Basic ESL tructure is very simple; thin film with high square resistance between two conductive stators. If low bias voltages are used it is also very simple to get working and to sound good. ESL has few unigue properties that cannot be accomplished with any other available method. Push-pull structure results very low distortion especially to crucial midrange, there is no cone breakup problems. One element can easily produce very wide bandwidth without compromises like there are in wideband dynamic elements. Low weight diaphragm results excellent transient response and fast decay of sound.
Constant charge is generated with high voltage supply. Polarization voltage can be generated with voltage doubling ladders or switch-mode converters. Required output current is very small if panel is properly insulated and is not used in high humidity levels. Normal speaker level signals have to be raised to increase sensitivity. That is in most cases accomplished with audio transformer. High voltage amplifiers can be used as well if available.
One problem of the panel is poor sensitivity. Sensitivity can be increased by increasing polarization voltage, transformer ratio or both. With voltages as high as 5-7 kV it is essential to insulate stators properly. Frequency response is much due to stator dimensions. High panel is vertically extremely focusing but often they are also rather wide compared to normal tweeters. That makes them highly focusing in horisontal direction also. It can be reduced with bending stators (Martin Logan), driving them differently with different frequencies (Audio Static) or electrically delay some parts of the stator like QUAD has done. Or they can just be made so narrow that focusing is not so disturbing. Unfortunately that makes them still less sensitive and makes it unsuitable for low frequency use. Beaming is not disturbing in very wide panels like Portal bacause beam is so wide.
Full range operation
Usual speakers have at least two elements and they still have difficulties in producing the lowest frequencies. At least three dynamic elements are needed to cover frequencies from 20-40Hz to 20kHz with normal SPL and distortion requirements.
Electrostatic element has lightweight membrane that is capable of producing frequencies above 20kHz. 12 to 15 um polyester film can be used before diaphragm mass would become limiting factor at highest frequencies. Normally panels are build using about 6-10um films. Diaphragm is uniformly driven so speed of sound in diaphragm material is not important like it sometimes is in wide range dynamic elements with single point (voice coil) exitation. Upper frequency limit of electrostatic element is more often limited by stator construction or electrical implementation than diaphragm mass.
Low frequency limit of panel are set by dipole operation and bass resonance. Dipole operation will decrease sensitivity in far field 6dB/oct after certain frequency set by panel and frame physical dimensions. Larger panel has lower roll-off frequency and listener is closer to panel compared to panel size. Increased air loading decrease bass resonance frequency. Portal has resonance peak at 23 Hz. Both magnitude of the peak and its frequency can be further decreased with additional damping. At low frequencies spacer thickness limits excursion especially at resonance frequency and spacers prevent ideal pistonlike operation, reduces effective area and increases distortion.
Near field frequency response of panel element is in theory perfectly flat. In practice there is bass resonance and small variations due to construction. Stator structure can cause resonance peak typically around 20kHz. Transformer spread inductance and panel and transformer spread capacitances will cause second high frequency resonance. Bass resonance can be damped and resonance frequency can be shifted several ways. High frequency resonance caused by mechanical construction can be shifted above hearing band and transformer can be designed so that resonance frequency is well above 20 kHz and has low Q-value.
Far field frequency response in flat panel element is far from straight. Large area uniformly driven panel is extremely directional. Operation can be simulated with very simple panel model. Frame structure should be included in simulations. Even if norman panels are used in dipole mode normal 6dB/oct dipole compensation cannot be used because listening position is somewhere between near and far field. For optimal equalization accurate listening distance should be known.
Structure we have used
Our panel is at least fourth generation developed at TUT. Common problem has been stator insulation that is now handled with 1 mm thick polyethen coat on perforated steel plate. Some of the tweeters and headphones we have made use epoxy laminate as a stator. Polyethen insulation is successfully tested with 15 kV and panels use currently about 6-7 kV polarization voltages without problems. Good insulation makes it possible to increase sensitivity.
Polyethen is not the best choice for the insulation, but there are not many methods available that would produce reliable results. Different paints, lacquers, and resins have been tested with less encouraging results. Things get little easier if sharp edge that results from perforating process can be removed with a way or another. Electrolytic polishing is one of those methods that can be used.
Polyethen layer is done by dipping statorplate into a hot liquid polyethen. Hole should be rather large to prevent surfacetension to close it. About 6mm works fine. Larger holes should be avoided to prevent unequal field and decrease of sensitivity.
Spacer thickness varies between types of elements. Panels that are designed to operate in midbass or bass region use 3mm spacers. Midrange and high frequency elements have been typically made with 1mm spacing.
Materials selected to spacers has very small relative permittivity factor. Small permittivity is necessary for keeping waste capasitance small. Area below spacers increase capasitance but produce no sound, thus its effect should be minimized. For example PVC has rather high relative permittivity of 4.6. Teflon PTFE and polyethen have low permittivities but they cannot be glued very well. Stiff foam materials are well suited because they are mostly air. By using stiff foam tapes simplifies construction significantly.
We have used 6 and 8um mylar in diaphragms. Everything below 12um is suitable. If thickness is above that weight of the film becomes so high that it may limit high frequency response. Film is treated with graphite. Several other materials and treatments have been tested but none of them have brought any significant improvement. It is crucial to make surface resistivity as constant as possible and as high as possible without jeopardising reliability. High resistivity is crucial because of constant charge operation and distortion.
We have used very large area contact to diaphragm to ensure even distribution of charge even if there happends to be breaks in the graphite treatment. Diaphragms are heat shrinked with heat gun. None of the panels we have made during the years have had any problems with diaphragm.
All transformers are selfwounded. See page: Full range ESL transformer.
We have used both switch-mode converters and voltage doubling ladders in our panels. Ladders are very well suited for this application. One ladder version is in front of the transformer at full range ESL transformer. Switch-mode converters allow easy adjustment of voltage that may be beneficial in prototyping stage. Voltage doubler ladder should be isolated from mains networks for safety.
Some of the panels made use passive equalization network at high voltage side. It has the additional benefit of decreasing panel capacitance. Low pass filter can be formed by adding resies resistance. Dipole correction requires resistor and capacitor to be connected in parallel and this network in series with the panel. Values can be easily calculated if panel capacitance is known.
Bass resonance of the panel can be varied with diaphgram tension, mass, Young's modulus and separation of spacers. With suitable construction and material selection it is possible to build a panel which has only very mild resonance peak. Large panels have their resonance at bass or subbass region. In fullrange operation this phenomena is evident and usually result unpleasant one tonal bass. Higher power levels cause severe distortion when diaphragm hits the stators. Diaphragm displacement amplitude at the resonanse can be very high. Most panels require some external damping. Adequate damping can be made by adding cloth bag over the element that acts as a resistive element. It also keeps larger dust particles away from the panel. Same damping can be accomplished by adding cloth only to backside of the panel. Portal has currently double cloth layer behind the panel. All hybrid panels have cloth on both sides.