I vaguely remember reading about heterodyning speakers in the mid 90s - the physics does check out, and such technology should be able to deliver perfectly flat response along the entire audible spectrum and with a tiny footprint. I guess they never managed to make it work or cheap enough or safe enough (yes, it’s also supposed to be flat at the harmful subsonic frequencies)

IIRC the idea is to have two crystals, one at a constant e.g. 100khz, and the other at (100+x)kHz for x corresponding to the sound you want. By physically connecting them, you get the sum (ultrasonic, lost energy but not a problem) and the difference - which is the sound you want - with most of the physics across half an octave so easily flat. Something along those lines.