Electret Driver Principle
Electret Driver Principle
To put it simply, electret headphones are electrostatic headphones. More specifically, they're a type of electrostatic headphone. The only difference is in the method by which a fixed charge is placed on the diaphragm. An electret foil or film is a thin polymer sheet that has been given, and which will retain, an electrostatic charge of a few hundred volts. It would be as if the hard-rubber comb you used to do simple static-electricity experiments at home (picking up tiny bits of paper, making a small stream of water bend) could stay charged indefinitely.
The only mechanical difference between an electrostatic headphone and an electret electrostatic headphone is the material the diaphragm's made of and whether or not there's an electrical connection to it from the outside world, namely from a bias-voltage supply.
From an engineering standpoint, the optimum diaphragm material is a polyester called boPET, biaxially-oriented polyethylene terephthalate film, aka Mylar or Hostaphan or Melinex. Problem is, boPET doesn't make a very good electret. So an engineer must either accept a compromise in the sound, or he can re-engineer the typical electrostatic headphone layout by metallizing a boPET diaphragm and applying the audio signal to it and relegating the electret material to the stators where its inferior mechanical properties won't matter. This inside-out or back-electret design was used by only two manufacturers: Toshiba (most notably the HR 910 and 810; Rotel marketed an OEM version of the 810) and Audio-Technica (the ATH-8, aka the Signet TK33).
This is not to say that a diaphragm-electret headphone need be bad sounding. The 1976 Sony ECR-500 had unique engineering that makes it a favorite among electrostat fans more than 30 years after it was designed.
Worthy of note:
1) The only practical electrostatics with the ability to plug right into the headphone jacks of typical home audio equipment without adapter boxes or auxiliary amplifiers are all electret types.
2) The newest (which is to say, the last) electret 'phones were made about 20 years ago.
3) Despite persistent rumors of imminent electret failure, there are many electret headphones, many more than 30 years old, that still function perfectly. Still, a smart prospective buyer always asks if the channel balance of the headphones he's about to buy is perfect-- not a bad question to ask about any electrostatic headphone purchase. High voltage, however it's generated, always presents an extra layer of risk to reliability.
4) All Stax electret 'phones will plug into any Stax transformer boxes or amplifiers, past or present.
Keep in mind that except for the Toshiba and Audio-Technica back-electret models, all electret headphones were designed as entry-level models. It would be fair to say that they were designed to be Electrostatic Heaphones For The Masses, thus it would be unfair to compare them directly to their much more expensive brandmates.