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An analytical study was presented on active control of sound transmission into a vibro-acoustic enclosure comprising two flexible plates.Two types of actuators were used,I.e.Acoustic actuator and distributed lead zirconate titanate piezoelectric (PZT) actuator instead of point force actuator.Using the modal acoustic transfer impedance-mobility matrices,the excitation and interaction in the coupled sound transmission system can be described with clear physical significance.With the control system designed to globally reduce the sound field,different control system configurations were considered,including the structural actuator on the incident plate,actuator on the receiving plate,acoustic actuator on the cavity,and their combinations.The effectiveness and performance of the control strategy corresponding to each system configuration were compared and discussed.The role and control mechanism of each type of actuator were of particular interest.It was shown that the incident plate actuator is effective in controlling the cavity-dominated modes and the structural modes dominated by the incident plate and receiving plate.Two main control mechanisms are involved in this control configuration,I.e.,modal suppressing and modal rearrangement.For control system configuration with only acoustic actuator in the enclosure,the mechanism involved in this arrangement is purely modal suppression.Desirable placements of structural actuators in terms of total potential energy reduction were also discussed.