Advanced control for modifying the acoustic impedance at the diaphragm of a loudspeaker
Résumé
Conventional techniques for acoustic impedance control operate either by feedback control of acoustic quantities (pressure and/or velocity) or by plugging an electrical network at the terminals of an electroacoustic transducer, thus modifying the acoustic impedance of its diaphragm. The general idea is to absorb the incident sound energy or to contain it, by simply controlling the dynamics of loudspeakers. The direct application is to better control the low-frequency sound field in enclosed or semi-closed spaces, improve the sound insulation between adjacent rooms or confine the noise emitted by industrial machines. Using a loudspeaker as a noise control device has several advantages: it is readily available at almost low cost, and already well-suited for rudimentary acoustic treatments. It naturally behaves as a damped harmonic oscillator which resonance can be altered through basic electrical means. With active control, sound absorption within the frequency range around the loudspeaker resonance can be significantly improved. The underlying problem often results in performance degradation outside this range, in the form of over-reflections. In this paper we propose means for compensating such undesired effects. For illustrative purposes, computed results and measurements obtained in impedance tube are provided to show the performances of a controlled loudspeaker in terms of absorption and stability.
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