I've been learning about modes, and one thing I was interested in at first was to change modes while improvising (to get to know them better technically as well as to have a better understanding of what their flavour is). For instance, I improvised in minor pentatonic, and used its ambiguity with the relative minor modes to try to explore their differences. This lead me to question how to properly change modes in a convincing way, and even how to properly change key.
I don't know anything about the theory of modulation, but I wanted to try and experiment to figure out how it could be done.
I first picked two keys at random : E aeolian and G# aeolian.
I basically did what I think is a common-chord modulation. My idea was to pick the common notes between the two keys, and use them to transition from one key to the other. Here, the common notes are B E F#, which are a Bsus4 triad, so I used that chord to transition between the two keys. Going toE aeolian I let Bsus4 resolve to Bm, and going toG# aeolian I let Bsus4 resolve to B.
I used this very simple progression (on a loop) and improvised in E minor pentatonic/E aeolian, going to G# minor pentatonic/G# aeolian and back, to keep it as simple as I could.
Em Am | Em Am | Bsus4 B | G#m C#m | G#m C#m | Bsus4 Bm
My observation is that I managed to change key in a way that sounded pretty good (at least to me) when the change was from E to G#, but not the other way around, I found it much more difficult to sound even decent.
I thought it could be because Bm triad is B D F# and F# is not in Em pentatonic, while the B triad is B D# F# which are all in G# pentatonic. So I changed F# to the closest note in Em pentatonic, G, which means I'm left with a G triad. So I tried this :
Em Am | Em Am | Bsus4 B | G#m C#m | G#m C#m | Bsus4 G
But the chord themselves didn't sound that good (even if I tried to play G in first inversion to keep B at the bass, to have some sense of resolution for the suspended chord), and when I tried to improvise the problem remained.
I thought it could be because of the resolution sus4 -> minor which isn't as appealing as sus4 -> major, so I tried this progression :
Em Am | Em Am | Bsus4 B | G#m C#m | G#m C#m | Bsus2 Bm
Bsus2 isn't part of both keys, but at least the resolution Bsus2 -> Bm sounds better. But alas, improvising a melody that sounded good when going from G# aeolian to E aeolian was still a challenge.
Coming back to my initial progression
Em Am | Em Am | Bsus4 B | G#m C#m | G#m C#m | Bsus4 Bm
I don't get how this doesn't work. I thought it could also be that B doesn't have the same function in both keys, but it happens to be that B is V in E aeolian and III in G# aeolian. Functionnally it should work, even if I'm only mostly going from Em penta to G# penta and back, shouldn't it ? Even if that's the issue, I've tried to change B to G, which is III in E aeolian(see first point above) but it doesn't work !
Since I've never tried modulating while improvising before, I doubt it could be my improvising itself that is the issue. If it is, why is it so easy for me to change from E to G# but not the other way around ? And even if it's just my ear that is not accustomed to this change, why is there a difference really ?
I'm also thinking that maybe from E to G# your going up a major third while from G# to E you're going up a sixth, but I'm not sure I understand why it could be that that makes that much difference. Obviously E and G# are as far away from each other as G# and E are in the circle of fifths.
Is it maybe because of the relative functions of the common notes in both keys ? In E aeolian, E F# B are I, II, VI, and in G# aeolian they are II, VI, VII. I'm not sure were to go from there if that's the reason.
TL;DR : Using a common-chord modulation from E aeolian to G# aeolian and improvising, going from E to G# sounds good pretty easily, but not the other way around. Why is that ? I'm using the same method to come up with the chords, and I've never improvised in such a modulation context before, so there is no apparent reason why one change should be easier than the other.
