It probably goes without saying but a drone is a constant note used to underpin melody. You can hear the effect of adding a drone in this video, in which I play a snippet of a medieval estampie on a rudimentary homemade double whistle.
In popular usage we tend to think of drones as boring. We talk about people droning on monotonously, and if drones consisted of pure sine waves they would, I think, drive us mad. I wonder how much of this you can stand?
But drones, as used in traditional music, are harmonically complex. Compare the irksome sin wave with the rich unfolding timbres of a tampura, used in Indian classical music. No wonder it is called a river of sound.
Furthermore, however much we are taught as, say, bagpipe or hurdy-gurdy players to keep our drones steady, there are always subtle variations in timbre and pitch as the wheel turns or the bag empties and fills. In other words, drones breathe (which is why synth programmers have to add filter sweeps and other harmonic variability to pure sin waves to make them sound more 'organic').
I suspect the origin of the drone, as with all music, is the human voice. By varying the shape of the mouth and by directing sound into the various resonant cavities of the body we can make pleasingly undulating 'wah-wah' sounds that support a second melodic voice. But always there is the problem of having to inhale: the vocal drone is punctuated and interrupted, unless many voices cascade together.
So drones mimic human breathing through their rising and falling but at the same time achieve what the human voice is incapable of: continuous sound. I think this endless unfolding is what gives the drone its alluring pull and is why it is associated in many cultures with trance.
Perhaps by transcending the biological limitation of having to breathe, drones (in combination with certain repetitive rhythms and melodies) have an actual physiological effect upon us, drawing us into a state of intensity or trance. Certainly that sense of continuous unfolding mirrors the internal, unfolding sensorium of the psychedelic experience. The one seems to map the other.
Of course, there's another more prosaic reason why drones have been seized upon by psychedelic musicians from the 1960s onwards. The musical cultures with which they're typically associated - North Africa, the Middle East and India - are also those that are also typically orientalised by the West as exotic, primitive, other, and more innately concerned with spirituality. Thus in the 60s slapping a bit of sitar on your track became the easy, and rather lazy, shorthand for saying - 'hey man, we're freaks too.' In the same way, the didgeridoo served the same function in the 90s.
Nevertheless I think the very nature of drone music (and don't forget, up until the Middle Ages we had it too) invites metaphors that appeal to the psychedelic imagination. The drone provides a ground for the melody. The old word for a drone is burden. It bears the music, in both senses of carrying it and giving birth to it (quite literally, in that all the notes of the scale are present within the harmonics of the drone).
As Pythagoras is said to have discovered, some notes played against a drone form pleasing, or consonant, intervals. They do so because their frequencies form exact harmonic ratios (1:1 unison; 2:1 octave, 3:2 fifth, 4:3 fourth). Others are more dissonant and the human ear 'wants' these notes to resolve up or down to a more stable, consonant note.
To play any scale, mode, rag or maqam against a drone is therefore to negotiate a series of pushes and pulls. The intrinsic consonance and dissonance between note and drone creates narrative, endless variations of home and away, or there and back again. Or even up and down, for there is also a sense of verticality within drone music.
So, when a master bansuri player like Hariprasad Chaurasia, teasingly introducing the notes of this rag, finally arrives at the octave, it is a sublime moment. We have reached a higher place. Then he takes us higher still, before gently wafting us back down to earth again.
And that is why I think drones are so popular in psychedelic music. For if psychedelics aren't about that fundamental yearning for transcendence, what then?