Abstract: A device for converting electric into acoustic vibrations and vice versa, provided with a diaphragm, comprising at least one layer of a piezoelectric polymer material, which is polarized in a transverse direction and which at either side is provided with electrodes, the diaphragm being undulated in at least one orthogonal section, whilst each flank of the undulation is polarized in the same direction over its entire width and in the operating condition the direction of the applied electric field relative to the direction of polarization in adjoining flanks are alternately parallel and anti-parallel. Two types of embodiments are possible.