Electrostatic loudspeakers explained: how they work and what living with them involves

An electrostatic loudspeaker moves the air with a membrane far thinner than a human hair, driven over its whole surface by an electric field. Owners describe a clarity and speed that cone loudspeakers find hard to match; they also learn that an electrostatic panel has its own needs for placement, amplification and climate. This guide explains how an electrostatic loudspeaker (ESL) works, what it asks of the amplifier, the room and the climate, how long panels are commonly reported to last, and how our two electrostatic models approach the design.

How does an electrostatic loudspeaker work?

An ESL has three essential parts: a diaphragm, two stators and a bias supply.

• The diaphragm is a plastic film a few microns thick, tensioned on a frame and given a very thin conductive coating of high electrical resistance. In Menelaus and Atreus it is 2 microns thick.
• The stators are two perforated conductive plates or grids, one on each side of the diaphragm and a small distance from it. Sound passes through their openings.
• The bias supply charges the diaphragm to a high, steady voltage, typically several thousand volts at a tiny current. Both models use 5,000 V from an integrated multiplier, powered from the mains.

The music signal from the amplifier passes through step-up transformers that raise it to hundreds or thousands of volts, and it is applied to the two stators in opposite polarity. The charged diaphragm is attracted towards one stator and repelled by the other, following the signal. Because the force acts evenly over the whole surface, the diaphragm moves as one piece instead of being pushed at its centre like a cone, and because it weighs almost nothing, it starts and stops with the signal. The high-resistance coating keeps the charge on the diaphragm nearly constant as it moves, which keeps the force proportional to the signal and the distortion low.

Full-range electrostatics reached the home with the Quad ESL in 1957, and a few makers have built them ever since. Electrostatic headphones work on the same principle; see our article on electrostatic headphone amplifiers.

Dipole radiation and placement

A panel has no box, so it radiates equally to the front and the rear, with the rear wave in opposite polarity. This dipole pattern is a figure of eight: strong in front and behind, with a null at the sides. Three practical consequences follow:

• Distance from the wall behind. The rear wave reflects from that wall and returns delayed. Owners commonly place panels at least about 1 m from it, often further, for depth and a clearer image.
• Fewer side-wall reflections. Less energy reaches the side walls, which many listeners hear as a clear, stable image.
• Bass cancellation. At low frequencies the front and rear waves meet around the edges and cancel, so a dipole needs a large area, or help from a woofer, for deep bass.

A flat panel also becomes more directional as frequency rises, so the treble is concentrated in a narrow beam. Makers answer with curved or segmented panels; both Demograf models combine curved and flat panels, and the curved ones focus the sound on the listening position. Toe-in, seat height and the distance to the wall behind are worth adjusting a few centimetres at a time; our articles on getting the most from your loudspeakers and the stereo image describe the method.

Impedance and amplifier demands

To an amplifier, an electrostatic panel seen through its step-up transformer looks largely like a capacitor, and a capacitor’s impedance falls as frequency rises. The amplifier must therefore deliver current into a low impedance at high frequencies and stay stable with a capacitive load, which some designs handle better than others. Sensitivity is also usually modest.

Our two electrostatic models illustrate this: their impedance is rated at 2–4 Ω, falling to about 1 Ω at 20 kHz, with a sensitivity of 85–86 dB. An amplifier with a high output impedance, such as a single-ended triode amplifier, would lose treble into that load, as our guide to loudspeakers for single-ended amplifiers explains. Other electrostatics present gentler loads, so always check the curve of the model you have in mind.

Why Menelaus and Atreus come in active versions

Both models can be driven passively by an amplifier that is comfortable with a 2–4 Ω load. Both are also offered as active systems, in which an external analogue crossover from Demograf Audio feeds four amplifier modules, Class D or Class AB, engineered specifically for these loudspeakers, and a preamplifier sets the volume. Our guide to passive and active preamplifiers explains the choices.

Hybrids: electrostatic panels with dynamic woofers

Deep bass from a dipole panel needs a large area and enough membrane travel, and travel is limited by the narrow gap between the stators, so many designs hand the bass to a cone woofer. The difficulty is integration: a woofer in a box radiates differently from a dipole panel, and owners of some hybrids describe a woofer that sounds slower than the panel above it. Crossover point, level and placement decide whether a hybrid works.

Menelaus is such a hybrid. Open-baffle panels, curved and flat, 180 × 15 × 5 cm, play everything from 600 Hz up, so there is no crossover in the 2–5 kHz region where many loudspeakers divide the range. Below 600 Hz a Demograf Audio subwoofer with a 12 or 15 inch driver and electromagnet drive takes over, down to 10 Hz, and its level and frequency can be adjusted to the room.

Atreus takes the other route and stays electrostatic throughout. Its curved and flat panels cover 600 Hz to 20 kHz, and the bass comes from an isobaric electrostatic section, six membranes between eight 3 mm steel stators, that reaches 20 Hz. Each loudspeaker weighs about 60 kg, 50 kg of which is the steel of the bass stators.

Humidity, dust and the Gulf climate

An electrostatic panel depends on keeping a high voltage where it belongs. In humid air, surfaces become slightly conductive, so the bias charge can leak across the diaphragm’s edges and supports; output may drop, and some panels hiss or crackle until the air dries. The bias field also attracts dust, and dust that absorbs moisture creates further leakage paths, as does salt-laden sea air.

In coastal Gulf cities, where outdoor humidity is high for much of the year and dust storms are common, these points matter more than in a temperate climate. A closed, air-conditioned room usually solves most of them. The advice is simple:

• Keep the listening room air-conditioned and the relative humidity moderate, roughly 40–60%. A dehumidifier helps where air conditioning alone is not enough.
• Do not place panels near open windows or sea-facing doors, or in the direct airflow of an air-conditioning outlet.
• After delivery, especially if the crates come from a cool truck or store into warm, humid air, let the loudspeakers reach room temperature for several hours before switching on, so that no condensation forms on the panels.
• Keep windows closed during dust storms, and cover the panels with a breathable dust cover when they are not used for long periods.
• Switch off and unplug the loudspeakers before cleaning. Remove dust gently from the grille with a soft brush or a vacuum cleaner held at a distance, and never touch the membrane.

How long do electrostatic loudspeakers last?

There is no single answer. Owners commonly report panels lasting well over a decade, and some classic designs have run for several decades with service; others needed attention sooner. The usual causes are a diaphragm coating that loses conductivity with age, membranes damaged by overload or accident, arcing when a panel is driven too hard, and dust and humidity. Heat, direct sunlight and tobacco smoke shorten a panel’s life; a clean, dry, air-conditioned room lengthens it. The bias supply and step-up transformers can be serviced like any other circuit.

Menelaus and Atreus need no routine service; drivers, crossovers and terminals are repaired by the atelier for as long as parts exist. Treat the 5,000 V bias supply with respect, and leave service to the atelier or a qualified technician.

Are electrostatic speakers better? Advantages and drawbacks

That depends on your priorities. Compared with conventional cone loudspeakers, electrostatics trade some practical ease for a particular kind of clarity.

Advantages

• A diaphragm of almost no mass, driven over its whole area, which owners commonly describe as fast, transparent and free of colouration.
• No box, and so no cabinet resonances.
• A large radiating surface that presents voices and instruments at a natural scale.
• Dipole radiation, with less sound sent to the side walls.

Drawbacks

• A demanding load: low impedance in the treble, capacitive behaviour and modest sensitivity.
• Limited bass and maximum level unless the panel is very large or helped by a woofer.
• Placement: distance from the wall behind and a room of reasonable size, at least 20–25 m² for Menelaus and Atreus.
• A mains connection for the bias supply, sensitivity to humidity and dust, and considerable size.

Why are electrostatic loudspeakers rare?

The drawbacks explain most of it. Electrostatics are large, need space behind them and an amplifier chosen for them, take many hours of hand work to build, and reward a dedicated listening room more than a shared living space, while most shops and amplifiers are built around box loudspeakers. A small number of makers keep building them because their owners value what they do well.

Which Demograf Audio piece fits

For electrostatic midrange and treble with deep, adjustable bass from a dynamic subwoofer, choose Menelaus. For a loudspeaker that is electrostatic from 20 Hz upwards, choose Atreus. In their active versions the crossover and amplifier modules come from Demograf Audio. Each is built for 100, 120 or 230 V, including a 230 V version with a type G plug for the Middle East. For a passive pair, choose an amplifier comfortable with a 2–4 Ω load rather than a single-ended triode amplifier.

To add weight in the lowest octave to other full-range panels, the active Zeus, a 15-inch field-coil subwoofer covering 10–300 Hz, accepts RCA or speaker-level inputs. For the electrostatic sound in headphones, Tantal has a built-in STAX-type output. Tell us your room size, its climate and whether you prefer an active or passive system, and we will suggest a configuration.