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 currentdriven 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 lowfrequency 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

Room Modal Equalisation with Electroacoustic Absorbers

The sound quality in a room is of fundamental importance for both recording and reproducing processes. Because of the room modes, the distributions in space and frequency of the sound field are largely altered. Excessive rise and decay times caused by the resonances might even mask some details at higher frequencies, and these irregularities may be heard as a coloration of the sound. To address this problem, passive absorbers are bulky and too inefficient to significantly improve the listening conditions. On the other hand, the active equalization methods may be complicated and costly, and the sound field might not be well controlled, because of the added sound energy in the room. Another approach is the active absorption, which consists in varying the impedance of a part of the enclosure boundaries, so as to balance the sound field thanks to the absorbed sound power into the active boundary elements. The thesis deals with the design and optimization of electroacoustic absorbers intended to specifically reduce the effect of the unwanted room modes. These active absorbers are closed box electrodynamic loudspeaker systems, whose acoustic impedance at the diaphragms is judiciously adjusted with passive or active components to maximize their absorption performance in the domain in which it is located. Several topologies merging sensor- and shunt-based methods are proposed resulting in an efficient and broadband sound absorption at low frequencies. A multiple degree-of-freedom target impedance that is assigned at the transducer diaphragms is then optimized to lower the modal decay times at best. The performance of the electroacoustic absorbers for the modal equalization is investigated in actual listening rooms, and their audible effect is subjectively evaluated. The overall combination of concepts and developments proposed in this thesis paves the way towards new active absorbers that may improve the listening experience at low frequencies in rooms

A multi-tone sound absorber based on an array of shunted loudspeakers

© 2018 by the authors. It has been demonstrated that a single shunted loudspeaker can be used as an effective low frequency sound absorber in a duct, but many shunted loudspeakers have to be used in practice for noise reduction or reverberation control in rooms, thus it is necessary to understand the performance of an array of shunted loudspeakers. In this paper, a model for the parallel shunted loudspeaker array for multi-tone sound absorption is proposed based on a modal solution, and then the acoustic properties of a shunted loudspeaker array under normal incidence are investigated using both the modal solution and the finite element method. It was found that each shunted loudspeaker can work almost independently where each unit resonates. Based on the interaction analysis, multi-tone absorbers in low frequency can be achieved by designing multiple shunted loudspeakers with different shunt circuits respectively. The simulation and experimental results show that the normal incidence sound absorption coefficient of the designed absorber has four absorption peaks with values of 0.42, 0.58, 0.80, and 0.84 around 100 Hz, 200 Hz, 300 Hz, and 400 Hz respectively

Thin broadband noise absorption through acoustic reactance control by electro-mechanical coupling without sensor

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Dual frequency sound absorption with an array of shunt loudspeakers.

Transformer noise is dominated by low frequency components, which are hard to be controlled with traditional noise control approaches. The shunt loudspeaker consisting of a closed-box loudspeaker and a shunt circuit has been proposed as an effective sound absorber by storing and dissipating the electrical energy converted from the incident sound. In this paper, an array of shunt loudspeakers is proposed to control the 100 Hz and 200 Hz components of transformer noise. The prototype under tests has a thickness of 11.8 cm, which is only 1/28 of the wavelength of 100 Hz. The sound absorption performance of the array under random incidence is analyzed with the parallel impedance method, and the arrangement of array elements is optimized. The test results in a reverberation room show that the proposed array has sound absorption coefficients of 1.04 and 0.93 at 100 Hz and 200 Hz, respectively, which provides potential of applying this type of thin absorbers for low-frequency sound control

Design of active Multiple-degrees-of-freedom electroacoustic resonators for use as broadband sound absorbers

We present a novel control method achieving stable multiple-degrees-of-freedom electroacoustic resonators. Such broadband absorbers, composed of a feedback-controlled electrodynamic loudspeaker, have many practical applications to real-world acoustic engineering problems, such as low-frequency industrial noise reduction in the range of [20 - 200 Hz]. The proposed control architecture combines a conventional microphone-based feedback control loop and a current-driven direct acoustic impedance control scheme, proven to perform optimally in recently reported acoustic impedance synthesis methods. This paper presents a methodology for designing the transfer function to be implemented in the controller, after specifying a target multiple-degree-of-freedom acoustic resonator impedance. Numerical simulations presents the expected acoustic performances, confirmed by experimental assessments in an impedance tube

Design and experimental validation of an active acoustic liner for aircraft engine noise reduction

The use of acoustic liners in aviation industry is a quite common solution for reducing the engines acoustic emissions. Although the current solutions based on single or multilayer liners are efficient and compact for the mid and high frequencies, noise mitigation in the low frequencies would require large volumes, making the integration in the nacelle difficult. Moreover, the passive liners are tuned to attenuate fixed frequencies and are optimized for specific flights regimes. An active electroacoustic skin based on a distribution of loudspeaker and microphones is presented here. The acoustic impedance is controlled by an embedded electronic system and can be changed in real time. Compared to a conventional passive liner, it is shown that the resonance frequency of the active skin can be adjusted to better match the flight phase and that the performance is better at low frequency. An experimental campaign in a wind tunnel has been performed and is presented here

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

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