|Acoustics of the clarinet
Notes are the written pitch.
Here, the D#4 may be considered as produced by a variant of the E4 fingering, with cross fingering to make it flatter. The D#5 and B5 are produced by the second and third peaks in the impedance spectrum, which one could consider as the second and third resonances of a modified fingering for G3, with the open key acting as a register hole.
Sound spectrum of a Bb clarinet played using fingering for the multiphonic D#4, D#5 and B5.
For more explanation, see Introduction to clarinet acoustics
You can hear the multiphonic D#4, D#5 and B5 played.