[simne]

Amateurs in USSR 50 years ago made wireless and powerless headphones, which use wire lay on perimeter of room to transfer sound and power.

In headphones there is tiny coil.

It really work and very reliable, but result coil (size of room) have very large reactive resistance, so it is nearly impossible to transfer even high frequencies, only low (bass) and medium, so it workable for speech but music is heavily distorted.

[saidnooneever]

they also made microphones work like this. place em behind the panels of sockets on the wires. some accounts of such devices being found can be found in the book Spy Catchers (by some ex MI5 science officer during cold war). pretty interesting tech!

[anamexis]

Could you use multiple coils for different bands?

[simne]

You could transfer different bands via different coils on different frequencies, but unfortunately, capacity of information channel is limited by frequency. Because of this, radio using high frequency waves as carrier (radio or light, or even some sort of invisible rays), not coil, and have hassle with some sort of modulation of waves.

This mean, you could not transfer more information than half of maximum frequency.

https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Very good channels with very high signal to noise ratio, could handle more bits than Shannon limit (on engineers slang "channel is ringing", such example is fiber channel).

https://en.wikipedia.org/wiki/Shannon%E2%80%93Hartley_theore...

Most modern research also consider some digital techniques of sound (information) compressing, like use LLM as (de)compressor (google llm compression algorithms).