Achieving direct acoustic impedance control with only two microphones
Résumé
Active electroacoustic absorption is an efficient way to reduce noise propagation in cavities and ducts and is a promising technology for aircraft engine noise reduction. Hybrid sensor-/shunt-based impedance control has proved effective but requires a most accurate model of the electroacoustic actuator. Alternatively, by sensing the pressure on the electrodynamic absorber membrane simultaneously with its velocity, a target acoustic impedance can be achieved on electroacoustic actuators, without requiring a thorough estimation of the electroacoustic model parameter. In the proposed implementation, a microphone close to the loudspeaker membrane senses the sound pressure acting on it, while another microphone in the backing cavity of the loudspeaker measures a signal proportional to the membrane displacement, which after derivation is an estimator of the membrane velocity. These two quantities are used as input to a digital control function, used for feeding back the loudspeaker. The great advantage of this control scheme is that no model of the electrodynamic loudspeaker is required except the backing cavity specific acoustic compliance, which can easily be obtained by either measuring the membrane velocity once, or via the volume of the cavity. The proposed method is compared to state-of-the-art techniques, showing the interest for active acoustic impedance control.
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