[en] A seismic vibro-acoustic source element 10 includes an enclosure 20 having first and second openings 22,24. First and second pistons 30,32 close the first and second openings. An actuator system 40 inside the enclosure actuates the first and second pistons to generate a wave having first frequency. A pressure mechanism 70 attached to the enclosure control a pressure of a fluid 73 inside the enclosure such that a pressure of the fluid is substantially equal to an ambient pressure of the enclosure (i.e. the pressure of the water 50 outside the enclosure). The actuator may be electromagnetic. The pressure mechanism 70 may be used to drive the pistons 30,32 with an additional force at lower frequencies (e.g. less than 4 Hz). An array of such source elements may be provided. In an alternative arrangement there need only be a single piston which closes a single opening in the enclosure.