1. A tweeter loudspeaker or a loudspeaker designed to handle only the higher audio frequencies in which a flat metal diaphragm is driven directly by a varying high voltage applied between the diaphragm and a fixed metal electrode.
2. A speaker with a movable flat metal diaphragm and a non-movable metal electrode capable of reproducing high audio frequencies.

The diaphragm is driven by the varying high voltages applied across it and the electrode.