Electrostatic Loudspeakers Explained
How electrostatic speakers work, why they are prized for transparency, and what amplifier, room and placement they need.
Electrostatic loudspeakers replace the cone and magnet of a conventional driver with a vast, almost weightless diaphragm. The result, at its best, is a clarity and speed that few box speakers match.
How they work
A thin plastic film, coated to hold an electrical charge, is stretched between two perforated metal panels called stators. A high polarising voltage charges the film, and the music signal, stepped up to hundreds or thousands of volts by a transformer, is applied to the stators. The film is pushed and pulled across its whole area at once, so it moves as a single piston with very little mass.
Strengths
- Low moving mass, for exceptional transient detail and low distortion in the midrange.
- No box to colour the sound, so there are fewer cabinet resonances.
- A large radiating area that can sound open and life-sized.
Limitations
- Bass and level. A thin diaphragm can only move so far, so deep bass and very high volumes are hard. Many designs add a conventional bass driver (hybrids) or use very large panels.
- Dipole radiation. Sound comes from the back as well as the front, so placement away from the rear wall matters a great deal.
- A demanding load. Electrostatics look like a capacitor to the amplifier, with impedance falling at high frequencies. Choose an amplifier that is stable into difficult loads.
- Mains power and humidity. Most need a mains supply for the polarising voltage, and some are sensitive to damp.
Setting them up
Start with the panels at least a metre from the rear wall, toed in towards the listening seat, and experiment from there. Small movements change the balance between bass reinforcement and image depth. Keep the listening height close to the panel's designed height, because many full-range panels beam treble narrowly.
Famous names
Quad's ESL of 1957 established the genre. Today names such as Quad, MartinLogan and Sound Lab carry it forward with full-range and hybrid designs.