MartinLogan Expression ESL 13A hybrid electrostatic speakers.
Although the loudspeaker business is dominated by moving coil transducers, the electrostatic loudspeaker has some advantages if used wisely. John Watkinson looks at this alternative technology.
The long-lived moving coil drive unit can be made to work well, but it is inherently a three-stage device. First the magnet and the coil convert the input signal into a force and then the force is transmitted along the coil former to a diaphragm and then across the diaphragm. In a moving coil motor, the magnetic field, the current and the motion are all mutually at right angles. The field is radial, the current is tangential and the motion is axial. That is how electrodynamics works.
Electrostatic speakers are not like that as they seek to move the diaphragm directly. The term is something of a misnomer, since it is intended to distinguish from electrodynamics, but what goes on is far from static. There is still a field, but it is an electric field. The input signal is a voltage and the result is motion. Instead of the mutual orthogonality of electrodynamic motors, all three aspects of electrostatic drive are directed along the same axis, essentially the one towards the listener.
The drive mechanism is basically the force experienced by an electron in a voltage gradient. As quantum devices, the charge on all electrons is absolutely identical so the force produced by a given voltage gradient is always the same. What is more it is a perfectly linear function of the field. That is the good news about electrostatic loudspeakers; the drive mechanism is absolutely linear and phase linear, which means that the reproduction of transients will be very accurate. This is an area in which the majority of moving coil designs perform miserably.
The charge is applied all over the diaphragm by a polarising power supply that creates a high voltage. To stop the charge wandering about the diaphragm, which would make its behaviour very irregular, the diaphragm has to have very high resistivity, so it is made by covering an insulating plastic film with a very weakly conductive coating.
Figure 1 shows that the basic principle is very simple. The electric field that makes the diaphragm move is applied by electrodes, one each side of the diaphragm. The voltage required in a practical speaker is too great for a conventional amplifier to provide directly and so step-up transformers are required. The diaphragm moves towards and away from the electrodes. These are two of the down sides of the electrostatic loudspeaker; the linear transduction process can be compromised by imperfect transformers and the sound created at the diaphragm has somehow to get through the electrodes, which must be perforated in some way. If too much of the area is punched out, the field strength will fall.
Figure 1. The basics of an electrostatic speaker are the polarising voltage generator, that charges the diaphragm, and the step-up transformer that provides differential drive to the stators. The output of the transformer is hazardous when sound is being reproduced. (Click to enlarge).
Both stators need to be perforated to allow the diaphragm to move and as a result all electrostatic loudspeakers are dipoles. The same sound comes out of the back, only inverted. That is one of the great advantages of the electrostatic speaker over the legacy moving coil speaker and one of the reasons why they have their adherents, because the sound from the back is as accurate as the sound from the front and the ear can recognise it as reverberation after it has bounced around the room. Unlike conventional speakers, electrostatics don’t require heavy acoustic treatment, but they can’t be placed flat against a wall.
The down side of electrostatic speakers is that they can’t be put in an enclosure. The diaphragm is so light that the stiffness of the air spring in an enclosure would drive the resonant frequency sky high. The result is that they are unable to radiate low frequencies unless they are physically very large. One solution for those with large houses is to seal the speakers in holes in a wall between two rooms.
One of the common myths about electrostatic loudspeakers is that the high polarising voltage is dangerous. In all modern electrostatic speakers, the polarising voltage is obtained from a very low-power electronic voltage multiplier which has a high output impedance. If one touches that voltage, it will get your attention, but not enough current can flow to do much harm. In contrast, the signal voltage applied to the stators has come from an amplifier that uses negative feedback to give it negligible output impedance. Even after the step-up transformers the source impedance is still very low. The voltages on the stators when the speaker is reproducing sound can be lethal. All practical electrostatic loudspeakers need to have adequate means to prevent inadvertent contact with the transformers, the stators and the associated wiring.
Another common myth about electrostatic speakers is that they are inefficient. Actually the light diaphragm means that the speaker itself is extremely efficient, which is confirmed by the fact that they never get hot. The source of the myth is twofold. A typical electrostatic loudspeaker needs its associated amplifier to produce more voltage than would be needed by a moving coil speaker, so the volume has to be turned up a bit more. In conventional speakers, that is a symptom of low efficiency. Conventional speakers have a coil that has a resistance and it gets hot. However, an electrostatic speaker does not contain such a resistance and it presents a completely different load to the amplifier. The parallel electrodes of an electrostatic speaker form a capacitor and that is what the amplifier sees.
A conventional amplifier driving a pure capacitance delivers no net power. An electrostatic speaker has a small resistive component due to the sound radiation resistance, otherwise it is purely reactive. A conventional amplifier dissipates power in its own output devices when driving a capacitive load. So in fact it is the amplifier that is inefficient, not the speaker. Switched mode amplifiers are a good match for electrostatics because they can work well with reactive loads and when energy comes back to the amplifier it goes back into the power supply instead of into heat.
The use of a high resistance diaphragm means that the charge is uniform all over and so the force will also be uniform over the entire area affected by the stators. The motion of the diaphragm is mass controlled, and as it is of constant thickness, the result is that the entire area of the diaphragm subject to the electric field from the stators remains flat and moves without breaking up. The only flexing of the diaphragm is at the extreme edge where it is supported. The result is that an electrostatic speaker can operate over an extremely wide range of frequencies, eliminating or minimising the use of traditional crossovers that simply don’t work well.
In a moving coil speaker, the diaphragm can move a long way and the whole moving mass is so large that the adjacent air mass makes precious little difference to the motion. In complete contrast, the diaphragm of an electrostatic speaker has very limited travel and is so light that the air mass adds to the diaphragm mass and has to be taken into account. The travel has to be limited because otherwise the spacing between the stators would be so great that the field strength would fall.
Figure 2. In the QUAD electrostatic speaker designed by Peter Walker, a series of delay lines provides signals to stators in the form of concentric rings. This has the effect of making the sound radiate as if it had come from a sphere. (Click to enlarge).
It is fundamental to electrostatic speakers that they can only create volume velocity by moving a large diaphragm area a short distance. Unfortunately that directly contradicts the requirement for a small radiating area for good directivity at high frequencies. The result is that with very few exceptions, electrostatic loudspeakers are compromised by poor directivity characteristics, specifically high frequency beaming.
There have been various approaches to the problem. Attempts have been made to make curved panels, but keeping a curved membrane under tension is impractical. Very tall thin speakers have been made in an attempt to make the listening position independent of height, but they dominate the lounge.
One of the first people to understand the importance of directivity was Peter Walker, who founded Quad. His ESL-63 electrostatic loudspeaker had a unique system in which the diaphragm was driven by stators divided electrically into a series of rings. Figure 2 shows that, using delay lines, the sound fed to the centre ring would reach the next ring a little later so that a flat diaphragm acted as if it was a pulsating sphere. The result is magnificent. Recently the principle of the phased array electrostatic speaker has been re-visited by electro-acoustician Tim Mellow, who has used the basic delay line idea of Peter Walker but instead simulates an oscillating sphere in order to broaden the directivity pattern.
Figure 3. These are electrostatic speakers designed by Tim Mellow. They use a delay-and-ring structure similar to that of the QUAD, but incorporate moving coil LF units in sealed enclosures.
Instead of trying to reproduce the entire audio spectrum with an electrostatic panel, Mellow’s speakers, Figure3, include moving coil woofers to make them smaller than most electrostatic designs and more acceptable in actual living spaces. In order to match the phase linearity of the electrostatic panel, the woofer enclosures are sealed, so that there are no timing errors due to the use of ports.
Links to any of the many other Watkinson articles, including the preceding 10 loudspeaker tutorials can be found via The Broadcast Bridge home page search box. Simply enter the name, Watkinson into the search box.
John Watkinson Consultant, publisher, London (UK).
John Watkinson has a new book readers may wish to view. In The Art of Flight, John Watkinson chronicles the disciplines and major technologies that allow heavier-than-air machines to take flight. The book is available from Waterstones Book Store.
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