Why do some perfect 4ths sound 'right' , and others sound quite dissonant (in the lower ranges on the guitar specifically).

I forget the exact term, but basically intervals will start to sound muddy and unclear at lower pitches. For fifths, this is apparent at the Bb below the low E on a guitar, so I never tune a 7 string or a baritone guitar lower than that. For fourths, this seems to happen at the lowest F on a guitar (a fifth above that low Bb).

This happens for all intervals (even octaves). It's helpful to go to an 88 key piano and figure out which intervals start going muddy first (it's the same on basically any instrument).

This is also likely the reason bassists only play single notes besides some occasional chord stuff higher up the neck (think Cliff Burton).

/r/musictheory Thread