An active equivalent of the λ/4 resonance absorber with a porous plate or foil of flow resistance ρc is described. The porous layer is backed by a loudspeaker driven such that zero sound pressure is maintained in the air gap between foil and speaker. The simultaneous control of magnitude and phase has been achieved in analog technique by a combination of open-loop and closed-loop control, utilizing single sideband modulation of the compensation signal with a weak infrasonic auxiliary signal. The on-off ratio of the reflection coefficient reaches −60 dB for sinusoidal sound, and −20 dB for broadband noise.

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