Method
Vocal fry is created by narrowing the glottal closure and dropping to the lowest vocal range. In doing so, the voice is emitted as a low-pitched, raspy croak. This can in turn be projected through the tuba or euphonium, as with the normal singing voice.
Necessary information
Given the tensioned nature of this technique, it can be fatiguing (if not damaging) to sustain this technique at loud dynamics. Additionally, the mouthpiece rim opening can serve to limit the volume of this technique by in turn limiting the ability of the mouth to open and project properly. As a result, this technique is often best utilized at lower dynamics, although, as is usually the case, this can be stretched in certain cases.
Many performers may find it difficult or painful to perform this technique, as the proper method for producing it can be difficult to achieve while keeping a seal on the mouthpiece. Additionally, this is not something that many people can achieve consciously, so it is advised that the composer check first with any intended performer (or, if there is no intended performer right away, the composer is well-advised to use this technique sparingly).
Notation
The majority of the times that the author has encountered vocal fry in a score, it has been notated as a generic vocal effect with either the direction “vocal fry” above the staff, or with a similar notehead dedicated to just the use of fry. This includes X-head and hollow noteheads. As always, the key is to be consistent and clear with the notation being used in a particular piece of music.
Relative Difficulty
Professional
Works to consider (bolded titles are particularly representative examples of this technique)
Tube space – Dmitri Kourliandski
Digestion of Memory – Élise Roy
The Composer's Guide to the Tuba 