A modulating device for breaking ink jets in an ink jet printing machine into trains of uniform droplets comprises a nozzle plate (24) provided with a substantially linear array of nozzles and forming one wall of an ink cavity (23); and acoustic generator in the form of an elongate body (26) projecting towards the nozzle plate to transmit acoustic vibrations into ink into the cavity. The body is shaped and tuned to vibrate substantially only in the longitudinal mode and at a resonant frequency which is within ten percent of a frequency to excite natural resonant vibrations in the ink in the cavity between the end of the body and the nozzle plate, the width of the body being less than the length of the nozzle array or of that part of the array associated with that body.