An ink jet apparatus comprises a chamber (200) having a diaphragm (210) preloaded to a deformed position when the transducer (204) is in its de-energised state. Upon energisation, the diaphragm (210) returns to a substantially planar condition so as to permit filling of the chamber (200) from an inlet prior to firing a droplet from a chamber orifice (202) when the transducer (204) is de-energised and the diaphragm (210) again assumes it preloaded, deformed condition.