ELECTROSTATIC PHONES
Another type of headphone needs a special mention at this juncture—those using the capacitive or electrostatic principle.
A very thin, partially conductive plastic membrane, is supported just clear of a metallic surface being, at the sametime, attracted towards it by an electrostatic charge. An audio signal voltage, applied to either element, adds to or subtracts from the charge, causing the membrane to vibrate and produce sound waves.
This obviously parallels the earlier explanation of a magnetic earphone (Fig. 3.8) but with the vital difference that a plastic membrane can have much less mass and stiffness than a metal diaphragm, thereby minimising problems with system resonance and uneven response over the working range. ...
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