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Electrostatic loudspeakers 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. For low distortion operation, the diaphragm must operate with a constant 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 power supply and the diaphragm (resistor not shown in image). The diaphragm is usually made from a polyester film (thickness 2-20 µm) with exceptional mechanical properties, such as PET Film . 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, and its resulting movement drives the air on either side of 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 extremely light weight of the diaphragm, and exemplary 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 acoustic 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 relative to the listener. 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 the unfeasible physical requirements to reproduce low frequencies with a vibrating taut film), and 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 that 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. 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. The directionality of electrostatics is also a disadvantage in that it means the 'sweet spot' where proper stereo imaging can be heard is relatively small, restricting the number of people who can fully enjoy the advantages of the speakers simultaneously. Commercial speakers The first commercial design was the Quad ESL-57 designed in England by Peter Walker, founder of Quad Electroacoustics of Huntingdon, in 1955 and put into commercial production in 1957. This highly-regarded and elegant-looking loudspeaker, originally intended for mono use, remained in production until 1985 and 54,000 units were sold. In 1981 Quad introduced the ESL-63 as a successor to the ESL-57. This model remained in production until 1999 and sold 35,000 units. The ESL-63 attempted to address amongst other things the deficiency in bass reproduction of the 1957 design and its extreme directionality. In 1999 Quad introduced the ESL-988 and the ESL-989, currently in production. Other manufacturers currently producing electrostatic loudspeakers include Martin-Logan and Sound Lab in the United States. Martin-Logan build hybrid designs with conventional subwoofers. Final and Audiostatic are two less well-known Dutch manufacturers. Quad revealed a new electrostatic loudspeaker at the Milan audio show in September 2005. This retained the vertical rectangular shape of the 989 but marked a return to the slight backward tilt of the original ESL 57, supported by a strut behind the speaker. |
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