An apparatus, comprising: a marine seismic source that includes: a first plate (104, 150, 204, 250) that defines a first axis perpendicular to the first plate (102, 202, 302, 402) and a first outer edge, the first outer edge being circular and having the first axis passing through its centre; a second plate (104, 150, 204, 250) that defines a second axis perpendicular to the second plate (102, 202, 302, 402) and a second outer edge, the second outer edge being circular and having the second axis passing through its centre, the first axis being coaxial with the second axis; an interior cavity; a first pole piece (210, 260, 310, 360, 410, 460, 610, 660, 710, 760) attached to the first plate, the first pole piece within the interior cavity; a first groove (212, 262, 312, 362, 412, 462, 612, 662, 712, 762) defined in the first pole piece; a second pole piece (210, 260, 310, 360, 410, 460, 610, 660, 710, 760) attached to the second plate, the second pole piece within the interior cavity; a second groove (212, 262, 312, 362, 412, 462, 612, 662, 712, 762) defined in the second pole piece; and a wire coil (270, 370, 470, 570, 670, 770) disposed in the first groove between the first and second plates such that the first and second pole pieces are configured to attract one another when an electric current is passed through the wire coil; the apparatus being characterized in that: it includes a hoop (108, 408, 608, 708) disposed around and fixed to the outer edge of the first and second plates, wherein the hoop is configured to separate the first and second plates so that the interior cavity is defined between the first plate, the second plate, and the hoop and such that the first and second plates bend toward each other along the coaxial first and second axis when an electric current is passed through the wire coil. a first plate (104, 150, 204, 250) that defines a first axis perpendicular to the first plate (102, 202, 302, 402) and a first outer edge, the first outer edge being circular and having the first axis passing through its centre; a second plate (104, 150, 204, 250) that defines a second axis perpendicular to the second plate (102, 202, 302, 402) and a second outer edge, the second outer edge being circular and having the second axis passing through its centre, the first axis being coaxial with the second axis; an interior cavity; a first pole piece (210, 260, 310, 360, 410, 460, 610, 660, 710, 760) attached to the first plate, the first pole piece within the interior cavity; a first groove (212, 262, 312, 362, 412, 462, 612, 662, 712, 762) defined in the first pole piece; a second pole piece (210, 260, 310, 360, 410, 460, 610, 660, 710, 760) attached to the second plate, the second pole piece within the interior cavity; a second groove (212, 262, 312, 362, 412, 462, 612, 662, 712, 762) defined in the second pole piece; and a wire coil (270, 370, 470, 570, 670, 770) disposed in the first groove between the first and second plates such that the first and second pole pieces are configured to attract one another when an electric current is passed through the wire coil; the apparatus being characterized in that: it includes a hoop (108, 408, 608, 708) disposed around and fixed to the outer edge of the first and second plates, wherein the hoop is configured to separate the first and second plates so that the interior cavity is defined between the first plate, the second plate, and the hoop and such that the first and second plates bend toward each other along the coaxial first and second axis when an electric current is passed through the wire coil.