Contributions aux études sur le couplage électroacoustique dans les espaces clos en vue du contrôle actif

The aim of this work was to analyze the global behavior of a loudspeaker exciting a room in the frequency band of its first modes, for the purpose of active control applications. First, the loudspeaker in free field was studied. The characterization of a loudspeaker on the base of equivalent circuits has been established, showing the preponderant importance of the load impedance, determined by near field measurements. Then, the steady state has been modeled and the study has been set on the characterization of the wall impedance, which determines the eigenfrequencies and the amplitude of their modes, as well as charge impedance. The latter which modifies the volume velocity compared with the one in free field, has been characterized. The previous results enable the precise computation of the pressure field in a closed space and excited by a loudspeaker in steady state, according to the wall impedance. For active control purposes, transitory sounds are particularly important, therefore the behavior of a sound field in a room excited by several parameterized sounds has been studied in the time domain. The room response to the source activation could therefore be modeled. This response consists of a free and a transitory state, both of which are qualified through coherent assumptions and observations. Through simulation and systematic experimentation, the loudspeaker-room system has been characterized for a stationary as well as for a transitory sound, revealing the constraints affecting modal active control. Many experiments showed the efficiency of modal active noise control. Observations presented the importance of low frequencies in the annoyance of airplane noise inside rooms. Directives have been given to install active noise reduction system. The main part of this work constitutes an electro acoustical knowledge basis to perform modal active noise control.

Duct modes damping through an adjustable electroacoustic liner under grazing incidence

Romain Boulandet, Hervé Lissek, Gaël Matten

This paper deals with active sound attenuation in lined ducts with flow and its application to duct modes damping in aircraft engine nacelles. It presents an active lining concept based on an arrangement of electroacoustic absorbers flush mounted in the duct wall. Such feedback-controlled loudspeaker membranes are used to achieve locally reacting impedances with adjustable resistance and reactance. A broadband impedance model is formulated from the loudspeaker parameters and a design procedure is proposed to achieve specified acoustic resistances and reactances. The performance is studied for multimodal excitation by simulation using the finite element method and the results are compared to measurements made in a flow duct facility. This electroacoustic liner has an attenuation potential comparable to that of a conventional passive liner, but also offers greater flexibility to achieve the target acoustic impedance in the low frequencies. In addition, it is adaptive in real time to track variable engine speeds. It is shown with the liner prototype that the duct modes can be attenuated over a bandwidth of two octaves around the resonance frequency of the loudspeakers.

2018

Active low-frequency modal noise cancellation for room acoustics: an experimental study

Xavier Falourd, Hervé Lissek, Pierre-Jean René

Low frequency background noise in rooms generated by numbers of human activities - traf-fic, railway, airport and industrial noise - creates major disturbances from loss of speech in-telligibility to stress and fatigue. To overcome such situations classical solutions generally consist in deploying resonator-like absorbing materials to reduce both the acoustic level and the reverberation time. Unfortunately these traditional acoustic treatments are not efficient enough to attain A-weighted level specifications imposed by regulation laws. In the context of cancellation of low frequencies in rooms, an experiment was performed to qualify and quantify performances of a low-frequency noise cancellation active system. So as to minim-ize and increase performance in noise reduction, active solutions are actually studied to attain and fulfill high expectations in terms of integration with high efficiency in opposite to pas-sive solutions. We propose an experiment based on active sources to counteract modal low frequency noise for quasi-stationary to permanent noise emission. We present some results using a step-by-step approach to understand the global behavior of the room excited in the frequency band of its first modes. Through coherent assumptions and observations we show the efficiency of low-power active system with a priori constraints affecting modal active control.

2009

Optimization of electric shunt resonant circuits for electroacoustic absorbers

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

Sound absorption can be achieved in a relatively simple manner with passive or active electroacoustic absorbers. Electroacoustic absorbers are basically loudspeakers, the membrane of which is used as an absorber/reflector of sound, and the electric terminals of which are connected to a dedicated electric load. This electric shunt can be a simple electric resistor, or an active device employing feedback control on acoustic and/or electric quantities (aka. direct impedance control). Whilst the latter allows significant broadening of acoustic absorption performances that could not be obtained with simple passive resistors, it is also possible to find passive shunt alternatives that allows interesting acoustic performances without the use of expensive and energy consuming electric disposals. A simple configuration of shunt electric resonator is proposed as an alternative to active shunt for broadening the performances of passive electroacoustic absorbers.

2012