Multiphonics with voice and vowels
This playing technique is performed mostly the same way as regular multiphonics, but in addition to playing and singing different tones, the performer changes the oral cavity with different vowels. The timbres produced are polyphonic, but because of the vowel change, the difference in timbre is the perception that becomes the focus for the listener.
Summation and difference tones are still present, but they normally creates the background when listening. Even if it is possible to sing all sorts of vowels, it is important to notice that similar vowels like a and o will sound approximately the same, and will probably not cause the desired musical effect.
This technique can be notated in two systems: the singing note in the top system, the playing note in the lower system, and the vowel in the middle.