Flextensional acoustic source with sound-emitting surfaces (1, 2, 3, 4) adapted to be excited into vibrational movement, and for use in seismic exploration at sea. The source comprises a drive unit (21) being through the intermediary of at least one push element (22A, 22B) adapted to impose vibrational movement to the sound-emitting surfaces. Between at least one push element (22A, 22B) and the associated sound-emitting surface(s) (1, 2, 3, 4) there is provided a non-linear spring element (27, 28) the spring constant of which is controllable. Two oppositely acting push elements (22A, 22B) are arranged in axial alignment and preferably centrally in the source, and in addition thereto there are provided at least two tension elements (39, 40) with associated controllable, non-linear spring elements (35, 36, 37, 38) and being coupled to the sound-emitting surfaces (1, 2, 3, 4).