Extrêmement basse fréquence | EPFL Graph Search

L’extrêmement basse fréquence ou EBF (en anglais, extremely low frequency ou ELF) est la bande de rayonnement électromagnétique (radiofréquences) comprise entre (longueur d'onde de ). Dans le domaine des sciences de l'atmosphère, une définition alternative est généralement retenue : de 3 Hz à 3 kHz. La fréquence du courant alternatif dans les réseaux électriques (50 ou 60 Hz) est située dans la bande de ELF, ce qui fait des réseaux électriques une source involontaire de rayonnement ELF. Utilisations vignette|300px|Vue aérienne (1982) d'une installation de transmission en ELF de l'U.S. Navy (Clam Lake, Wisconsin) dédiée à la communication avec les sous-marins en immersion profonde. En raison de la difficulté de construire des antennes pouvant rayonner ces ondes longues, ces fréquences ne sont que très peu utilisées par les télécommunications. Ces ondes se propageant dans l'eau de mer, elles se montrent utiles pour la communication avec les sous-marins ou des robots s

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

Sound absorption at low frequencies still remains a challenge in both scientific research and engineering practice. Natural porous materials are ineffective in this frequency range, as well as acoustic resonators which present too narrow bandwidth of absorption, thus requiring alternative solutions based on active absorption techniques. In the present work, we propose an active control framework applied on a closed-box loudspeaker to enable the adjustment of the acoustic impedance at the loudspeaker diaphragm. More specifically, based on the proportionality between the pressure inside the enclosure and the axial displacement of the loudspeaker diaphragm at low frequencies, we demonstrate both analytically and experimentally that a PID-like feedback control approach allows tuning independently the compliance, the resistance and the moving mass of the closed-box loudspeaker to implement a prescribed impedance of a single-degree-of-freedom resonator. By considering different control combinations to tailor the resonator characteristics, a perfect absorption (with absorption coefficient equal to 1) is achievable at the target resonance frequency, while enlarging the effective absorption bandwidth. Moreover, the proposed feedback control strategy shows an excellent control accuracy, especially compared to the feedforward-based control formerly reported in the literature. The mismatches between the performance of experimental prototype and the model, likely to result from the control time delay and the inaccuracy in estimating the loudspeaker parameters, can be compensated directly by tuning the control parameters in the control platform. The active resonators implemented through the reported control scheme can be used to build more complex acoustic devices/structures to enable high-efficiency broadband sound absorption or other types of acoustic phenomena such as wavefront shaping.

2022

Low frequency sound field reconstruction in a non-rectangular room using a small number of microphones

Hervé Lissek, Vu Thach Pham

An accurate knowledge of the sound field distribution inside a room is required to identify and optimally locate corrective measures for room acoustics. However, the spatial recovery of the sound field would result in an impractically high number of microphones in the room. Fortunately, at low frequencies, the possibility to rely on a sparse description of sound fields can help reduce the total number of measurement points without affecting the accuracy of the reconstruction. In this paper, the use of Greedy algorithm and Global curve-fitting techniques are proposed, in order to first recover the modal parameters of the room, and then to reconstruct the entire enclosed sound field at low frequencies, using a reasonably low set of measurements. First, numerical investigations are conducted on a non-rectangular room configuration, with different acoustic properties, in order to analyze various aspects of the reconstruction frameworks such as accuracy and robustness. The model is then validated with an experimental study in an actual reverberation chamber. The study yields promising results in which the enclosed sound field can be faithfully reconstructed using a practically feasible number of microphones, even in complex-shaped and damped rooms.

2020

Low Frequency Sound Field Reconstruction in Non-rectangular Room: A Numerical Validation

Hervé Lissek, Vu Thach Pham, Etienne Thierry Jean-Luc Rivet

Characterization of sound fields in rooms has always been a challenging task. A faithful reconstruction of sound fields in rooms generally would require an impractically high number of microphones. At low frequencies, where sound field can be modeled as a finite superposition of modes, a combination of Matching Pursuit (MP) and Least-squares optimization can help interpolate the Room Impulse Responses using a relatively small number of measurements. Our research focuses on using this paradigm to obtain the low-frequency information of a room in a broader sense, where the spatial distribution of sound pressure in a non-rectangular reverberating room can be reconstructed and visually analyzed. Several evaluations are performed along the progression of the framework to confirm its validity. The modal parameters estimated by MP are also compared with those obtained by the Rational Fraction Polynomial method (global curve fitting). Finally, the reconstruction of sound field, which leads to the visualization of the spatial distribution of sound pressure at any chosen frequency in the range of study, is validated visually and numerically using a finite element method software for a non-rectangular room model. This demonstration provides an extensive look at the high robustness and reliability of the framework as a whole, which is crucial in terms of its practical implementations for room acoustics practitioners.

2018

PID-like active impedance control for electroacoustic resonators to design tunable single-degree-of-freedom sound absorbers

Xinxin Guo, Hervé Lissek, Maxime Volery

2022