As with all the brasswind instruments, articulation on the tuba and euphonium is achieved by interrupting the air stream with the tongue while buzzing into the instrument. Compared to the smaller instruments like the trumpet and horn, the tuba family of instruments require a much larger amount of resonant space within the oral cavity. It is thus of great importance that tubists and euphonists from a young age learn to keep the tongue lowered within the oral cavity. When tonguing, the tip and/or front part of the tongue usually strikes around the point where the teeth and roof of the mouth meet. As the performer moves into the higher range of the instrument, the tongue usually strikes farther back on the roof of the mouth. Conversely, when playing in the low range of the instrument, the tongue moves forward in the oral cavity; in the lowest ranges, and especially on the tubas, the tongue may even be striking the tip of the teeth and the lips themselves. These physical distinctions are not fixed, though, and depend almost entirely on the individual physiology of the performer. Although the exact physiological composition of experienced performers varies wildly, the end result (clear articulation) is primarily an auditory issue, and as such may be accomplished by many similar yet slightly-different physical methods.
Compared to its higher-pitched relatives in the brasswind family, the tuba and euphonium articulation process takes quite a bit more physical effort to achieve the same result. This does not mean that fast, clean articulation is impossible: as a general rule, tubists and euphonists both develop the skill to cleanly and rapidly articulate individual notes, regardless of range. The main limitation is less the amount of effort required, and more the simple acoustical issues facing the euphonium and tuba, (especially the latter); at a certain point, no amount of clean articulation can make the low frequencies of the pedal range speak more quickly than is possible given the laws of acoustics.
Brasswind players almost uniformly learn to single, double, and triple tongue, and one of the goals of the professional brass player is to have a clean amount of overlap in speeds between the three styles of tonguing. The single tongue is often denoted by the “dah” or “toh” syllable (an onomatopoeia representing the sound made when speaking this type of articulation), while the double tongue is achieved by adding a second articulation further back on the roof of the mouth (“dah-gah”, “toh-koh”, or similar). The triple tongue can be represented with two different techniques: by two forward articulations with a backward articulation in the middle (“dah-gah-dah”), or two forward articulations followed by one backward articulations (“dah-dah-gah”). The choice of which to use is left to pedagogical preference, and in professional usage, there is no difference between the two methods of triple tonguing.
It bears repeating the issue of articulation in the extreme low range of the tuba and euphonium. Although this issue is less extreme with the euphonium, the use of rapid, detached articulations in the low range of the tuba can become a problem in certain contexts. The low frequencies of the tuba pedal range (reaching down nearly to the threshold of discernible pitch) are ill-suited to rapid, detached performance, unless a non-pitched rhythmic sound is desired. At the same time, the amount of air required to perform at an adequate volume in the pedal range of the tuba and euphonium both makes sustained, detached articulations difficult in many circumstances. If such a style of performance is desired within the low range of the tuba, it is the author’s suggestion that either 1) the composer be prepared to lower the speed of the section of the work in question, or 2) that the composer acknowledge the slim likelihood of such a figure being accurately reproduced.
The Composer's Guide to the Tuba 