Basically the same as the electrostatic loudspeaker but without an outside power-supply. Works with charged electrons attracting and repelling each other. 


Solid electrically insulating, or dielectric, material that has acquired a long-lasting electrostatic polarization. Electrets are produced by heating certain dielectric materials to a high temperature and then letting them cool while immersed in a strong electric field. An electret is an analog of a permanent magnet.