Electrostatic loudspeaker

Schematic showing an electrostatic speaker's construction and its connections. The thickness of the diaphragm and grids has been exaggerated for the purpose of illustration.

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 (Extra High Tension or Voltage) power supply and the diaphragm (resistor not shown in the diagram here).

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 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 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 will be unacceptable non-linear, thus causing harmonic distortion. Using grids on both sides cancels out this source of 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, 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. This type of speaker is therefore a high-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 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 and so avoid distortion. The transformer is almsot always specific to a particular electrostatic speaker.

Advantages of electrostatic loudspeakers include the extremely light weight of the diaphragm, and exemplary frequency response (both in amplitude and phase) because the principle of generating force and pressure is not as prone to resonances as in the operating principle of the more common electrodynamic driver. Musical transparency can be better than in electrodynamic speakers because the radiating surface is much lighter mass than most other drivers and so more responsive to the applied signal.

Since most electrostatic speakers are tall and thin designs without enclosure, they act as a vertical dipole line source. This makes for rather different acoustic behaviour in rooms compared to conventional electrodynamic loudspeakers. Planar (flat) drivers tend to be very directional giving them good imaging qualities, on the condition that they have been carefully placed relative to the listener and the sound-reflecting surfaces in the room. Curved panels have been built, making the placement requirements a bit less stringent, but sacrificing imaging somewhat.

Disadvantages include a lack of bass response (due to phase cancellation from a lack of enclosure, and the difficult physical requirement to reproduce low frequencies with a vibrating taut film with little excursion amplitude), and sensitivity to ambient humidity levels. While bass is lacking quantitatively, it can be of better quality ('tighter' and without 'booming') than that of electrodynamic (cone) systems. Phase cancellation can be somewhat compensated for 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 using a dynamic loudspeaker to handle lower frequencies with the electrostatic diaphragm 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, little box colouration, and (ideally) flat frequency response. However, there is often a problem with integrating such a woofer with the electrostatics. This is because the line source electrostatic loudspeaker's SPL decreases by 3dB for each doubling of distance. Whereas, the cone speaker's SPL decreases by 6 dB for each doubling of distance because it behaves as a point source.

The directionality of electrostatics can also be 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

Arthur Janszen was granted U.S. patent 2,631,196 in 1953 for the first practical electrostatic loudspeaker.

Among the first full-tange electrostatics, and also among the most respected, were the ESL series from Quad Electroacoustics, of Huntingdon, England. These were shaped somewhat like the curved panel of an automobile door leaning slightly backward. They were widely admired for their clarity and precision, but like most full range electrostatics, were not good performers at low frequencies. ESLs were designed by Peter Walker, founder of the company, and David Williamson. The first in the series was the ESL-57, based on U.S. patent 1,983,377 developed by Edward W. Kellogg for General Electric in 1934 [1]. It was introduced in 1955, put into commercial production in 1957, and discontinued only in in 1985. In 1981, Quad introduced the ESL-63 as a successor to the ESL-57. It attempted to address both the deficiency in bass reproduction of the ESL-57 and its extreme directionality at high frequencies. It remained in production until 1999. In 1999 Quad introduced the ESL-988 and the ESL-989, both currently in production. A new model has been intoduced as of late 2005, which returns to the slightly back-tilted stance of the original designs.

Other manufacturers currently producing electrostatic loudspeakers include Innersound, Martin-Logan and Sound Lab in the United States and AudioStatics in the Netherlands. Innersound and Martin-Logan build hybrid designs with conventional subwoofers.

Among electrostatic full-range speakers which are no longer made are the KLH 9, one of the earliest US full-range designs, several Acoustat models, and the Infinity Servo-Static and its successors which used a dynamic subwoofer at low frequencies.

Specialized electrostatic high frequency drivers (ie, tweeters) are still in common use by many manufacturers.

External links

• The Audio Circuit - An almost complete list of manufacturers of electrostatic loudspeakers including DIY speakers, materials and parts, and 'how do they work' sections.