Editing Electrostatic loudspeaker (section)

==Design and functionality==

The speakers use a thin flat [[Diaphragm (acoustics)|diaphragm]] usually consisting of a plastic sheet coated with a [[conductive]] material such as [[graphite]] sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids. For low distortion operation, the diaphragm must operate with a constant [[electric charge|charge]] on its surface, rather than with a constant [[voltage]]. This is accomplished by either or both of two techniques: the diaphragm's conductive coating is chosen and applied in a manner to give it a very high surface [[resistivity]], and/or a large value resistor is placed in series between the EHT (Extra High Tension or Voltage) power supply and the diaphragm (resistor not shown in the diagram here).{{citation needed|date=July 2017}} However, the latter technique will still allow distortion as the charge will migrate across the diaphragm to the point closest to the "grid" or electrode thereby increasing the force moving the diaphragm; this will occur at audio frequency so the diaphragm requires a high resistance (megohms) to slow the movement of charge for a practical speaker.{{citation needed|date=July 2017}}

The diaphragm is usually made from a [[polyester]] film (thickness 2–20&nbsp;μm) with exceptional mechanical properties, such as [[PET film (biaxially oriented)|PET film]].{{citation needed|date=July 2017}} By means of the conductive coating and an external high voltage supply the diaphragm is held at a DC [[potential]] of several kilovolts with respect to the grids. The grids are driven by the audio signal; front and rear grid are driven in [[antiphase]]. As a result, a uniform [[electrostatic field]] proportional to the audio signal is produced between both grids. This causes a force to be exerted on the charged diaphragm, and its resulting movement drives the air on either side of it.

In virtually all electrostatic loudspeakers the diaphragm is driven by two grids, one on either side, because the force exerted on the diaphragm by a single grid will be unacceptably non-linear, thus causing [[Total harmonic distortion|harmonic distortion]]. Using grids on both sides cancels out voltage dependent part of non-linearity but leaves charge (attractive force) dependent part.<ref>''The theory of electrostatic forces in a thin electret (MEMS) speaker'' Eino Jakku, Taisto Tinttunen and Terho Kutilainen, proceedings IMAPS Nordic 2008 Helsingør – 14–16 September</ref> The result is near complete absence of harmonic distortion. In one recent design, the diaphragm is driven with the audio signal, with the static charge located on the grids (Transparent Sound Solutions).

The grids must be able to generate as uniform an electric field as possible, while still allowing for sound to pass through. Suitable grid constructions are therefore perforated metal sheets, a frame with tensioned wire, wire rods, etc.

To generate a sufficient field strength, the audio signal on the grids must be of high voltage. The electrostatic construction is in effect a capacitor, and current is only needed to charge the capacitance created by the diaphragm and the stator plates (previous paragraphs referred to as grids or electrodes). This type of speaker is therefore a high-[[Electrical impedance|impedance]] device. In contrast, a modern electrodynamic cone [[loudspeaker]] is a low impedance device, with higher current requirements. As a result, impedance matching is necessary in order to use a normal [[electronic amplifier|amplifier]]. Most often a [[transformer]] is used to this end. Construction of this transformer is critical as it must provide a constant (often high) transformation ratio over the entire audible frequency range (i.e. large bandwidth) and so avoid distortion. The transformer is almost always specific to a particular electrostatic speaker. To date, [[Acoustat]] and Beveridge built the only commercial "transformer-less" electrostatic loudspeaker.{{citation needed|date=July 2017}} In this design, the audio signal is applied directly to the stators from a built-in high-voltage valve amplifier (as valves are also high impedance devices), without use of a step-up transformer.