There are cases where a double or triple drone is useful. Double organ-points are common in the classical literature. Some drone-instruments have a double drone. A special case of drones takes a low tone and by filters or variable resonators accentuates this or that member of its harmonic series. The inverse process, of sounding faintly a high tone and playing a subharmonic series melody below it, would be well worth experimenting with.
Drones are especially valuable when someone is learning to sing in a non-twelve scale.
The timbre or harmonic constitution of a drone-tone is extremely important, since the new intervals will not be defined or clearly set off one from another unless the drone has a long series of harmonics. Preferably it should have no vibrato at all, for the purposes of this article. While you are learning to hear new intervals, you need something reliably steady to measure Then you will feel more sure of where all these strange melodies are going.