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Voices can be "blended" or be independent. Independent voices are very distinct, the listener can hear each voice separately. On the other hand, "blended" voices are those which sound like a single voice, the listener can't tell apart instruments which's voices are blended. An example of "blended" voices are parallel perfect consonances(perfect 5th octave, unison), direct 5ths and such.

If one wants to write voices that are very independent, then they should follow the rules of counterpoint.

Through out my journey through music I noticed that melodies that have very big difference in pitch (like more than 1.5 octaves apart) don't interact normally with each other: they don't blend very much even if played in parallel perfect consonances. Also intervals with such kind of huge pitch differences don't seem to be very dissonant and disturbing, for example a major second is very dissonant and disturbing in close position, but a major second plus 2 octaves is much less dissonant and less disturbing.

Is true that voices and intervals with huge pitch differences interact less with each other: they blend less, are less disturbing and such? Can many counterpoint and harmony rules be omitted while writing voices with huge pitch differences?

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    Yes, it's true. Take a simple major seventh chord - CEGB. Play the B and C next to each other, and it doesn't sound too good. Play them almost an octave apart, and it's fine. To do with the frequency clashes. Check out beats.
    – Tim
    May 3 at 15:39
  • I wonder if people with perfect pitch find it harder to hear two separated melodies as independent (since I've heard they can have trouble telling the difference between octaves) May 4 at 17:41

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You are correct, more or less. One thing that needs to be said: Dissonance does in its root mean "something that sounds apart", while consonance means "something that sounds together. This means that in fact the defining characteristic of dissonance is not blending together, but to be recognizable as different sounds.

Thus we see that we can define two different ideas of dissonance: Dissonance in the classic way by low harmonic congruence and dissonance by tonal distance.

The basic principle behind this is more or less the same: If two notes are close together and show high harmonic congruence the brain is not easily able to tell them apart. If on the other hand the harmonics of the two notes do not match at all the brain has little problem differentiating them, especially if they are apart by a wide tonal range.

Of course all of this is not precise, so if two notes are sufficiently close to being very congruent they will sound consonant with potentially inducing a beat.

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The point about closer voices blending more readily together is certainly true. The one about dissonances I wouldn't really agree with, unless we're talking extreme differences (four octaves or more apart).

It is true that some dissonances become already less pronounced when occuring in voices a bit apart (1-2 octaves) than if they're close to each other, but this is not universal. The main counterexample is the perfect fourth, which if you play it as such sounds just perfectly good. But put one or two octaves in between, and it rubs much more, a phenomenon that has led to the (IMO wrong) classification of the fourth as a dissonant interval in general.

Here's a table of which intervals are more consonant in close vs wide positions. Of course this is only a rough overview; actually it always depends a lot on context how different intervals are perceived.

Interval close wide
minor 2nd very dissonant dissonant
major 2nd dissonant consonant
minor 3rd consonant reasonably consonant
major 3rd consonant very consonant
perfect 4th consonant somewhat dissonant
tritone usually dissonant dissonant
perfect 5th very consonant very consonant
minor sixth consonant slightly dissonant
major sixth consonant quite consonant
minor seventh somewhat dissonant somewhat dissonant
major seventh somewhat dissonant quite consonant
octave very consonant very consonant

So, your point holds true in the sense that some severe dissonances become much less jarring in wide spacing, but the tritone sounds also dissonant in wide spacing (it depends much more on context than spacing), and minor thirds and sixths also tend to work worse in wide spacing.

Why is this? Well, it's down to physics, to how overtones align. A major second (in Pythagorean tuning) has a frequency ratio of 8:9, both pretty high numbers so it's hard to latch onto. But a major ninth has a frequency ratio of 4:9, which is much clearer. Similar story for the major seventh (close 8:15, wide 4:15) and to some degree also for the major third (close 4:5, wide 2:5) and perfect fifth (close 2:3, wide 1:3), but these are already clearly consonant even in close position.
But for the fourth (close 3:4, wide 3:8) and minor sixth (close 5:8, wide 5:16) it's the opposite way around: the wide voicing has higher numbers in the frequency ratio.

For the tritone and minor 2nd and sometimes minor 7th, the frequency ratio is ambiguous, so for them context is more important than spacing.

Now, when you go to very large spacings, all of this becomes less important again, in particular with instruments of a relatively mellow timbre (which have less overtone content relative to the fundamental) or with strong inharmonicity, because there you simply don't have much of any high-numbered overtones that could cause dissonances to obviously clash. So e.g. tuba and flute can play together in very strange harmony without it sounding obviously dissonant, but it will still sound strange simply because the voices are so disconnected.

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  • "The point about closer voices blending more readily together is certainly true": For most voices and instruments, C2 blends better with E3 than with E2.
    – phoog
    May 4 at 10:34
  • @phoog I disagree. The wide voicing will often harmonize better, yes (as per the effects I wrote), but even a major third sounds more homogeneous in close voicing (except with some unusual timbre combinations). May 4 at 11:05
  • I suppose that depends on your definition of "blend," but it's certainly harder to make C2 and E2 sound good together than C2 and E3. Your comment is the first appearance of the concept of homogeneity, which is a different matter.
    – phoog
    May 4 at 11:28
  • Yes, it does depend on the definition of “blend”, which shouldn't have much to do with “sound good” IMO. Blending is all about making a combination of voices sound as a cohesive unit. You can have good blending that sounds good (extreme: barbershop quartet), and good blending that sounds bad (narrow cluster on strings), and bad blending that sounds good (flute+harp) and bad blending that sounds bad (kazoo+lute). May 4 at 14:40
  • A road surface of loose gravel is more homogeneous than one of gravel mixed with asphalt. Which is blended?
    – phoog
    May 4 at 14:57
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One slight obstacle to a straightforward answer is that the word 'dissonant' can have different meanings in different contexts. It may simplify the discussion to put that term on one side for a moment.

When two notes are in the same frequency range and display a high degree of harmonic congruence (to borrow Lazy's terminology) - possibly because there is a simple ratio between the fundamental frequencies - the brain will 'try to hear' the combination of notes as a single sound, giving a characteristic smooth and harmonious effect.

When two notes are in the same frequency range but there is a low degree of harmonic congruence, then it's more likely that some of the harmonics will clash and cause beat frequencies, giving a characteristic sense of 'roughness'.

When notes are further apart, then both these phenomena are diminished. It is easier for the ear to distinguish notes far apart in pitch, even if they are (say) an exact number of octaves apart, as the strongest harmonics of each note no longer overlap; and the beating phenomenon that causes roughness doesn't occur to the same degree, as more of the strongest harmonics of each note will be further apart than the critical band.

Simplifying massively, and focusing on the 'momentary sensation' aspect of the meanings of the words 'consonant' and 'dissonant', perhaps you could say

  • notes close in pitch but harmonically congruent: subjective effect is positively consonant
  • notes close in pitch but not harmonically congruent: positively dissonant
  • notes far apart in pitch: independent

(though it's important to reiterate that this is a simplification, and it does remain possible to discuss consonance and dissonance of notes distant in pitch from each other when we are considering other aspects of consonance and dissonance, such as expectation of resolution)

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"If one wants to write voices that are very independent, then they should follow the rules of counterpoint."

Well, sort of. You continue to obsess on the 'rules' of 17th century vocal part-writing. There were other styles even then, and there are MANY other styles now.

You seem to have made some observations about how large intervals sound and behave. Good. Use them while writing your music. Yes, the perceived harmonic interaction between, say, tuba and piccolo is quite different than that between soprano and alto (human) voices. Partly because of the distance, partly because of of the very different tone colours.

(Funny how tuba/piccolo duets have almost become a cliché. An urge towards ultimate novelty has led many people to the same destination.)

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