In the middle is a light flexible membrane, which is held under tension and clamped at its perimeter between insulating ring spacers. The spacers separate the membrane from the rigid stators or ring electrodes located on either side of it at a distance d. The membrane is circular with a radius a and has a conductive coating that is charged by a polarizing supply with a dc voltage E _P . The polarizing supply is connected via a high-value resistor R _P to prevent the charge on the membrane from varying significantly when the alternating signal voltage e ˜ in is applied to deflect the membrane to either side of its central position. This keeps the force acting on the membrane linear, and the constant-charge principle was an important step forward in making commercial electrostatic loudspeakers viable [1,2]. We take the input voltage e ˜ in to be that across the entire secondary of the push-pull step up transformer in Fig. 15.1, which is still the most common way to develop the large signal voltage required to drive an electrostatic loudspeaker. As the membrane moves, it produces sound that passes through the perforations in the stators. The most common membrane material is polyester (PET or Mylar), but polyimide (Kapton) has also been used. If the loudspeaker is required to produce low frequencies and therefore large membrane excursions, the conductive coating on the membrane is likely to have a high resistance to prevent the charge migrating to the central part, which will be closest to the stator at maximum excursion. Such coatings are usually in the form of graphite, Elvamide, or indium-tin-oxide (ITO).