I mentioned this technique many years ago when I wrote my post on clarinet articulations. However, I didn’t go into too many details in terms of how it works, what pitches are available, etc.
Basically, by removing the mouthpiece and the barrel, tilting the clarinet slightly and blowing across the opening, one can produce a charming set of flute-like pitches.
There are a few things to keep in mind: this does actually take some practice. The lower you go in the first register, and the higher in the second register, the harder the notes are to produce. In the low register, these tend to go very airy for me – perhaps someone with more flute-playing experience might be able to do a better job of this. It also uses a different kind of endurance: the most time I’ve ever spent on this technique has been in making this blog post, and after about half an hour of doing the technique, the right hand side of my face started to turn numb and I had to take a significant break before I could carry on. So it might not make sense to use this technique for the entirety of your piece, or your clarinetist may struggle with rehearsing it.
(Remember that everything on the blog is written in transposed pitches!)
So that you can see what this looks like when I do it, here’s a video I put on Instagram earlier:
There is definitely an easy range, and I would say that this lies between these pitches, using chromatic fingerings only:
The bottom pitches are the resulting sounds, and the top line shows the fingered pitches (everything is transposed for Bb clarinet here!). You should always show both, by the way. No clarinetist should be expected to work out what fingering to play, but if you just treat this as a kind of scordatura, and show both the fingered pitches and the resultant sounds, everything should work out splendidly.
In this range, I can get a nice strong tone here (as in the example from Instagram), and the pitches are more or less stable. Below that, it gets increasingly airy as the pitches gradually become more and more difficult to produce. So I would generally recommend sticking to this easy set of pitches.
The entirety of what’s possible I’ve split into two ranges, one for the low register, and another for a second set of pitches possible by overblowing. Here are the pitches (again, bottom set are the resulting sounds of the fingered pitches on the top line).
This sounds as follows (I’ve tried to play the line as slowly as possible, which as you can hear does affect my ability to get as clear and singing a sound as in the Instagram example):
The high register has some overlapping, but I think the sound quality is slightly different:
And sounds thus:
Some clarinetists can only achieve this technique by keeping the barrel on. I, however, can’t manage to do it at all with the barrel attached, so I’ve had to rely on another composer-clarinetist team (in this case Matthias Krüger and the excellent Gilad Harel from Meiter Ensemble) for sending me the resulting pitches of my easy reduced range – so if you’re writing a piece for someone specific, do make sure you ask if they can do this, and in what format!