The Electrostatic speaker is based on that two equally polarized charges repel each other while two oposite charges attract each other.

Electrostatic speaker element:

    .....           electrode 1
    :  )---------===-=-=-=-=-=-=-=
    :  ):  ____  #               #
  o--) ): |    | # foil membrane #
Si  :) )--|-Up+|-----------------# frame
  o--) ): |____| #               #
    :  ):        #               #
    :  )---------===-=-=-=-=-=-=-=
    :...:           electrode 2

The power supply Up is supplying the polarizing voltage(about 5000V). 'Si' is the signal from the driving amplifier, it is transformed up by T1 to a voltage high enough to move the membrane, also the elecrodes will always have an opposite charge. The electrodes are made from a perforated metal plate or a metal mesh. When electrode 1 gets a positive electric field it will push the membrane and the negative field from electrode 2 will pull on the membrane(The charge change according to Si). The moving membrane will make the air vibrate, and thus make sound.

A variation on the electrostatic speaker uses the an electret membrane. This speaker doesn't need a polerizing voltage and is most used in headphones.

Since the electrostatic speaker needs a polerizing voltage, it uses more power than a dynamic speaker. The membrane is relatively large but very light giving these speakers good frequency response at booth low and high frequency. The electret version is cheaper but has a somewhat lower sound quality.