How do they work? How does the cone vibrate when a signal is coming?
I have seen thousands of electret microphones but have never seen an electret earpiece.
Very high quality electrostatic speakers were made in England. Their loudness was low and they used a stepup transformer to activate the diaphragm.
Aren't those earpieces that come with crystal radio kits electret speakers, or is it just a highly wrapped coil?
Also wouldn't the bass reproduction be absolutely horrible on those speakers? If it's transformer coupled you'd lose all the low frequencies.
Using the search term "electrostatic loudspeaker" on Google, I get about 80,000 hits, the first of which is:
Electrostatic loudspeaker - Wikipedia, the free encyclopedia