Sometimes, singing in tight vocal harmony, certain chords locked just right will produce a hair-raising effect: the air buzzes, the sound gets "fuller," goosebumps, psychosomatic tears. I think it comes from overtone overlaps? Sharing rabbit hole and questions so far:
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Combination tones. Hindemith wrote about them in "Craft of Musical Composition."
If I sing frequency A while you sing lower frequency B, then the first-order combination tone produced is frequency (A-B).
Perfect 5th reinforces the bottom note, P4th the top (but octaves down).
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Isn’t another name for that “beat frequency” ? Or are you talking about something different.
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if you've never tried loudly singing into somebody else's mouth, and them singing back, starting at the same pitch, then exploring gently nudging the pitch up/down to get tighter and looser beat frequencies right in your jaw...
...you're uhhh in for a treat
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Oh gosh I haven't tried this but it sounds amazing