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How electrostatic loudspeakers work

Electrostatic Basics

The ESL operates as a result of forces due to applied static charges. Electrical similar charges repel and dissimilar charges attract. An ESL is created when a charged membrane or diaphragm is suspended between two perforated conductive plates called stators. The diaphragm is held between the stators by insulating spacers and the applied tension of the diaphragm. Depending on the voltage on the stators, the diaphragm will be pulled towards one stator and pushed away from the other - hence push pull operation of the drive descriptor action.

The perforations in the stators allow the diaphragm to couple its motion to the air in the surrounding environment. If the voltage on the stators is modulated by an audio signal, with the polarity of one stator in opposition to the other, the resultant movement of the diaphragm will track the audio signal and this motion being coupled to the air resulting in sound pressure waves.

The diaphragm should preferably be manufactured from an extremely thin and lightweight plastic material. It also must be made slightly conductive in order to distribute and retain a distributed polarising charge. This charge is provided by an HT supply providing an exceptionally high impressed voltage called a polarizing or bias voltage. The most common DIY surface conductive application involves a thin layer of graphite impressed into the surface of Mylar. The resistance is very high, since the intent is to distribute charge in the form of voltage rather than current the surface receives an evenly distributed surface charge that tends not to move, hence electrostatic operation. A high voltage is required because the force arising from the electrostatic forces are very small.

The stators must be physically very strong and ridged, in addition they should be acoustically transparent. They must also have a uniform inner surface area and dimensional characteristic to ensure an even, uniform electric field over the surface area of the active diaphragm. There are three main types of stator: perforated metal, wire grid, and conductive plastic. Each has a specific advantage. However, perforated metal and wire grid are the commoner forms.

The spacers are as important as the diaphragm and the stators. The spacers hold the membrane centered between the stators uniformly. Uniformity is extremely important since any changes in distance will result in a varying force being applied to the active diaphragm and result in non-uniform diaphragm movement. In addition, the spacers must provide perfect insulation, even in conditions of high ambient humidity. Since the resistance on the diaphragm may be greater than 100Meg ohm and the voltages may rise above 10kV with little current delivery capability, tracking due to dust motes and humidity or indeed any leakage represents a substantial loss of volume output and possibly a dangerous situation depending on system characteristics.

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Advantages of Electrostatic over Dynamic Drivers

Electrostatic units have many advantages over standard dynamic loudspeakers. Dynamic loudspeakers create sound by using an electromagnetic coil mechanically coupled to a diaphragm (usually a coneified or dome structure) supported and centralised in a strong magnetic field. This "Motor" electromagnet is driven with an AC audio signal and the frequency time-varied induced magnetic field that results causes a force to be applied to the diaphragm due to interaction with the static magnetic field. The motion of the diaphragm being coupled to the air as is an ESL produces sound waves. However the cone and the electromagnetic coil have a mass and therefore suffer from inertia. Although the diaphragm, in theory, should adopt a position in direct relation to the input electrical signal, the inertia induces an error in the systems overall capability to follow the exciting signal, over or undershoot adding coloration and veiling to the sound. The magnetic field produced by the driver motor assembly is non uniform outside the limitations of the small gap and as heat builds up due to power dissipation, the characteristics change. Furthermore, the driven diaphragm is not uniformly ridged and adds distortion as a result of the intense forces at work on it. In addition the loading is complex as the inductive resistive characteristics are frequency dependent and make designing a suitable crossover fraught with difficulty

This also explains why moving coil loud speaker arrays suddenly come to life at a specific volume setting as the driving voltage overcomes the base line inertia of the driven elements.

ESLs essentially are not affected by any of these problems. At around 1 mil thick, the diaphragm weight is about 1/280 of that of air. When the diaphragm mass is compared to the mass of the air volume of the listening environment, the diaphragm is essentially mass less. This extremely low working weight almost completely eliminates distortion caused by overshoot or undershoot. Also as the electrical field is to all intents uniform between the two parallel plates the resulting or arising motion of the diaphragm is therefore theoretically perfectly linear, until the mechanical limits of the diaphragm is reached, any distortion shall be of en extremely low order. The bias voltage creates a constant force in one direction that causes the diaphragm to be attracted slightly towards one stator. However, this is very small when the diaphragm tension is correct and should not represent a problem. What is important is that due to the uniform electrical field, the diaphragm responds linearly as a moving plane to the applied drive signal. Since the entire surface is being driven, uniformly, there are no losses or distortions caused by any form of mechanical coupling. Only a very narrow zone around the perimeter is restricted from movement due to the proximity to the insulating spacers, and this is normally about 1/4" which is insignificant compared to the overall active area and size of the ESL diaphragm. As an aside since the ESL does not have a low theoretically value of working resistance, the unit does not dissipate power since it is an applied voltage dependent device as opposed to current / voltage transducer.

Design Issues with Electrostatic Loudspeakers

As in any system, ESLs have an extremely complex set of Engineering weightings to be balanced in the design consideration stage these are very much dependant on the set in desired design parameters. However, there are some aspects of electrostatic loudspeaker construction and design that are deemed to be extremely challenging and potentially dangerous. The ESL does not dissipate power because of its capacitor construction, it stores a potential charge. Therefore, although the amplifier doesn't in theory provide power, the amplifier must dissipate and sink the energy it derives from the lower level input sources which very often results in power amplifier drive problems, this factor causes most standard consumer amplifiers to oscillate in the ultrasonic range or simply self destruct. Also, the stator voltages involved in driving the diaphragm range from 1kV to 10kV. Although the current for the diaphragm bias supply may be a few micro amps, the current in the high voltage audio section can be significant higher due to the capacitive loading effects of the ESL and the stored energy in the driving systems. In addition to the interesting electrical properties, the ESL also represents a challenge acoustically. It must be operated as a dipole, which means both front and rear must free to radiate into the environment. This can cause the lower sound register to cancel out by interaction from the front to the rear of the driver, which may require some form of acoustic baffles or wave guides. Room placement is critical because the sound may be highly directional. The diaphragm motion is very very small, and this requires a very large surface area to provide sufficient sound pressure listening levels. Below its resonance point, an ESL output is virtually non-existent, but unlike dynamic speakers in standard enclosures, there is usually no resonance peaks or dips. Also, because the ESL is incapable of producing sound below its resonance point, bass extension is virtually non-existent for most systems.

Biasing and Supply

A conventional system involves the connecting of the stator plates to the terminals of a center tapped step up transformer with the diaphragm biased with reference to the center tap of this transformer. An amplifier drives the primary windings of the step up transformer, which causes the stators to be driven in perfect push pull opposition to each other, the applied AC audio signal results in the DC voltage polorised diaphragm seeking to adopt a neutral energy position and gives rise to an Audio signal reflecting the applied voltage. This is a simplified descriptor of such a system using a step-up transformer.