Extended range clarinets and their twelfths
The low Eb key on a clarinet (or bass clarinet) equipped with one is there first and foremost to produce a low Eb. It will throw the twelfth (the Bb in the middle of the staff when played with the register or double register key), but the note that it produces is both different in character and slightly more difficult to produce than the notes immediately above (mid-staff B and C) (but see expansion of this below). For that reason, in over forty years of owning both clarinets and bass clarinets so equipped, I have only used it (in actual performance, rather than experimentation) a mere handful of times.
One (the most frequent, by the way) is when playing the oboe solo opening the "Bacchanale" movement of "Samson and Delilah". It's an "oriental-flavored" bit that is good if a situation requires a little bit of ad lib exotic-sounding music, and as such I usually end up playing it on the clarinet. (You don't want to hear me play it, or any other tune, or even a tuning A, on the oboe...)
It runs through the entire "clarinet" range of the instrument down through mid staff B, with one grace note like venture down to Bb in the horrid (in conceptual terms) throat tones. As the little deviation down to the pinch Bb is both awkward and difficult to execute with the same facility as the rest of the bit, I will (in that instance only) use my "long Bb" with the extra Eb key. It allows for the entire phrase to be played with the same lilting light touch as it has when played on the torture rack soprano oboe.
Other than that (or some other similar circumstance), the only time that I touch it is for playing the extended range Eb (which on both horns, soprano and bass, was placed there only for the transposed E of the every-so-slightly deeper pitched A clarinet) should I be transposing an A clarinet or bass clarinet part.
There is one other use for the low Eb key though, and it's one that I really appreciate. The long B on a horn equipped with a low Eb key always sounds more "consistent" than it does on instruments not so equipped. This is true both of soprano clarinets and of the occasional "old school" bass clarinet that I have owned or experimented with. Putting this note through a tone hole rather than through the bell really smoothes out the "clarinet" register.
Having said all of that, I agree that having some of the throat tones duplicated at the bottom of the "clarinet" register is certainly an excellent idea. We already have that done for us with the B and C at the top end of the "chalmeau" register (register and side trill key fingerings), even though (again) the tone quality leaves something to be desired.
(Well, as perceived by the player of the instrument, at least. I have had impromptu "taste tests" held to see if "non-clarinet people" can detect the difference - sort of a blind test only with ignorance (on the part of the listeners) taking the place of the screen. They can't tell the difference, folks, regardless of the fingering. So, like breath noise (so easily perceived by the player (and on closely miked recordings), but rarely heard by the listener, it may all be in our heads.)
My guess is that makers of "full Boehm clarinets", the few of these that have ever existed (and the makers of the analogous modern bass clarinets of every stripe), have not spent that much time tuning and fiddling with the low Eb note on these horns to make it also as functional as a Bb in the middle of the staff with the addition of the register key. The difference between a low E and low Eb is there in all cases (but see below), slight though it may be, and the difference between the twelfths of these notes (mid-staff B and Bb) is much more striking. But, spending the time equalizing and grinding when it isn't (in the eyes of the maker) really all that important for producing a note that is already pretty good played in the normal fashion.
(In this, it is similar to the alternative Eb key for the LH little finger. When it is provided, it simplifies things a lot, but a lot more get by (due to their training and tradition) without it pretty well.)
Other examples of this exist as well. The aforementioned C and B with the "trill keys" are one. These notes come through about as well as they can, but they are never (in most cases) as good as the "long" fingerings for the same note. (But yet again, see below) On the saxophone, the much hated (by me, anyway, with my huge ham hands) palm keys provide another example. Sure, you can play a mid-staff D with the first palm key, but few serious sax players will do so when it's possible to sub in the "long" D in its place. As for the saxophone, so too for the clarinet (for some of us).
What's the point of all of this. Well, it filled in my time before breakfast this morning, so that alone works for me. And, it may piss off some of you - like the grain of sand that starts the formation of a pearl - irritation has its rewards. I know that the "flat" response of the horn when playing a throat Bb the normal way has irritated me for nigh on half a century now, and the solution is right there in front of us if only the makers would take note and the teachers would recognize it for what it is.
Anyway, enough bitching on my part; the notes follow.
NOTES ON ABOVE FOLLOW:
Regarding the differences between C, B and the "faked" Bb afforded by a low Eb key:
Clarinet players are not as aware of this as are saxophone players (who also play clarinet). The temptation to put that RH little finger down to play C in the staff is always there in the back of my mind. Hell, it works on the clarinet - why not on the sax. Sax players (who have never played clarinet) don't give this a second thought. Nurture versus nature, I guess.
Regarding the difference between low E and low Eb:
This is rare enough (due to the paucity of "full Boehm" instruments) that most don't worry about it. On the bass clarinet, it is pronounced (but not a real issue, due to the rarity of parts pitched in A in most player's universes).
It comes from the perceived difference in the liberation of the sound from the low E on the bottom joint of the instrument, while the low Eb issues from the different bell. I have never been able to get another bass clarinet player (who could actually play the horn as well as I) to play these two notes while I had my head down between their knees to listen up close and personal at the point where the sound was issuing from the horn. (I wonder why this was?)
However, on an extended range bass, the difference is pronounced. There, since everything from the low E down to low C# comes from the side of the horn, there is much more perceived consistency. (The low C, issuing from the bell, has the same problem as the low Eb on the smaller horn.)
And, I have noticed (but have not had an opportunity to evaluate first hand) the practice of some makers (Buffet for one) to have the low Eb issue from a tone hole (usually situated at the beginning of the bell), instead of through the bell. This has been a practice of some German makers for many years now, and it is nice to see the practice bleeding over into the French world. All too often, the practices of the German clarinet world have been ignored by the French school of playing - it's high time that this stops.
Regarding the difference between trill fingering C and B and their "clarinet" register analogs:
While it is hard to evaluate this adequately on a soprano clarinet, the owner of an extended range bass clarinet can try this experiment: finger the low Eb, D, Db and (barely) C with the addition of the double register key. You will find that you can choke out the equivalent Bb, A, Ab and G of the "chalmeau" register of the horn, although the tones get progressively less desirable as you descend the scale.
But, if you have access to a third hand, you may be able to finger the same notes down on the bottom joint with some side trill key on the upper joint (it varies from horn to horn) in place of the register key, and the Bb through G will come through much clearer (although still not perfect). This is because you have added an additional "register vent" further down on the instrument that allows these long notes to speak clearly, just as every double register key bass clarinet maker has already provided on the more highly developed bass clarinet.
The location and size of the register vent on a "bog standard" soprano clarinet (and on a "student model" bass clarinet) has been determined by a series of compromises. It has to work both as a tone hole for the stuffy "pinch" Bb, as well as the vent for a whole series of tones ascending upwards on the horn, from the middle of the staff (where it acts for the entire tube of the clarinet) all the way up to high E (when used for various alternate fingerings ascending from high C above the staff). Really, considering all that it has to do, the clarinet makers have done a wonderful job with stealing a little facility here, a little tone clarity there, and a little positional cleverness there. (This last can best be seen on a single register key basset horn - that key arm is one long, thin piece of metal.)
However, craft and artifice can only go so far. Once their abilities have been exhausted, complexity has to take their place. One striking example of this is the "double register key" that is required on any working bass clarinet, "pro" or "student".
(I have seen a single register key bass clarinet, although it was not in playing condition. I don't envy those few who chose to expose themselves back before Sax thought of the double key - just coaxing out the tones below G above the staff when playing in the "clarinet" must have required a lip to do justice to a Hapsburg prince.)
The double key register systems trade complexity for compromise, since the limits of compromise have been reached in such a horn (probably due to bore diameter - I leave it to the scientific among you to explain this). Instead of one register vent opening high up on the body, they add a second vent. Then, the upper vent can be sited and sized to act only as the vent for those notes for which it is designed, while the lower opening serves for notes lower in the scale.
"Student" horns differ in that the upper vent is still called upon to handle the entire "clarinet" register on the instrument, while the lower one only comes into play with the middle Bb. The mechanism is not so much less complicated for this, but it does avoid the long and exposed linkages on the back of the instrument, so easy to damage when in the hands of a student player (or non-player).
With this (and the equally clever systems on the saxophone - thank you Uncle Adolph for inventing this cleverness, wherever you are), there has been laid down a blueprint for further improvements. Maezzo noted the superiority of the trill key for B as a vent for the throat Bb and moved in that direction. (His "error", if you can call it that, was to depart from tried and true clarinet technique (with his "improved" fingering system), instead of just going the automatic key method (like on a "student" bass clarinet). Selmer had a different approach, which went so far as to add another vent (this up on the barrel) to improve performance and intonation in the "altissimo" register. We all "half-hole" above high C (or, in the case of bass players, "finger roll").
And Don Berger, the patent expert over on the Clarinet boards, could probably cite a dozen or more similar efforts by others to fix the problems. All have failed, partially because of "tradition" and partially because of expense/complexity.
Me? Well, I'm happy enough that the two hole approach was taken on the bass clarinet. I can live without automatic provisions for the register vents (but still really appreciate the automatic keywork on both the bass and the saxophone). And, I hardly ever play the throat Bb without using the trill key as the second tone hole (in place of the register key). Somebody ought to do something about that...