We lazily call the MartinLogan ElectroMotion ESL stereo loudspeakers "electrostatic", when in truth they are hybrids, since the bottom 4.5 octaves are handled using a conventional dynamic driver.
Virtually all loudspeakers you see are dynamic. They have wire coil wrapped around a cylinder, which is surrounded by strong magnets. A signal through the wire makes a varying magnetic field, which makes the coil move. A diaphragm is attached to the cone, and creates pressure waves in the air. That is, sound.
Each of these MartinLogan speakers has one of those in an enclosure at the bottom. Each carries the audio load from the specified low end of 42 hertz up to 500 hertz, which is in the low mid-range of the sound spectrum. The frequencies above that are carried by an electrostatic panel.
This is a slightly curved sheet of transparent material, about the thickness of cling wrap, stretched between two perforated metal panels. Instead of magnetic force being applied, an electrostatic one is used. This is the same kind of force apparent when a rubbed plastic comb attracts a small piece of paper. Internally, the amplifier signal is stepped up by a transformer to thousands of volts, and applied to the metal panels (don't worry, they're insulated so you won't be electrocuted), while a high static voltage is applied to the thin sheet of material held between them. This sheet vibrates evenly across its surface, driven by electrostatic forces generated in proportion to the audio signal.
It is difficult to do bass with electrostatic panels, due to low-frequency phase cancellation and the need for greater fore-aft movement space, which is why MartinLogan uses a conventional driver for this purpose.
Each loudspeaker comes with a low-voltage wall-wart power supply. This is plugged into the back to provide the necessary high-voltage static charge to the thin film diaphragms. Little energy is used by this; power draw was about 0.7 watts when in standby mode, and 1.4 watts when in use, regardless of volume level.
All loudspeaker brands sound different, and these differences are clearly obvious to even a casual listener. But we'd venture the opinion that these loudspeakers raise that difference to a new order.
Before getting into that, we should note the ways in which they are the same. Tonally, their balance is beautiful. At normal to loud listening levels, there wasn't the slightest whiff of harshness or sound colouration. They were wonderfully clean and detailed. Some silly member of the orchestra playing the Carmen Suite tapped a foot on the floor, and this was revealed with embarrassing clarity through the veil of sound of massed violins.
The bass — conventionally produced, remember — was good, down as far as it went. MartinLogan has resisted any temptation to have it at too high a level, so the sound melded smoothly across the woofer and the electrostatic panel. The woofer seemed nicely designed to manage the same output levels as the panel.