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Electrostatic Headphones: How They Work, and Why They Sound Better
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Electrostatic headphones and speakers operate on a completely different and more advanced principle than conventional "dynamic" or "moving coil" headphones and speakers.

Conventional speakers and headphones stick a coil of wire inside a magnet, and glue this coil to a stiff cone or dome that's held in place with a springy suspension. Current passes through this coil, and electromagnetism creates force on the coil while in the field of the magnet. The resulting force vibrates the coil, and since it's glued to to a heavy cone, moves the whole mess in and out. This primitive method was invented back in the 1800s, and is still used today because it's cheap and works reasonably well for most purposes.

Electrostatic systems allow us to place a force on an ultralight film without touching it. It's the same thing as electrostatic ("static") cling on clothing and dry-cleaning bags. Electrostatic forces are applied evenly across the entire film. It's magic; sound comes out without any physical contact, much less any interference from coils, stiff cones or any of the problems that plague conventional systems.

Electrostatic headphones use extremely thin diaphragms. For comparison, a very thin dry-cleaning plastic bag, the type that blows away just by breathing on it, is ten to twenty times thicker than the material used in electrostatic headphones.

Electrostatic systems work by placing a static (non-moving) electric charge on a film that floats between two perforated metal plates. When audio voltages are applied across the plates, static cling and repulsion causes the entire film to move all by itself. This film is so thin that it weighs less than the air around it, and has no resonances or energy storage which leads to the coloration inherent in moving coil speakers.

It's called "static" because the electric charge on the film does not move. It takes a minute or so for the charge to build when first turned on, but since the electrons hold their places on the film, the forces applied to them by the electric field allows these electrons to move the film, instead of moving themselves around the film.

Most music and movie sound is recorded in studios using large-diaphragm condenser microphones that work on the same principle, sideways. They measure the position of a large, electrically-charged and almost weightless diaphragm next to a perforated plate; they don't glue a coil to anything. Electrostatics use many of the same principles of how our music is recorded in the first place.

The rated harmonic distortion of the world's finest dynamic loudspeaker, currently the $24,000-the-pair B&W 800 Diamond, is rated for <0.5% distortion from 80Hz - 100 kHz at 90dB at 1 meter. But wait - that's not THD, Total Harmonic Distortion, that's only for the second and third harmonics, not the total of all harmonics. Distortion from 45Hz - 100kHz is rated as <1%, and distortion below 45 Hz gets so ugly that B&W won't disclose it, and that's the world's state-of-the-art.

By comparison, the Stax SR-Lambda Professional of 1982 was rated at 0.007% distortion at 400 Hz at a live-concert level 100 dB SPL. Looking at its actual distortion curve, we can see that its distortion is about -67 dB (0.05%) from 30 Hz on up, at 100 dB SPL. Not only is this about one-thirtieth the level of the world's best conventional loudspeaker, this is measured at 100 dB at the ear, or at ten times the apparent acoustic power, and down to 30 Hz, not 45 Hz! Even at 100 dB SPL, these electrostatic headphones have only about 0.1% distortion down to 20 Hz!

This lack of distortion is only one of many reasons why electrostatics sound so much better than ordinary transducers.

So if electrostatics are so superior, why aren't they more popular? Easy: they require a high-voltage power supply, which usually means an amplifier that plugs in the wall, and there are no inexpensive electrostatic headphones.

Stax in Japan are the most popular electrostatic headphones, having invented them back in 1959. Stax makes these in very limited quantities, and I know of no USA dealers. You have to get them through eBay, often directly from Japan or used.

 

Let's compare the two systems:

 

 
Conventional
Electrostatic
Name
Electrodynamic
Electrostatic
Name means
Something moves
Not moving
Name refers to
coil
charge on diaphragm
Force comes from interaction with
Magnetic field
Electric field
Field generated by
magnet
voltage across plates
System works based on Magnetic fields generated by currents flowing through wires.
Pure electric fields flowing through space.
Linear field?

No, magnets and magnetic structures must be very carefully designed to preserve some semblance of linearity across the small space through which the coil moves.

The fields generated by the coil aren't exactly linear, either.

Yes.

The voltages across the two plates are naturally linear across the space between the plates.

Weight of motor Insulated coil of wire held together with glue. Zero: clinging electrons are moved through electrostatic force.
Linear motor force? No, varies with position of coil, linearity of coil and variations in magnetic field across the gap. Yes. Perfectly linear push-pull action across the entire gap and across the entire diaphragm.
Motor force applied to Coil glued to center of cone or dome. Applied evenly across entire radiating surface.
Radiating surface Cone or dome, usually paper or heavy, stiff plastic. Vanishingly thin film.
Weight of radiating surface

Heavy, has to support itself and the coil and all the force from the coil glued to it at just one point.

Typical speaker cone weights measured in ounces.

Virtually weightless: weighs less than the air around it. All parts of the film are driven equally to make sound, so no one part has to do much of anything by itself.

Typical speaker diaphragm weights measured in milligrams, headphone diaphragms in micrograms.

Does radiating surface have any resonances?

Many. Tap a cone, and it has a sound from waves bouncing around inside the cone from center to edge, all through the audio band.

At low frequencies, the interaction between the springy suspension and the air in a box adds its own low-frequency resonances, which were first well described by Neville Thiele and Richard Small. Speaker and headphone designers spend a lot of time working around these resonances.

None.

The film is microscopically thin and has nothing to resonate. Even if it did, the air around it would damp it, as the air is much heavier than the film.

Does radiating surface always move as one?

No. Numerous breakup modes occur at different frequencies. Cone or dome motion only approximates piston motion at the lowest frequencies.

Yes.
Acoustic energy storage (leads to short-term echoes, time smears and resonances that muddy the sound.) Many: kinetic energy in the piston motion of the coil, spider and cone motion, and vibrations inside the cone and dome materials.
None.
Cabinet problems

When drivers are mounted in boxes, half the sound goes from the back of the driver into the box. It then bounces around inside the box, and some of it comes back out through the cabinet, or even reflects back out through the driver itself.

Even with all the distortion inherent in dynamic drivers, mounting them in boxes colors and degrades the sound even more.

None: no cabinet.

Film floats free between perforated plates.

Crossovers?

Yes, if woofers and tweeters are used.

Crossovers add their own coloration, frequency, phase and group delay problems.

No.

 

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