Electrostatic speaker

Electrostatic speakers use a thin flat diaphragm usually consisting of a plastic sheet impregnated with a conductive material such as graphite sandwiched between two electrically conductive grids, with a small air gap between the diaphragm and grids.

The diaphragm is usually made from a polyester film (thickness 2-20 µm) with exceptional mechanical properties, such as Mylar. 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 counterphase. 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, in turn driving the air surrounding it.

In all but a few modern electrostatic loudspeakers the diaphragm is driven by two grids, one on either side, because the force exerted on the diaphragm by a single grid is non-linear, thus causing harmonic distortion. Using grids on both sides cancels out non-linearity. The result is near complete absence of harmonic distortion.

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

To generate a sufficient field strength, the audio signal on the grids must be of high voltage. Current is only needed to charge the capacitance between both grids. This makes this type of speaker a high-impedance device. In contrast, a modern electrodynamic cone loudspeaker is a low impedance device driven by current. As a result, impedance matching is necessary in order to use a normal 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.

Advantages of electrostatic loudpseakers include very fast transient response due to the extreme light weight of the diaphragm, and examplary frequency response (both in amplitude and phase). Transparency is usually better than in electrodynamic speakers because of the large radiating surface.

Since most electrostatic speakers are tall and slim designs without enclosure they act as a vertical dipole line source. This makes for totally different acoustical behaviour inside rooms. Planar (flat) types tend to be very directional which gives them superb imaging qualities, on the condition that they have been carefully placed inside the room. Curved panels have been built, making the placement requirements a bit less stringent but sacrificing stereo imaging somewhat.

Disadvantages include a lack of bass response due to phase cancellation from a lack of enclosure, and a sensitivity to ambient humidity levels. While bass is lacking quantitatively it is often of much better quality than that of electrodynamic (cone) systems. Phase cancellation can be remedied by electronic equalization (a so-called shelving circuit boosts the region inside the audio band where the generated sound pressure drops because of phase cancellation).

The lack of bass is often remedied with a hybrid design utilizing a dynamic loudspeaker handling lower frequencies with the electrostatic diaphram handling middle and high frequencies. However some purists feel that this detracts from the purity of the "electrostatic sound" and insist on a pure electrostatic design. Many feel that the best low frequency unit for hybrids are transmission line woofers or horns, since they possess roughly the same qualities (at least in the bass) as electrostatic speakers, i.e. good transient response, almost no box colouration, and flat frequency response.

Electrostatic directionality is also said to create a more personal, as opposed to democratic, listening environment.

Current manufacturers include Martin Logan, Sound Lab in the United States and Quad in the United Kingdom. Final and Audiostatic are two less-known Dutch manufacturers.

There are active experimenters creating their own electrostatic speakers.