Re: Well mixing should not occur
"If there are two pure sound sources in air at two different frequencies - then the "beat" frequencies will also be heard if they are within the audible range. That's the basis of polyphonic singing."
Well but that's strictly not "mixing". What you have here is your ear/brain-system interpreting 2 tones at close distances as one tone which is changing in intensity in a certain way. That's simply because that's essentially the same thing.
The ear/brain-system interprets the signal in certain ways. It's a bit like that triangle pointing downwards in this picture:
You could argue that it's objectively not there. You could however also argue that it is there, because all 3 corners are plainly visible.
The same can be said about beat frequencies. Yes, they objectively exist if you look at the envelope of the resulting signal, but no, if you look at the spectrum it does not exist.