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I wish to understand the connection of music theory to the physics world, or to waves.

In this context, I quite understand notion of dominant chords as containing tritone, which is dissonant. What I don't understand is the stability of the chords.

I read somewhere :

the 1st, 3rd and 5th degrees of the scale are relatively stable. The 2nd and 6th degrees are somewhat unstable. The 4th and 7th degrees are very unstable

But I am missing the explanation to these claims. Is it related to the degree in the overtone scale?

Additionally, What does it mean to be stable? Does it mean you feel uncomfortable if it is not resolved to the root? Or that some other chord should follow? Or some feeling?

  • I can sort of figure out why a wave that its period is very different from another one sounds dissonant when played together and makes mess in terms of superposition. – user2679290 Sep 15 at 20:58
  • @piiperi do you always require that answers to any question on any topic include a step-by-step method for you to personally verify every fact or observation referenced? I think OP's question is fine as it is. And if you're skeptical about whether described phenomenon exist, then research them yourself... Often how to do so is self-explanatory, if you require expansion or disagree with a point then you can always say so in response. Pre-emptively making demands you never would in another field seems... strange. Almost like you're skeptical of musical theory in general. – Some_Guy Sep 15 at 20:59
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    You may want to look a book like Physics and the Sound of Music by Rigden or On the Sensations of Time by Helmholtz. There are attempts to explain certain "subjective" qualities of western music in terms of physic and make them more "objective". – ggcg Sep 15 at 22:55
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    I read somewhere that the moon is made of green cheese. I can't find any explanation of that either. And some guy called Debussy used to write harmony that sounds perfectly stable where every chord contains a tritone. I go with the "yet another bedtime story" interpretation of the question. – guest Sep 15 at 23:36
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    @guest behind this question there is a general sense of confusion over terms and concepts. That's why people come to this site - they feel confused and need clarification. They won't even be able to properly explicate their question to begin with. AFAIK the StackExchange vision is to build an information source where search engines could bring people. But how can this work when people cannot even explicate what it is they're trying to ask. – piiperi Reinstate Monica Sep 16 at 10:18
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"1st, 3rd and 5th degrees of the scale are relatively stable"

That cannot refer to chords but individual notes or pitches, when there's an established tonic pitch in the listener's mind. The major chord built on the 5th scale degree is definitely not "stable" relative to the tonic, because that's the dominant chord. If your tonic is C, in C major, even if it's only G major and not G7 (which would be the proper "dominant seventh" chord for C), it makes you want to hear a C as the next chord to make it feel like the harmony resolved.

So it's a misunderstanding. The somewhere you're quoting is talking about individual pitches when there's a tonic, which is the first scale degree. "Stability" means lack of sense of pressure to change. If you have established C as the tonic, and you play an F - the 4th scale degree, or a B - the 7th scale degree, it makes you feel tense: things are not right until you hear a C, E or G note. That's the meaning of "unstable". If you hear a G note, which is the 5th scale degree, you feel like, "that's not perhaps an ideal final solution, but it's ok, let it be at that". That's "stable".

All of this depends on there being an established tonic note.

What comes to the physics side and "consonance" and "dissonance" and all ... how do you explain that the fourth scale degree is "very unstable", when the interval between the root and the fourth is a perfect interval and a perfect consonance? http://www2.siba.fi/muste1/index.php?id=65&la=en So you "resolve" the tension caused by the fourth scale degree by moving it to the third scale degree, how come? The interval between the root and the third is said to be more dissonant than the fourth, so you resolve an instability by increasing "dissonance"? It must be slightly more complicated than a simple physical measurement between frequencies.

I guess a proper answer to this question would need to define all the terms: pitch, scale, scale degree, interval, root, chord, stability, instability, tension, resolution, dissonance, consonance, ... hopefully someone writes a better answer that sets everything straight. :)

However, I wouldn't put too much weight on these explanations, because their actual real-life utility is limited. Music theory describes feelings and phenomena occurring in practice, and the descriptions can be helpful in finding your way around when composing or playing music, or when talking and reasoning about it. But if you sit at a piano and play these notes, scales, intervals and chords, you can experience first hand how the things affect your feelings, without needing a degree on theoretical concepts.

Anyway, the title of this question is a misunderstanding. The quoted text is not talking about stability of chords.

Here is some further reading about stability from this site:

I'm still not sure how you could utilize these "why" explanations. Would any of them affect your music making? Maybe they can help you to see patterns and relate things to them.

  • Isn't that "instability" defined by the ratio of the frequency of a given note to the frequency of the root? After all, OP wanted some physics to be thrown in. – Pyromonk Sep 16 at 3:29
  • I guess you are right. Although there is no much difference presumably between the triad root chord and the root alone, as the note raises additional harmonics. – user2679290 Sep 16 at 7:05

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