For quite a long time manufacturers of ESL's have been divided in two camps : either using flat, conductive sheets(e.g Quad, Martin Logan) or stretched wires(like Acoustat, Audiostatic). Each way has it's own advantages and field of application. For example, Martin Logan has succeeded in building optically interesting, "airy" designs which is obviously a very important selling point. On the other hand Quad focused in making full range designs without the use of crossovers, giving up transparency for other design considerations. Some other manufacturers like Acoustat even used many stretched wires to build conductive electrodes, so called stators.
Building an electrostatic loudspeaker has many challenges. First, it should be protected from dielectric breakdowns, as voltages involved are in the order of thousands of volts. This is also an important safety counter-measure. One way of doing so is to provide insulating layer of plastic. Perforated metal sheets can be powder coated. However coating stators to resist high voltages is a difficult task. Designs employing already insulated wires are typically more robust to breakdowns. The main reasons are rounded shape of wires and more uniform insulating layer.
Another quite interesting idea used was to "sandwich" panel within a thin film and fill inner space with inert gas. However the gas was much heavier than air, limiting high frequency response as well as escaping confinement over time. A different approach used is to manufacture sheets from printed circuit boards and place plastic side towards the membrane. Because dielectric constant of PCB material is relatively high(typically around 4) loss of sensitivity due to increased distance to the membrane is only around 1/4 as compared to if it was air. The spark must travel much higher distance to outer side of stator thus likelihood of arching is reduced. This is the solution used in famous Quad ESL 63 and some other models.
Other challenge of building an ESL is high directivity. When wavelength exceeds dimension of sound source it starts to become increasingly directional. For example, if radiator width is 20 centimeters it becomes more and more directional when frequency exceeds about 1.7 kHz. There are several solutions for this problem. The first approach is using separate panels for different frequencies, like used in famous Quad ESL 57. Second is to create an imaginary point source by delaying sound propagating through membrane. Again, this was very successfully used by Quad in model 63 and above. The third approach is to curve the whole speaker, as employed by Martin Logan. This way of construction has an additional advantage : it makes perforated metal much more rigid, so supporting structure is greatly simplified, although stability of curved & stretched membranes is somewhat difficult to achieve.
The last approach I am going to discuss is specific to stators built from stretched wires. Wires can be grouped electrically and then fed with different frequencies. This is called electrical segmentation. Each segment may be optimized so that it's width is less than wavelength radiated, thus achieving good polar response along horizontal axis. A tall, narrow element can be built to improve dispersion in vertical direction. This is called a line source. Surely, it's only an approximation to it because of finite width of the speaker. However for practical purposes this approximation works quite well. A line source has an additional advantage of reducing reflections from floor and ceiling. The conclusion is that resistance to arching and simple means of achieving good polar response are two important advantages of a wire ESL.
Disclaimer. The author of this site is not responsible for, and expressly disclaims all liability for, damages of any kind arising out of use, reference to, or reliance on any information contained within the site. While the information contained within the site is periodically updated, no guarantee is given that the information provided in this website is correct, complete, and up-to-date.