Click a note to start the drone. Click it while holding up/down arrow to play an octave higher/lower. All drone notes are in just intonation relative to each other.
I made this project because in Harmonic Experience, by W. A. Mathieu, there is a lot about singing over a drone. I couldn't find a drone that worked, unless you count my electric toothbrush, and I wanted the flexibility to change the drone note, which can't be done on a toothbrush. By the way, I highly recommend Harmonic Experience.