there’s also the duofluctus by sergey filatov, which produces sound from two strings being resonated by an alternating magnetic field, created by magnets on a spinning disc, which is then amplified via two pickups for a nice stereo effect.
I was very inspired by the duofluctus when making a sound sculpture built into a pillar together with a friend and collaborator last year. It had no microphones or pickups, only the box for acoustic amplification. this is what is looked like:
here’s what it sounded like: https://vimeo.com/373890551
the speed of the spinning disc enhances different overtones. the duofluctus has a knob which changes the speed. the sculpure I made had an arduino built in which varied the speed on the motor.