The Resonance Tube
The "resonance tube" is a device found on all modern clarinets, covered by the register key pad.
The resonance tube has gone through some changes over the years. Let's check it out!
A unique aspect of the clarinet is that it has not one, but two holes on the rear of the instrument - one covered by the thumb, and the other covered by a key operated by that same thumb.
Early clarinets had a rear register key that covered a hole that was cut flush, directly into the body, like any other tone hole. The register key was in this way just like any other key on the instrument, and enjoyed no special treatment.
Instrument making technology improved as the years progressed, and more and more keys began to be added. One of the developments that came with that was the introduction of the resonance tube.
The reason for its existence is due to a simple compromise between a correctly sized hole and a clear upper register, as will be explained soon.
This tube is longer than the body wall is thick, forcing it to either protrude in or out of the bore.
The resonance tube came into play when so many keys had been added that the so-called "Simple System" key system was instituted. This precursor to the Albert System pioneered a register key that protruded out of the clarinet bore. In addition, the hole for this was in front of the clarinet, and was closed by a "wrap-around" register key, which (usually) gracefully curved its way around the body of the instrument to the front. This out-protruding resonance tube with wrap-around register key was the new golden standard, and was used in all Simple System, Albert System, and some earlier Boehm System clarinets.
The reason for the outward protrusion was due in large part to intonation concerns. Technology had improved so much that actual scientific care could now be taken towards producing clarinets with the best and most accurate intonation possible. It was discovered that a flush register hole would produce throat-tone notes that were slightly too sharp. In order to correct this, the protruding resonance tube was invented, allowing the throat-tone notes to be lowered into tune. In a perfect world, the Bb tone hole, which is first note that can be played with use of the register key, would be as large as the A or Ab tone holes. Unfortunately, if this was done, the upper register would exhibit an airy and unpleasing tonality. In order to compromise, a tube was added to the register hole in order to create the "illusion" of a larger tone hole for the Bb. The tube - which was still too small in diamater to really "work," helped the Bb be played in tune and still sound (relatively) pleasing. Since the physical hole was still narrow, the upper register could be played clearly.
It essentially came down to this: either the clarinet has one "bad" note, or an entire register of "bad" notes. The brains chose to sacrifice the one for the good of the whole.
The position of the register hole was moved from the back to the front due to physical concerns - it was unfeasible for a rear-hole to be reasonably opened by the thumb. Simple lever physics meant that the only way for the hole to be able to be opened far enough with the thumb was to move the hole to the front, allowing the thumb to have more leverage to lift the key higher.
It is unknown why the resonance tube extended outward instead of inward.
By about 1920, the wrap-around register key was all but dead, possibly due in some part to the fact that the Boehm System had almost fully replaced the Albert in most Western settings. Although early Boehm System clarinets often had wrap-around register keys, by the mid-1920s that practice was all but extinct.
Instead, the register key returned to the rear of the instrument - back to its roots. And instead of the resonance tube protruding out of the body, it now protruded inwards. It is unknown exactly why this occurred. Perhaps it had to do with the fragility of a long, curvy key that had a tendency to easily get bent if the musician was not careful in assembling or transporting the instrument. Or perhaps it was because it was more difficult to keep the key in good adjustment.
Whatever the reason, the key now returned to the back. This time, the brains decided to push the resonance tube into the bore of the instrument. This allowed the key to be reasonably actuated by the thumb while retaining its position on the rear of the clarinet. This key was now shorter, stronger, and much less likely to be bent out of adjustment.
However, in spite of all of the advances, the "bad Bb" has never been fully solved on a large scale. Even with the longer resonance tube, the simple physics dictate that the Bb will always sound more airy and less pleasing than almost every other note on the clarinet.
There have been several attemptes to fix this while retaining the integrity of the rest of the clarinet. All have failed to catch on. To read more about these failed - but brilliant - attempts to reform the Bb, please check out these articles: