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Electrostatic Loudspeakers (ESLs) work on the principle of static electricity generating an attractive or repellent force - like when you rub a balloon on your hair and the hair is attracted to the balloon. Two parallel plates have a thin diaphragm suspended between them. A charge is placed upon the diaphragm and the audio signal is placed on the plates. The diaphragm is attracted to the front or back plate depending on the polarity of the audio signal at that moment. The movement of the diaphragm creates the sound.
Compared to the electromagnetic force that is generated in a traditional speaker, the electrostatic force is very weak. For this reason, the surface area of the ESL tends to be large, the diaphragm must be very light, and the voltages involved must be high, much higher than traditional speakers.
Transformers are used to step up the audio signal 70 or more times in voltage, and the charge on the diaphragm is created by a power supply generating 2500 to 5000 volts DC. If this conjures up images of Dr Frankenstein and huge electrical discharges flying through the room, it is not that bad. We only are concerned with charge, not electrical current. The currents involved are very low. The diaphragm has a resistance in the Gigohm range and it can take many minutes for the charge to dissipate once the power supply is turned off.
The Projects section on this site has a discussion on the complete Electrostatic Speaker if you are interested in more information on the subject.
Follow along to discover how you can make one of these high voltage supplies.
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