What Is The Acoustic Scale?
I discovered the Acoustic Scale quite by accident. I was trying to write a riff for a new song. I tried to play a melody that I first thought was in Lydian Mode (when I was hearing it in my head), but the problem was one of the notes was wrong. So I moved that note down a half-step, figuring that I was just playing Lydian Mode incorrectly. Bingo - there was my melody, and it sounded great.
...Except when I wrote down the notes, I realized that the "wrong" note that now sounded "right" was outside of Lydian Mode. What I had on my hands was a melody that featured a sharp 4th (the defining feature of Lydian Mode), but it also had a flat 7th (the defining feature of Mixolydian Mode). But I knew of no mode that contained both.
A search for common scales revealed that my melody was utilizing something called "The Acoustic Scale," which I had never heard of until yesterday. This scale is also known as "Lydian Dominant" (#4 = Lydian, b7 = Dominant), and "the Bartok Scale."
What It Sounds Like
You don't really get a good sense of this scale by playing it through. It has a subtle charm to it (initially). Because it is basically a half-Lydian and half-Mixolydian cross-breed, I think the human brain has a tendency to interpret the first half of the scale as being typical Lydian, and the second half as typical Mixolydian. I think this happens psychoacoustically, so quickly that it becomes difficult to become aware of the Acoustic Scale as its own distinct tonality.
One starts to understand it when one starts to build harmonic content atop it, rather than simply expressing it modally. For a good sense of what this sounds like, start by listening to the man who first mastered it:
Once you've digested the appetizer, try the main course.
Discussion And Application
The Acoustic Scale has a really pleasant sound to it that begs for exploration. The challenge is figuring out how to build beautiful harmonic content from it. Jazz gets away with it because it only ever uses it as a passing thing. Bartok got away with it because he was specifically defying convention.
Perhaps the core challenge to utilizing the Acoustic Scale pertains to what I said in the previous section. It seems as though the brain wants to split the scale into two. We are so familiar with Lydian and Mixolydian phrases that, for any Lydian Dominant melody, the brain freaks out a little bit. If you can keep your mind on task, though, you can learn to appreciate its inherent charms.
There are some really interesting turns of phrase you can employ. My ear automatically gravitated to the v --> v/sus2 --> II transition. This sounds a little bit like IV-iv-I with a twist. A newcomer to the mode will want to gravitate toward the v and the vi along with the I and the II, but the problem with that is that you run into a problem of variety. You can come up with a good melody that spans the I and the II; you can come up with a good melody that spans the v and the vi... but when you put all 4 chords together, there only seem to be a few that "work."
The VII+ is a nice addition, but in the context of guitar music, the tendency to play it as a 1/5+ interval screws with the tonality of the overall composition.
But, in a rock context at least, it's common to waver back and forth between two key signatures. If you lean heavily on the harmonic minor scale of the v then you get some interesting changes. Lean on it too much, and you get sucked into that tonality, but throwing in the occasional G major when you're playing in E makes for some good times.
I have yet to finish the job of exploring this mode, but I already love the breath of fresh air it brings to the composition process. I highly recommend exploring this beautiful scale and finding your own approach to it. I hope to revisit this topic again soon, with an audio example of my own.