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The problem with the above explanation is that it starts at a lower level of abstraction than is warranted. Analogously, it explains MOSFET's rather than algorithms. The fundamental unit of Music Theory is the Interval (as in "frequency interval between notes"). A better explanation would have explained Intervals, how particular combinations of Intervals form various Scales, and how Scales influence Chord Progressions and Key Signatures. There are 12 unique notes in an Octave (yes, octave means 8; ignore this for now). On the 13th note, the octave repeats itself (counting is 1-indexed). But of these 12 notes, only specific subsets are combined into specific scales. The most popular scale is the Major Scale which follows the pattern "Tonic Major_2nd Major_3rd Perfect_4th Perfect_5th Major_6th Major_7th" (there exist other notations). The Major Scale imparts a generically-happy mood. But there's other scales, including: the Chromatic Scale; the Major Scale; 3 varieties of Minor Scales; 7 varieties of Diatonic Modes (including Major and Natural Minor); the Pentatonic Scales; etc. E.g. Smoke on the Water, Sunshine of your Love, and Fight for your Right (to Party) all sound similar because they use the Blues Scale. Once you familiarize yourself with the intervals, scales, and their various notations (which is its own feat), you can jump into the meat & potatoes of Music Theory. Which mostly consists of analyzing Chord Progressions and Key Signatures in order to find commonalities in mood. Chord Progressions and Key Signatures determine a song's general mood. It's too complicated for me to summarize quickly. But it's where all the interesting stuff happens. N.B. I've deliberately omitted various details for the sake of brevity. E.g. I could have mentioned Time Signatures, Rhythms, Tempo, Dynamics, etc. |
Guess what... A perfect fifth is a 3:2 ratio/interval of frequencies (+/- a small allowance for the historical/mechanical constraints that lead to well/equal tempering unless we're talking modern micro-tonalism in which case no adjustments are necessary). The inverse, 2:3, is a perfect fourth.
The example frequencies I gave correspond to concert-A, then E and D. I took a very simple/short path from intervals to harmonic function/progression to try to give a taste of the meat and potatoes before providing a link to additional sources.
I avoided music theory terms because that would've been a circular definition for any readers who had no understanding of even the basics.
In light of your post, I should've added a sentence or two about mood (major = happy, diminished = tense/scary) and compared that to how you can make a 'warm' picture using reds, oranges and yellows or a sombre picture with darker colors, etc.