|Acoustics of the clarinet
Notes are the written pitch.
Waves travelling down the bore are partially reflected at the first two open holes, which are respectively responsible for the first two notes in the multiphonic. Reflection at the next open hole would normally be expected to give a note of C#6, but in the cases of the higher notes the complicated cross fingering, and the influence of what are effectively register holes complicate the tuning.
Sound spectrum of a Bb clarinet played using fingering for the multiphonic F#4, G#5, D6 and G6.
For more explanation, see Introduction to clarinet acoustics
You can hear the multiphonic F#4, G#5, D6 and G6 played.