One of my favorite tips for producing woodwind multiphonics comes from J. Michael Leonard’s Extended Technique for the Saxophone. (Mine is an older edition, I think.) The book’s section on multiphonics gives two pages of instruction and and a one-page fingering chart with diagrams like this:
The “aha” moment I got from this was the small arrows, which the author says “indicate a relative primary focusing of the airstream.” To me this sounds like what I call voicing. The idea is that each of these multiphonics has a sort of key note within its chord, and if you focus/voice to favor that note, the multiphonic will speak.
Readers of this blog know I don’t like voicing gymnastics, at least for conventional playing technique. It’s better to find the optimal voicing for the instrument and keep it steady. Change it only when the acoustical quirks of the instrument demand, such as for a pitch “tendency” note or a slur that doesn’t respond well. I’m not sure if Mr. Leonard means for this fingering chart to imply that there are different focuses/voicings for different notes on the instrument, but voicing higher or lower to increase the success of certain multiphonic sounds works well for me. And, as a matter of convenience, I do use a similar arrow system to pencil in hints for multiphonics in repertoire that I play.