User:Grady/Harmonic similarity

This page serves to document a personal theory of mine that attempts to serve as a generalization of octave equivalence, as well as the various implications of the theory. I've seen other people express similar ideas, but I'm not sure if the concept in this exact form has been articulated before. If anyone knows if it has, I'd love to know more!

Quick aside: Naming

I wasn't exactly sure what term to use for this concept, but I settled on harmonic similarity for now, at least for the purpose of deciding what to name this page. Some other terms I've considered are harmonic relatedness, harmonic affinity, and harmonic correlation. I'm sure all of these terms have been used before, and some of them have certainly been used to express a similar concept as I'm putting forth here. If you have any opinions on what the most suitable name would be, I'd be glad to hear them!

Motivation

Harmonic similarity is a measure of how similar in harmonic function two pitches are. This is in contrast to consonance or concordance, which is generally defined as a measure of how harmonious or stable two or more pitches sound when played together, or perhaps less commonly a measure of how much timbral fusion occurs when playing them together.

Often, harmonic similarity between two notes correlates very strongly with perceived consonance: for example, two notes an octave apart are both highly consonant with each other and highly similar to each other. However, this is not always the case. For instance, two notes a major second apart (an interval of 9/8) are more harmonically similar to each other than two notes a major third apart (an interval of 5/4), but typically less consonant when played simultaneously. I'll be elaborating more on what I mean by this, but it aligns with how Western music theory would approach the question: two notes a major second apart are more closely related via the circle of fifths.

Definition

It should be intuitive that any equality or equivalence relation follows the transitive property. For example, if x = y and y = z, then x = z. Or in geometry, if A is congruent to B and B is congruent to C, then A is congruent to C. For a more practical example, if Alice's car is the same model as Bob's car, and Bob's car is the same model as Charlie's car, then Alice and Charlie also have the same model of car.

An extension of this is that any similarity or relatedness relation sort of follows a pseudo-transitive property. For example, if x ≈ y and y ≈ z, then you can probably say that x ≈ z, but the similarity in quantity between x and z might be less strong than that between x and y, or y and z. To extend the car example, if Alice's car is a similar color to Bob's car, and Bob's car is a similar color to Charlie's car, then Alice's car is probably a similar color to Charlie's car as well, but it depends on what your threshold is for defining two colors as "similar".

(Note that despite the choice of word, the concept of "similarity" in geometry is actually an equivalence relation by this definition, not a similarity relation: it means both objects have the exact same shape, not that they're close in shape.)

It follows from this that if we want to define a notion of two auditory pitches being similar or related in their harmonic function, said notion should also follow this pseudo-transitive property as well. This is the core principle that differentiates the notion of harmonic similarity from that of consonance, since it doesn't necessarily follow that if note X and note Y are consonant together, and note Y and note Z are consonant together, that notes X and Z ought to be consonant together to some degree.

Base similarity relations

Of course, the knowledge that harmonic similarity should follow this pseudo-transitive property isn't very helpful in determining which notes are harmonically similar without establishing some basic similarity relationships first. Essentially, two pitches have a basic similarity relation if the frequency of one is a low integer multiple of the other. The reason for this might be that we're highly accustomed to hearing harmonic timbres, so usually when we hear a note, we also somewhat prominently hear low integer multiples of that frequency as well, and over time this causes our brains to associate those higher harmonics as "similar" to the fundamental. The lower the integer multiple, the more prominently we hear it in those harmonic timbres, which corresponds to the fact that lower integer multiples represent stronger harmonic similarity relationships.

With this knowledge, we can use the pseudo-transitive property to discover higher-order harmonic similarity relationships. For example, two notes a perfect fifth apart (an interval of 3/2) aren't perceived as harmonically similar because the upper note is present in the harmonics of the lower note (which it isn't, assuming a typical harmonic spectrum). Rather, they're similar because two notes an octave apart (an interval of 2/1) are similar for that reason, and so are two notes a tritave, or perfect twelfth, apart (an interval of 3/1), and thus by the pseudo-transitive property, two notes a perfect fifth apart must be similar as well. For example, the note C4 is related to G4 (the note a perfect fifth above it) because both of those notes are related to G5 (or to C3).

Something to note about the base similarity relations is that the falloff in similarity with increasing integer values seems to be extremely fast. It's not exactly clear why this might be the case, but it's one of the core underlying assumptions of the theory. For example, the octave is a much, much stronger similarity relation than the tritave to most listeners, hence the notion of octave equivalence. A seeming caveat of this is that most listeners would rate the double octave (an interval of 4/1) to be a stronger similarity relation than the tritave. However, this is only because the double octave can be decomposed into two octaves, meaning this strong similarity can be explained with the pseudo-transitive property. In other words, the note C4 is very strongly related to the note C5, and C5 is very strongly related to C6, therefore C4 and C6 are also very strongly related. However, C4 is only moderately related to G5 (the note a tritave above it), and the tritave can't be decomposed into any simpler relationships.

Room for error

Since our ears are imperfect (and perhaps even because the overtones we hear that may have trained our mental map of harmonic similarity aren't perfect integer harmonics either), it makes sense to assign some margin for error to the notion of harmonic similarity by adding the assertion that two notes are harmonically similar if they're very close in pitch. This allows us to treat two notes that are an interval such as a perfect fifth apart in a tempered system like 12edo to be harmonically similar, even if the ratios are inexact.