Etienne Rivet

Broadband Low-Frequency Electroacoustic Absorbers Through Hybrid Sensor-/Shunt-Based Impedance Control

This paper proposes a hybrid impedance control architecture for an electroacoustic absorber that combines an improved microphone-based feedforward control with a current-driven electrodynamic loudspeaker system. Feedforward control architecture enables stable control to be achieved, and current driving method discards the effect of the voice coil inductance. A method is given for designing the transfer function to be implemented in the controller, according to a target-specific acoustic impedance and mechanical parameters of the transducer. Numerical simulations present the expected acoustic performance, introducing global performance indicators such as the bandwidth of efficient absorption. Experimental assessments in a waveguide confirmed the accuracy of the model and the efficiency of the hybrid control technique for achieving broadband stable low-frequency electroacoustic absorbers. An application to damping of resonances in a duct is also presented, and the application to the modal equalization in actual listening rooms is finally discussed.

Sensorless electroacoustic absorbers through synthesized impedance control for damping low-frequency modes in cavities

This paper presents a concept of sensorless electroacoustic absorber for damping the low-frequency modes in a cavity such as a duct or a room. Taking advantage of the reciprocity of the voice coil transducer, it is shown that a synthetic electrical admittance can be designed so that the loudspeaker diaphragm is matched to a target specific acoustic impedance. This electroacoustic device provides a relatively broadband sound absorption that can be used to dampen room modes regardless of the sound field in which the loudspeaker is located. A digital filter is used to replicate the frequency response of the synthetic load, and a voltage-controlled current source is needed so that the filter is seen as an electrical admittance. Unlike previous attempts to implement the synthetic load using an electrical network, greater flexibility and accuracy can be obtained. Experimental results confirmed the validity of this sensorless electroacoustic absorber (SEA) in a 1D sound field, showing that the dynamic range of the sound pressure level in a duct can be reduced by 15 dB from 50 Hz to 300 Hz compared to a hard surface panel. A discussion on the strengths and limitations of this concept is provided, in particular with a view to employing SEAs for modal equalization in actual listening rooms.

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.

On the Optimisation of Multi-Degree-of-Freedom Acoustic Impedances of Low-Frequency Electroacoustic Absorbers for Room Modal Equalisation

Low-frequency electroacoustic absorbers have recently been developed as a solution for the modal equalisation. Firstly investigated in waveguides, the technique consists in matching the acoustic impedance at a closed-box loudspeaker diaphragm to the characteristic acoustic impedance of air. Extending the results in a duct to rooms brings up several challenges. Some parameters, such as the position and orientation of absorbers, the total area, as well as the acoustic impedance achieved at the diaphragms may influence the performance, especially in terms of modal decay time reduction. In this paper, the optimal values of a purely resistive acoustic impedance at an absorber diaphragm, whose area varies, are first investigated under normal incidence and grazing incidence in a finite-length waveguide. The optimal acoustic resistance values are then investigated for a given position, orientation, and total area of absorbers in rooms of different size. From these results, the target acoustic impedances with multiple degrees of freedom are defined with a view to assign to the absorber diaphragms. These impedances are then optimised from a global criterion, so that these impedances approach at best the different optimal resistance values found to minimise the modal decay times. Finally, an experimental evaluation of the performance of the electroacoustic absorber in a waveguide is provided.

Acoustic energy harvesting using Electrochemical Double Layer Capacitors: Technical feasibility and performance assessment

This paper describes a first prototype harvesting the acoustic energy using electrochemical storage devices. In particular, the harvesting system is composed by an Electrochemical Double Layer Capacitors (EDLC) suitably coupled with an electroacoustic absorber by means of an AC/DC converter. The aims of the proposed work are i) to assess the technical feasibility to compensate the decay voltage via an acoustic harvester; and ii) to evaluate how such compensation can improve the performance of the targeted EDLC. Concerning the first aim, the experimental results show that the required sound pressures to charge the EDLC up to its nominal voltage range from 118 dB up to 122 dB (re. 20 μPa). The EDLC charging times range from couple of hours up to several hours. The charging powers range from few mW up to several tens of mW and the energy efficiency of the whole system ranges from 5% up to 35%. Concerning the second aim of the paper, further experimental results illustrate a non-negligible improvement of the capability of the EDLC to deliver high-pulsed currents. In particular, after compensating the EDLC decay voltage via the proposed acoustic harvester, the rise time required to deliver a high pulse current is even 170% lower than the one obtained without any previous decay voltage compensation. The associated current peak value and root mean square value are 20 % higher.

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 of a wave’s free propagation 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’s intensity can be achieved by adding a continuous gain–loss distribution to the disorder. Here we propose a practical discretized version of this abstract concept and implement it 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 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 localized 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.Perfect transmission of sound waves through a strongly disordered environment is demonstrated using a set of speakers that provide exactly the right input to counteract scattering by the disorder. These principles can also be applied to light.

Toward wideband steerable acoustic metasurfaces with arrays of active electroacoustic resonators

We introduce an active concept for achieving acoustic metasurfaces with steerable reflection properties, effective over a wide frequency band. The proposed active acoustic metasurface consists of a surface array of subwavelength loudspeaker diaphragms, each with programmable individual active acoustic impedances allowing for local control over the different reflection phases over the metasurface. The active control framework used for controlling the reflection phase over the metasurface is derived from the Active Electroacoustic Resonator concept. Each unit-cell simply consists of a current-driven electrodynamic loudspeaker in a closed box, whose acoustic impedance at the diaphragm is judiciously adjusted by connecting an active electrical control circuit. The control is known to achieve a wide variety of acoustic impedances on a single loudspeaker diaphragm used as an acoustic resonator, with the possibility to shift its resonance frequency by more than one octave. This paper presents a methodology for d...

Constant amplitude sound waves in non-Hermitian metamaterials

We investigate the possibility for acoustic waves to propagate with a constant amplitude in disordered media. We find that this remarkable property is possible if one adds a tailored distribution of gain and loss on top of the disorder, making the medium non-Hermitian. We present the theory of constant-amplitude acoustic waves in both cases of continuous and discrete media, and provide an experimental demonstration in a metamaterial at audible frequencies.

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

The paper focuses on the design of a built-in electroacoustic resonator for active noise reduction purposes. This concept basically encompasses a loudspeaker connected to a synthetic electrical load that enhances the ability of the transducer to dissipate a certain part of the incoming acoustic energy. The strategy is therefore to control the dynamics of boundaries in closed 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 its ability to achieve broadband sound absorption without the need for any sensor. The desired dynamic response of the loudspeaker for any sound disturbance is incorporated within the synthetic electrical load admittance (current/voltage transfer function). Computational results are provided to illustrate the benefits and potential of a built-in electroacoustic resonator compared to other options. Concluding remarks and discussions on foreseen future developments are then provided.