Method
The oral cavity can be adjusted in many different ways to affect the sound coming out of the instrument. By tightening or overly widening the space inside the oral cavity, the sound coming out of the tuba/euphonium likewise becomes narrow or diffuse in timbre. Additionally, the use of different vowels imparts different harmonic imprints to the resultant sound.
Necessary information
In standard performance, the tubist and euphonist both strive for a generally open and rounded “oh” shape inside of the oral cavity. Adjusting this shape can seem uncomfortable at first, but with only a small amount of practice this becomes a viable and achievable technique.
Both the tightening and widening of the oral cavity are both acceptable to perform as part of a composition, but the extensive use of these techniques can quickly lead to jaw fatigue and/or injury to the muscles of the embouchure.
Notation
Given this technique’s use as an overlay effect on top of other modes of performance, it is wise to pick a notation that can be combined with standard staff-based notation. For vowel modification, this can simply come in the form of a listing of the desired vowel shapes above the staff. For general vowel shape, some sort of overlay notation is acceptable, assuming it is clear in its intent. As with vowel modification in vocal music, it can be helpful to utilize the International Phonetic Alphabet for notating the intended symbol. It must be noted, though, that tubists/euphonists do not receive training in the use of IPA, and any use of this notation must be explained clearly and thoroughly within the notes for the work.
Relative Difficulty
Beginner to Advanced
Works to consider (bolded titles are particularly representative examples of this technique)
Tube space – Dmitri Kourliandski
Vox superius – Melvyn Poore
Three Essays – William Penn
The Composer's Guide to the Tuba 