What are Electrostatic Speakers?

Written by Published | Rapallo AV Team

There are many different speaker technologies out there in the market, each with their advantages and disadvantages depending on the way you look at them. 

So, this week we delve into what is an Electrostatic speaker.

Let us give you some background and information, as they could be right for your house or your HiFi application and you might not even know anything about them.

WHERE DID IT ALL COME FROM

In 1923, Bell Telephone Laboratories made the decision to develop a complete musical playback system consisting of an electronic phonograph and a loudspeaker to take advantage of the new recording medium. Bell Labs assigned the project to two young engineers, Chester W. Rice and Edward W. Kellogg.

Rice and Kellogg had a well-equipped laboratory at their disposal. This lab possessed a vacuum tube amplifier with an unheard of 200 watts, a large selection of the new electrically cut phonograph records and a variety of loudspeaker prototypes that Bell Labs had been collecting over the past decade. Among these were Lodge’s cone, a speaker that used compressed air, a corona discharge (plasma) speaker, and an electrostatic speaker.

After a short time, Rice and Kellogg had narrowed the field of “contestants” down to the cone and the electrostat. The outcome would dictate the way that future generations would refer to loudspeakers as being either “conventional” or “exotic”.

Bell Laboratory’s electrostat was something to behold. This enormous bipolar speaker was as big as a door. The diaphragm, which was beginning to rot, was made of a pig intestine that was covered with fine gold leaf to conduct the audio signal.

When Rice and Kellogg began playing the new electrically cut records through the electrostat, they were stunned and impressed. The electrostat performed splendidly. They had never heard instrumental timbres reproduced with such realism. This system sounded like real music rather than the honking, squawking rendition of the acoustic gramophone. Immediately, they knew they were on to something big. The acoustic gramophone was destined to become obsolete.

SPRING FORWARD 20 + YEARS

In 1947, Arthur Janszen, a young Naval engineer, took part in a research project for the Navy. The Navy was interested in developing a better instrument for testing microphone arrays. The test instrument needed an extremely accurate speaker, but Janszen found that the cone speakers of the period were too nonlinear in phase and amplitude response to meet his criteria. Janszen believed that electrostats were inherently more linear than cones, so he built a model using a thin plastic diaphragm treated with a conductive coating. This model confirmed Janszen’s beliefs, for it exhibited remarkable phase and amplitude linearity.

Janszen was so excited with the results that he continued research on the electrostatic speaker on his own time. He soon thought of insulating the stators to prevent the destructive effects of arcing. By 1952, he had an electrostatic tweeter element ready for commercial production. This new tweeter soon created a sensation among American audio hobbyists. Since Janszens tweeter element was limited to high frequency reproduction, it often found itself used in conjunction with woofers most notably, those from Acoustic Research. These systems were highly regarded by all audio enthusiasts.

HOW DO THEY WORK?

The speakers use a thin flat 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 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] 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.

The diaphragm is usually made from a polyester film (thickness 2–20 µm) with exceptional mechanical properties, such as PET film.[citation needed] 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 voltage dependent part of non-linearity but leaves charge (attractive force) dependent part. 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-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 (i.e. large bandwidth) and so avoid distortion. The transformer is almost always specific to a particular electrostatic speaker. To date, Acoustat built the only commercial “transformer-less” electrostatic loudspeaker.[not confirmed] 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.

THE PRO’S & CON’S

Like all types of systems there are advantages and disadvantages, and these are also based on the persons’ preferences, how they hear and what they like to listen to.

THE PRO’S

The advantages of electrostatic loudspeakers include:

  1. levels of distortion one to two orders of magnitude lower than conventional cone drivers in a box
  2. the extremely light weight of the diaphragm which is driven across its whole surface
  3. exemplary frequency response (both in amplitude and phase) because the principle of generating force and pressure is almost free from resonances unlike 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, thus contributing to the damping of resonance build-up by the air itself to a significant, though not complete, degree. Electrostatics can also be executed as full-range designs, lacking the usual crossover filters and enclosures that could colour or distort the sound.

Since many electrostatic speakers are tall and thin designs without an enclosure, they act as a vertical dipole line source. This makes for rather different acoustic behaviour 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.

THE CON’S

Now, looking at it from the other side of the coin, from researching these speaker types,  typical disadvantages we could find could include:

  1. Bass response, due to phase cancellation from a lack of enclosure, but this is not shared by all designs. There is also the difficult physical challenge of reproducing low frequencies with a vibrating taut film with little excursion amplitude; however, as most diaphragms have a very large surface area compared to cone drivers, only small amplitude excursions are required to put relatively large amounts of energy out. This relative lack of loud bass is often remedied with a hybrid design (MartinLogan) using a dynamic loudspeaker, e.g. a subwoofer, to handle lower frequencies, with the electrostatic diaphragm handling middle and high frequencies.
  2. The directionality of electrostatics can also be a disadvantage in that it means the ‘sweet spot’ where proper stereo imaging can be heard could be relatively small, limiting the number of people who can fully enjoy the advantages of the speakers simultaneously. But with MartinLogan, they have the curved panel, thus increasing this “sweet spot” with a wider range. Did you know that in 1992 Critical Mass introduced the first electrostatic speakers for use in the mobile environment (car audio). Critical Mass engineer and CEO Wayde Alfarone’s design capitalized on the directional nature of electrostats by creating separate sound fields for different seating locations in the vehicle.
  3. Attraction – they can have a tendency to attract dust, insects, conductive particles, and moisture. They also need protection measures to physically isolate their high voltage parts from accidental contact with little-humans and small hands, plus the family dog or cat.

BUT YOU HAVE TO HEAR THEM

As we all know, sound is subjective to the person. So come and listen @ Rapallo and see what you think. The sound stage, when setup correctly and powered by a good amplifier is something else. While they are a different style to normal box speakers, they may just work really well in your room.

So stand-out from the crowd and try something different.

Sources: MartinLogan.com / Wikipedia / Rapallo