Electrostatic loudspeaker

An electrostatic loudspeaker is a loudspeaker design in which sound is generated by the force exerted on a membrane suspended in an electrostatic field.

Design and functionality
The 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. 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 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 harmonic distortion. Using grids on both sides cancels out this source of non-linearity. 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 (Final Sound).

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 almost always specific to a particular electrostatic speaker. To date, Acoustat built the only "transformer-less" electrostatic loudspeaker. In this design, the audio signal is applied directly to the stators from a built-in high-voltage valve amplifier, without use of a step-up transformer.

Advantages
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 has much less mass than most other drivers and is therefore far less capable of storing energy to be released later. For example, typical dynamic speaker drivers can have moving masses of tens or hundreds of grams whereas an electrostatic membrane only weighs a few milligrams, several times less than the very lightest of electrodynamic tweeters. The concomitant air load, often insignificant in dynamic speakers, is usually tens of grams because of the large coupling surface, this contributing to damping of resonance buildup by the air itself to a significant, though not complete, degree. Electrostatics can also easily be executed as full-range designs, lacking the usual crossover filters and enclosures that could color or distort the sound.

Since most electrostatic speakers are tall and thin designs without an enclosure, they act as a vertical dipole line source. This makes for rather different acoustic behavior in rooms compared to conventional electrodynamic loudspeakers. Generally speaking, a large-panel dipole radiator is more demanding of a proper physical placement within a room when compared to a conventional box speaker, but, once there, it is less likely to excite bad-sounding room resonances, and its direct-to-reflected sound ratio is higher by some 4-5 decibels. This in turn leads to more accurate stereo reproduction of recordings that contain proper stereo information and venue ambience. 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 precision somewhat.

Disadvantages
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). Nevertheless maximum bass levels cannot be augmented because they are ultimately limited by the membrane's maximum permissible excursion before it comes too close to the high-voltage stators, which may produce electrical arcing and burn holes through it. Recent, technically more advanced solutions for lack of bass include the use of large, curved panels (Sound Lab, MartinLogan CLS), electrostatic subwoofer panels (Audiostatic) and long-throw electrostatic element allowing large diaphragm excursions (Audiostatic). Another trick often practised is to step up the bass (20–80 Hz) with a higher transformation ratio than the mid and treble.

This relative lack of loud bass is often remedied with a hybrid design using a dynamic loudspeaker, e.g. a subwoofer, 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 coloration, and (ideally) flat frequency response. However, there is often a problem with integrating such a woofer with the electrostatics. This is because most electrostatics are line sources, the sound pressure level of which decreases by 3 dB for each doubling of distance. A cone speaker's sound pressure level, on the other hand, decreases by 6 dB for each doubling of distance because it behaves as a point source. This can be overcome by the theoretically more elegant solution of using conventional cone woofer(s) in an open baffle, or a push-pull arrangement, which produces a bipolar radiation pattern similar to that of the electrostatic membrane. This is still subject to phase cancellation, but cone woofers can be driven to far higher levels due to their longer excursion, thus making equalization to flat easier.

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.

Because of their tendency to attract dust, insects, conductive particles and moisture, electrostatic speaker diaphragms will gradually deteriorate and need periodic replacement. They also need protection measures to physically isolate their high voltage parts from accidental contact with humans and pets. Cost-effective repair and restoration service is available for virtually every current and discontinued electrostatic loudspeaker model, e.g. Shackman ESL repair services.

Amateur-built speakers
Electrostatic speakers enjoy some popularity among do-it-yourself loudspeaker builders. They are one of the few types of speakers in which the transducers themselves can be built from scratch by an amateur. Basic hardware for complete ESL DIY projects is available all over the web. Such supplies include resistors and capacitors for RC-circuit frequency equalization, if necessary; step-up transformers; perforated metal sheets or grids and insulating plastics for the stators; polymer film and conductive paint (e.g. a liquid graphite suspension) for the membrane; simple tensioning equipment for proper membrane tuning; and a frame, usually of wood, to hold everything together.

Commercial speakers
Arthur Janszen was granted US patent 2631196 in 1953 for the first practical electrostatic loudspeaker.

Among the first full-range 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 US patent 1983377 developed by Edward W. Kellogg for General Electric in 1934. It was introduced in 1955, put into commercial production in 1957, and discontinued only 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. The latter goal is achieved by splitting the stators into eight concentric rings, each fed with a slight time delay compared to the ring immediately inwards, thereby attempting to simulate a point source. The ESL-63 remained in production until 1999. In 1999 Quad introduced the ESL-988 and the ESL-989, both currently in production. Two new models, the smaller 2805 and the larger 2905, have been introduced as of late 2005, which return to the slightly back-tilted stance of the original designs, albeit user-adjustable. Largely retaining the larger bass panels of the 98x models and concentric ring design of the ESL-63, the 2x05's feature heavier and far more rigid construction, and several electronic and transducer refinements.

Other manufacturers currently producing electrostatic loudspeakers include Solosound in The Netherlands, Martin-Logan and Sound Lab in the United States, Audiostatic, Final Sound and Metrum Acousticsin The Netherlands and Immersion from Australia, King's Audio in Hong Kong, China and Acoustat originally in the United States, since 2002 in China.

Sanders Sound Systems, Innersound, Metrum Acoustics and Martin-Logan build hybrid designs with conventional subwoofers. Final Sound build stand-alone electrostatic panels with freestanding bass-modules as an option. Final Sound also has two separate patents for producing electrostatic panels. Audiostatic, Quad and Sound Lab exclusively build full-range electrostatic panels. The only active electrostatic loudspeaker currently in production is the Audiostatic DCA-5.

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-Statik and its successors which used a dynamic subwoofer at low frequencies.

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