It is my understanding that unaccompanied singers (or players of fret less instruments) will naturally tend to sing (or play) just intervals, i.e. intervals whose frequency ratios are ratios of simple whole numbers, and that the complicated tuning systems invented throughout history are compromises made to accommodate fixed-pitch instruments that can’t adjust pitch on the fly. My first question is, is this true regardless of musical/cultural background? I understand the physical reasons why octave equivalence would be a universal thing, and why simple ratios of frequencies sound better together, but how far does physics go in explaining the notes humans use?
I have always been told that non-western music doesn’t use the same system of notes as in the western tradition; the example usually brought up are the microtones in Indian classical music. Before I ever looked into it, I assumed that they may not consider octaves to be equivalent, or that they would actually split the octave into many more notes than we do, and each one would have an important function like the usual western scales. However, it looks like the microtones are usually embellishments or elaborations on more “central” notes, and some scales in Indian classical music split the octave into seven notes that basically correspond to the western diatonic major scale.
So my main question is: Are there any cultures whose music would seem as truly foreign to me as, say, the Bohlen-Pierce scale? Or do most cultures really use simple just intervals like 3:2 (perfect fifths), 4:3 (perfect fourths), and so on, with some small variations?