So what's your theory?
I worked in Haiti right after the big quake in 2010, and when I came back I developed a...more active interest...in the idea of collapsing civilizations. I had a chance to spend a bunch of time down in the southwest going around in the old Chaco / Anasazi areas and studying thinking about collapse dynamics etc. I visited a few of the main sites in the NP system, but also saw a lot of petroglyph and pictograph sites as well. A lot of the sites where the actual art (rather than dwellings, towns) is located are very, very interesting places and it's a totally different thing.
Somewhere else here recently I posted some notes about my experiences taking drugs...back when I was playing music and taking drugs, I once took LSD and spent the whole trip setting up acoustic phenomena within my house. So I'd have various speakers and noisemaking things in various rooms, and I just had everything going and walking around and hearing all the different ways that things bounced around and merged and finding all these extremely interesting tiny details of how sound traveled and interacted. There are a lot of interesting phenomena that are normally unconsciously processed, like how you get a sense of a "space" from how things sound in a room, but have you ever tried to deliberately play around with influencing that process?
Somewhere during those years down in the southwest, I ran into this podcast about Steven J Waller's archaeo-acoustic phenomena at ancient art walls, and I completely buy into that idea. He's gone around documenting how the art was specifically put in places that have extremely rare auditory phenomena.
Speaking of crackpot theories, you may dig his theory on Stonehenge which is, in his claim, a mapping of acoustic interference nodes. If you put two flutes playing in a particular fundamental set of positions, you happen to get nodes exactly where they put the stones.
OK...what does all this have to do with harmony? I didn't have a clue until a couple of months ago. I was watching a YouTube video about chords and the pianist dropped this idea that when you are in a group that is performing, singing in harmony, you actually hear the overtone being sung by no actual singer. Like, there's a voice that appears. (Incidentally, this was a kind of a-ha moment all on its own about why certain religions use harmonic music to "manifest the divine" -- I mean, there's a scientific explanation, but it's a tidy idea that there's a "presence" from something you can't see.)
So the important detail is that the harmony parts are the same pitches as the overtones.
I started really thinking about all these things...and you get interesting stuff like overtone singing and, in eastern traditions, you get the two harmonic notes being oscillated between (but not struck simultaneously).
But getting back to acoustics, it's interesting that church music occurs in big, echoey live room spaces. If you put eastern music in an echoey space, you actually hear the harmony together. If you're doing a monophonic chant, you have these points in the chant where you're producing sounds that are heard on top of the echoes, and you get the harmony being produced, in part by the acoustic chamber effects.
So...that became a bit of a starting point...and low and behold, I'm not the first person to think about it this way.
Here's a fascinating paper: http://artship.org/inquiry_paper_cave_echoes.html
Even cave art documented these acoustic phenomena, and some people think that ancient cave paintings are written polyphonic musical notation. The whole paper is a run-down on polyphonic harmony going back into pre-history and ancient and even archaic times, and acoustic phenomena are like an unrecognized part of the story.
Mind blown! :)