1) Principles of Electrostatic Loudspeaker (ESL) Operation

by Mark Rehorst

November 2006

Here I will try to explain how they work and why they are so good.

You can learn more than you need to know about how ESLs operate by looking at any college physics text book.  It all boils down to this- like charges repel each other, opposite charges attract each other.  If you place positive electric charges on two objects then bring the two near each other, they will experience forces that push them apart because of the interactions of the electric fields that surround them.  If one of the charges is "negative" and the other "positive", the two charged objects will experience a force that pushes them together.  The force involved depends on the charges and the distances involved.  An ESL uses the force developed by the electric field between charges to move a very thin plastic diaphragm back and forth to produce sound.

The behavior of electric charges is similar to magnets when brought near each other, however the forces developed by electric fields are usually smaller than those we experience with magnets.  You can play with electric charges by rubbing a balloon on your head on a dry day.  When you lift the balloon, your hair will stand up to try to touch the balloon because the balloon has accumulated one polarity of charge and your hair has accumulated the opposite polarity of charge.  This is called triboelectric effect and occurs whenever two dissimilar materials are moved against each other.

ESLs are simply a way to keep electric charges separated but close together so they can do the work of moving a plastic film to produce sound.  The usual construction technique is to place a tight plastic film (the diaphragm) with an electric charge between two charged conductors (called stators) on which the charges alternate.  At one instant, the diaphragm is driven towards one of the stators, then when the polarity of the charge on the conductors reverses, the diaphragm is driven toward the other conductor.  The result is sound.

The charges are applied to the diaphragm and the stators using some simple electronics.  A very low current, high voltage power supply is used to charge the diaphragm.  Electronic components called transformers are used to apply the alternating polarity charges to the stators.  I'll explain more about those, later.

When an electric charge is applied to a metal object, the charge spreads out and the entire surface of the metal rests at a specific voltage (a measure of the quantity of charge divided by the distance separating two opposite charges) depending on the amount of charge applied and the distance to the opposite charge (charges normally come in opposite pairs- you don't usually have a positive charge floating around without an equal negative charge somewhere nearby).  Metals as said to "conduct" electricity because electric charge can move around freely on them.

Plastics don't normally allow electric charge to move, so they are called "insulators".  An ESL uses a plastic film for the diaphragm because some plastics are very strong and light weight.  The diaphragm won't move if it doesn't have an electric charge, so the first problem in making an ESL is how to apply charge to the nonconducting diaphragm.

In between metals which conduct electricity and plastics that don't there are other materials that conduct electricity very poorly.  Those materials are said to have high resistance to the movement of electric charge.  In an ESL, the diaphragm is coated with one of these materials.  An electric charge is applied to the coating and over time, it spreads out over the entire surface of the diaphragm.  The very high resistance of the coating prevents the charge from moving around on the diaphragm surface very quickly.  The charge is applied with a high voltage power supply that can develop as much as 5000 Volts.  All that supply needs to do is supply charge to the diaphragm which is very high resistance, so the current required from the high voltage power supply is measured in microamps.  It is not dangerous unless you happen to be in an explosive atmosphere.  Remember the triboelectric effect?  That is what is responsible for the "static" electric shocks you experience when you walk across a carpeted floor with rubber soled shoes and touch a doorknob.  You can easily generate more than 5000 Volts by walking across a room on a dry day.

The speakers most people have experience with use magnetic fields to move a paper, plastic, or metal "cone" back and forth to produce sound.  The most common configuration has a coil of wire mounted near the center of the cone, concentric with a very strong permanent magnet.  Current produced by an amplifier flows through the coil and sets up a magnetic field that interacts with the field of the permanent magnet and produces force that moves the cone back and forth.  The cone is not perfectly stiff, and if it is large, it is relatively heavy.  These properties affect the frequencies that can be reproduced and the distortion.  A very large, massive cone cannot be moved fast enough to reproduce high frequencies.  When the coil is driven with a lot of current, the cone flexes a little resulting in distortion.  Low frequency sound reproduction requires movement of a lot of air, so the speaker must be made large, and the cone will be heavy.  High frequency sound production requires that the coil and cone be made very light weight.  This is why so many speakers have separate drivers for the low, middle, and high frequencies.

Most electrostatic speakers use a single, large diaphragm to produce sound.  Even though it is large, it is very light weight because it is made of very thin plastic.  The very light weight means it can reproduce high frequencies very easily.  It can't move back and forth as far as a magnetic woofer, but what it lacks in distance is made up for in surface area, so it can reproduce relatively low frequencies also.  Distortion is very low because unlike a magnetic driver in which the cone is driven only from the coil at the center of the larger cone, the entire surface of the diaphragm is driven by the electric field between the stators.  So ESLs are capable of reproducing a wide frequency range with very low distortion.

As wonderful as ESLs are, they have some limitations.  The large surface area means the speakers tend to dominate whatever room they are placed in.  They usually are not as sensitive as "normal" magnetic speakers so they won't play as loudly.  The frequency response of ESLs is a function of their size.  The larger you make them, the lower they can go, but it is almost impossible to get flat response down below 50-60 Hz because the front and rear of the speaker are out of phase.  At high frequencies the radiation pattern narrows to a tight beam.  This results in rising response- about 6 dB per octave, and a "sweet-spot" that is suitable for one listener only.

The response issues can be overcome by electronic equalization.  The only way to overcome the low frequency response limitation is to crossover to a magnetic driver in a box at some low frequency.  This will allow the speaker system to play loudly and remain very low in distortion.  If you aren't concerned about flat response below 50-60 Hz (most magnetic speakers can't go lower than that either), you can use ESLs with no crossovers at all.  They sound amazingly lifelike!