Electrostatic Speakers

Electrostatic Principle

The electrostatic principle has inherent advantages that make the construction of loudspeakers with lower coloration, better transient response, and lower non-linearity distortion possible. Also, the radiation characteristics are more suitably related to room acoustics than other techniques. The diaphragm is an electrostatic loudspeaker that can be so light and flexible that the electric forces act directly on the air load to create sound waves. The effects of complex vibrational modes in the diaphragm can be almost eliminated. This makes the acoustic performance theoretically predictable with good accuracy and relatively simpler.

The History of Electrostatic Speakers

A few not-very-successful electrostatic loudspeakers emerged in the 1920s and 1930s. However, it was not until the 1950s that the virtue of the constant-charge polarization was appreciated. This, together with the availability of sheet plastic materials with the right characteristics for diaphragms, made satisfactory full-frequency-range electrostatic loudspeakers feasible. The British Quad loudspeaker, marketed in 1957, was the first commercial full-frequency-range loudspeaker design to appear. This was the outcome of pioneering work by P.J. Walker and D.T.N. Williamson.

Technical Issues

The theoretical principles underlying the operation of the basic electrostatic loudspeaker are essentially quite simple. However, many surprisingly difficult technical problems must be overcome before a thoroughly reliable production design can be achieved. Very high voltages and single-circuit impedances are involved. The diaphragm material must have carefully controlled mechanical and electrical characteristics that are not required in any other application.

The Advantages

Electrostatic loudspeakers have advantages, including distortion levels one to two orders of magnitude lower than conventional cone drivers in a box. The highly lightweight diaphragm has an exemplary frequency response in both amplitude and phase. This is because the principle of generating force and pressure is almost free from resonances, unlike the more common electrodynamic driver.

Disadvantages

Typical disadvantages include sensitivity to ambient humidity levels and a lack of bass response. This is due to phase cancellation from a lack of enclosure, but not all designs have this. The bass roll-off 3db point occurs when the narrowest panel dimension equals a quarter wavelength of the radiated frequency for dipole radiators. So, the bass roll-off for a Quad ESL-63, which is 0.66 meters wide, occurs at around 129 Hz, comparable to many box speakers. There is also the difficult physical challenge of reproducing low frequencies with a vibrating taut film with little excursion amplitude; however, as most diaphragms have a very large surface area compared to cone drivers, small amplitude excursions put out relatively large amounts of energy.