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.
It was limited at the bottom end for music that went below the usual four-string bass-guitar and kick-drum range of frequencies, so, if the bottom octave of audible frequencies — 20 to 40 hertz — are important to you, then consider adding a subwoofer.
As to levels, electrostatic loudspeakers have somewhat of a reputation for gorgeous delicacy, but not much of one for high volumes. This hybrid design dispelled that. With the dynamic bass driver carrying much of the load, the panels were able to unleash their own very high output levels with my various rock tracks, the drums and cymbals spiking through the mix to even higher levels.
But still they were different. It wasn't to do with tone, detail or distortion. It was to do with their stereo image. Electrostatic panels are dipoles; they produce the same sound from the front and back, except that it is out of phase. That means that very little sound is projected to the sides (it cancels out), but a lot is projected to the back, and reflected by the back wall back to the listeners after a delay.
The tall panels also reduced the up and down dispersion of higher frequencies more than is the norm with conventional speakers.
The net result of all of this was that rather than a precise and razor-sharp stereo image, they produced an airier, rounder sense of instruments between the speakers. But they also sometimes made the stereo image seem wider than the loudspeakers themselves, and added a great deal of depth. A depth which, we suspect, wasn't actually there in the recording.
All this was in turn heavily dependent upon the room; what surfaces were behind the speakers, and how far they were away from them.
We loved the result. But you ought to audition them before committing.
Electrostatic speakers are relatively rare and expensive. But these entry-level hybrids from MartinLogan show why people still love them.