They are solid state (SSTC) and the switching is usually done with isolated gate bipolar transistors (IGBTs). Basically you are turning it on and off real fast.
There are good designs (not true TCs) that make no spark noise at all and can reproduce music with high fidelity from about 100Hz-400kHz! The emitter is essentially a big plasma flame. I saw a demo in Hamilton (Bermuda) several years ago playing Moussorgsky's Pictures At An Exhibition and the tonal control and velocity was incredible! It was like the orchestra was floating 500 feet in the air and you could reach out and touch it! Supposedly it was a scaled down (!) demo using "only 5 megawatts of power".
I'm not sure what the breakdown was in power nor what kind of bass array was used because there was low registers (16Hz organ pedal notes) very clean and loud and I cannot fathom a flame producing those (although the fire was 60 feet tall). They were not answering my technical questions.
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