Why is Tonic Octave '8' in Music Theory?
We have a peculiar attribute in our hearing that makes 1 and 8 sound the same. They are not really the same but the phenomenon is psychological. It is in our heads. I think it has evolved because men and women sing and speak octaves apart yet we have to hear them as saying and singing the same. This unique perception of sound helps us to perceive the same note in different octave levels as the same, as long as it maintains the same function and relationship within a musical scale.
The Term 'Do' in Solfège
The term “do” as it's used in solfège (sounding like ‘do, re, mi’ in music) refers to a note's position and function in a tonal system. Scale degrees 1 and 8 in a diatonic scale are both called “do” because they share the same function and an analogous relationship with other degrees of the scale.
Chord Construction and Octave Notation
When what you are trying to do spans more than one octave, the higher note is an 8. For example, in jazz chord construction, 9th, 11th, and 13th chords are common. These chords contain a note more than an octave above the original root. You would count to the 8th and past it in setting up the chord.
Importance of Indicating Octave Changes
Some say it’s due to the convention of indicating octave changes. For the sake of this demonstration, D-D is an octave. When I teach music theory to vocal students often, their reference point is solfège. Therefore, when I discuss chord construction with them, intervals or chord inversions, I use some kind of system denoting the octave, such as 8, 9, etc.
If you are using only solfege and don't denote octave changes, it can leave it open for interpretation. Additionally, if someone doesn't know they have to play a do up the octave, the passage will indeed sound different. For example, D R M D 1231 implies the last do is the same as the first, which may not have been the intention.
The Tonic Note in Music Theory
Both “1” and “8” are called the tonic because they are exactly the same note. One is just an octave higher or lower. The tonic is the first note in a major scale and, therefore, is the key the music is written in. The only reason someone would call the tonic ‘8’ is to distinguish between octaves of the same key. There is otherwise no difference.
Conclusion
Why we call the tonic an “8” when the chord is presented with the tonic doubled at the octave is quite unusual in music theory. This practice helps to ensure clear communication and accurate interpretation of musical passages. Understanding the role of the tonic and its octave is crucial for any musician or music theorist, ensuring that every note is played and heard in the right context.