|Acoustics of the clarinet|
Notes are the written pitch.
Note how similar this spectrum is to that for F4. (Of course, the clarinet overblows a 12th.) The only substantial difference is the reduction and displacement of the first peak. The purpose of the speaker key is just that: to destroy the fundamental resonance so that the instrument will not 'drop down' to the lower register. See register hole for details.
For a general discussion of the clarino register, see the description on B4.
Sound spectrum of a Bb clarinet played using fingering for C6.
For more explanation, see Introduction to clarinet acoustics
You can hear C6 played.