This is for music people. If you know your intervals, you’d know that a perfect fifth is fairly open and most closely resembles an octave in its openess. I know that for an octave, the upper note is twice the frequency of the lower, and thus the two waves are in sync with each other and produce an “open” sound (open as in free of “beats” that arise from say quarter tones or in a minor 2nd interval). Is the upper note in a perfect fifth one and a half times the frequency of the lower? I guess also what I am saying is why is an open fifth so devoid of tonality (as opposed to a major third or minor third)? I realize that major and minor sixths are just upper inversions of thirds, which would explain each of those’s tonalities (not that I understand the tonality of thirds), but why does a fourth have a distinctive sound, while the fifth is so open. Hopefully I’ve made myself clear enough.
thanks for the quick and accurate answer 
looks like my gross speculation was correct
I know very little about music, but I would guess that the next most euphonic interval would be either the fourth (4:3) or the twelfth (3:1)? Or is it hard to tell at this point?
I’m not sure what you mean here by euphonic, however if you mean with the least “disruptance” or most “open” (as I described in my original post) you would be correct in that the fourth is similar to the fifth in that regard. I assume it is because of this lesser tonality that fourths and fifths are called perfect intervals, while seconds, thirds, sixths, and sevenths are called major (assuming these are all part of a normal major scale). A twelfth is a fourth, only an octave higher (which introduces slightly different tonalities due to the logrithmic relationship between piano notes and frequencies), which would indicate to me that it also shares similar characteristics with a fourth.
Ooops. Well, two things I need to amend then. First, when I said twelfth, I guess I meant thirteenth. I meant a fifth, only one octave higher, not a fourth. Second, when I said euphonic, I don’t know the right word for it, but it’s whatever property a sine wave has a lot of. Cleanness? Simplicity? I don’t know.
as a slight aside–
I used to play in a really loud band. After a time, I started to have hearing loss problems. I tried wearing ear plugs, but when ever I had them in I could not always properly distinguish between a root and its fifth.
I never really had a chance to bring that up until now. 
A natural overtone of any note is its fifth (well, perhaps an octave above its fifth or something like that).
Interesting.
Fascinating, in fact.
Just today, I observed this problem in two of my piano students: they, too, who are able to identify all intervals by ear (required for piano examinations at the conservatory), are struggling to tell the perfect 5th and the octave apart.
Nifty thread.
I teach them to make the difference through the use of solmization (and singing - d-s, versus d-d’), which seems to make a huge difference (thanks, Kodaly)