The electrostatic principle has inherent advantages which make possible the construction of loudspeakers with lower coloration, better transient response, and lower non-linearity distortion. Also, the radiation characteristics are more suitably related to room acoustics compared to using other techniques. The diaphragm is an electrostatic loudspeaker 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 in a relatively simple manner.
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 become 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.
The theoretical principles underlying the operation of the basic electrostatic loudspeaker are essentially quite simple. However, there are many surprisingly difficult technical problems to 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 not required in any other application.
Advantages of electrostatic loudspeakers include levels of distortion one to two orders of magnitude lower than conventional cone drivers in a box. Also, the extremely lightweight of the diaphragm with an exemplary frequency response both in amplitude and phase. This is because the principle of generating force and pressure is almost free from resonances, unlike the more common electrodynamic driver.
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 for a Quad ESL-63, which is 0.66 meters wide, this 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 relatively large amounts of energy out.