Liquid droplet forming apparatus

An ink jet head 10 has coaxially disposed inner and outer cylindircal tube 11, 12 with the outer cylindrical surface 23 of the inner cylindrical tube spaced from the inner cylindrical surface 22 of the outer cylindrical tube to form an ink cavity 24 therebetween from which ink is supplied through one or more arrays of ink jet nozzles 35. Only the outer tube is a piezoelectric material so that the outer tube vibrates radially when electrically excited to produce vibrations in the ink in the ink cavity whereby a stream of ink droplets is supplied from each of the ink jet nozzles. In embodiments having only multiple arrays of ink jet nozzles, if desired, only the inner cylindrical tube or both the inner and outer cylindrical tubes can be formed of a piezoelectric material. If both of the tubes are formed of a piezoelectric material, the operating frequencies, which are preferably resonant, of the two tubes must then be matched.