This is an experimental instrument I built that uses back-driven stepper motors to synthesize sound.

The operational principle is that when stepper motors are back-driven, they induce a pseudo-sinusoidal alternating current. If this signal is amplified and connected to a speaker, you can listen to it as sound. The pitch of the sound is determined by the frequency of the wave, which depends on how fast the motor is back-driven. Higher speed will be a higher note. I thought this sound was interesting, so I wanted to make an instrument that used this mechanism to make music.