Working on Shun-San got me thinking about small-interval multiphonics (those with an interval of an augmented second or less). The first line of advice on how to produce these comes from Robert himself, and can be viewed here. His advice is fantastic, spot-on and humorful, I recommend viewing it.*
*Although I don't agree with what Robert says in regard to offset G flutes or doing sit-ups, but that's another story.
In my Flute Talk article, I touch on the subject of small-interval multiphonics. This passage has elicited some raised eyebrows and questions. To begin, I'll site the passage:
Flutists often encounter difficulty with small-interval multiphonics because they are hung up on trying to produce a focus immediately. That is difficult to do when you are blowing in two directions at once. The irony of these small-interval multiphonics is, at first, you have to unfocus to get the sense of focus. Open up the embouchure hole and let both notes in. Initially there will be a lot of air, but with practice you can refine them. They will sound focused and rich because of the very low difference tone caused by a close interval. When you get the hang of playing these small intervals, it may help to focus on producing this difference tone rather than the individual notes themselves. That may seem strange but sometimes it works.
The first point of confusion may arise in that I assume the reader is already familiar with Robert Dick's advice: get to know the dynamic range of each note first. Then, keeping a constant airspeed, use the angle of the air to find both notes. If you don't research the gamut of air speed for each note, you'll never find the small range of speed that overlaps and works for both.
This is what I meant by having to unfocus to get the sense of focus. You need a constant airspeed and a wide angle at first that will let both notes in. Please forgive my artistic crudeness, but here hopefully you can see where the angles overlap. If your focus is too narrow at first, you may miss the range where the angles overlap.
Now, to explain that bit about the low difference tone. An explanation of difference tones can be found in Wiki. Often is is not an actual, distinct tone that I hear. Rather, it is just a low sort of humming sound, or it's as if something opens acoustically at the bottom - a feeling rather than a sound.
I hope this has been of help. Some of those multiphonics in Shun-San are hair-raising! Even someone like me who has been familiar with them for years needs to put in serious practice time on them. It is a good refresher!