loss distribution to the disorder. Here we show that this abstract concept can be implemented in a realistic acoustic system. Our prototype consists of an acoustic waveguide containing several inclusions that scatter the incoming wave in a passive configuration and provide the gain or loss when being actively controlled. Our measurements on this non-Hermitian acoustic metamaterial demonstrate unambiguously the creation of a reflectionless scattering wave state that features a unique form of discrete constant-amplitude pressure waves. In addition to demonstrating that gain-loss additions can turn localised systems into transparent ones, we expect our proof-of-principle demonstration to trigger interesting new developments not only in sound engineering, but also in other related fields such as in non-Hermitian photonics

Optimization of electroacoustic resonators for semi-active room equalization in the low-frequency range

At low frequencies, standing waves within the room cause large frequency-response variations in the listening environment, such as audio rooms or recording studios. This unwanted phenomenon has a significant impact on the sound quality of an audio system. Unfortunately, state-of-the-art soundproofing solutions cannot efficiently handle such low-frequency sound energy. To alleviate this problem, electroacoustic resonators can be used to damp room modes. This concept is based on the connection of direct-radiator loudspeakers to synthetic electrical loads allowing the passive dissipation of a certain part of the incoming acoustic energy of the sound field. Through judicious control of acoustic impedance depending of the placement of the electroacoustic resonators in the room, a significant damping of the dominant natural resonances can be achieved in order to meet the specifications of audio reproduction. This paper investigates the optimization and the spatial arrangement of electroacoustic resonators with a view to damp the low-frequency acoustic resonances in enclosed spaces

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