A theoretical prediction is undertaken for low-frequency duct noise control by a compact reactive mu%er. The mu%er achieves a pressure-release condition over a broad frequency band (from near zero to the "rst cut-on frequency of the duct) by four #ush-mounted pistons, one on each duct wall, with a structure to #uid mass ratio of the order of unity. The concept of a magnetic device is introduced to allow the sti!ness of the system to be made su$ciently small, leading to a high transmission loss near the DC frequency. Interactions among the pistons yield negative virtual mass which neutralizes the structural inertia. The inertia neutralization is nearly perfect over a wide frequency band, so that the high transmission loss extends to the "rst cut-on frequency of the duct. The near-"eld analysis reveals that the negative virtual mass derives from the plane wave mode of sound radiation due to the mirror e!ect of the opposite duct wall, an e!ect which can be maximized by the pairing of pistons on the two opposing walls. For relatively heavy pistons a minimum transmission loss of 14 dB may be obtained for plane waves, while that of relatively compact and light pistons could exceed 30 dB. The e!ects of residual system sti!ness required by the system stability, and of structural damping are shown to be insigni"cant except for a narrow band near the DC frequency.