Method
Contact mics are an effective tool in the amplification of a wide variety of mechanical sounds on the tuba and euphonium. By placing them on the body of the instrument itself, the myriad sounds that are normally inaudible to the listener are amplified and made audible. This includes valve noise, air sounds, and resonant frequencies brought out by the metal of the instrument. They can be placed in a number of locations, but due to its status as the main resonator of the instrument, are often placed on the bell.
A related technique involves using a wooden practice mute and placing the contact mic on the surface of the mute. This lets a relatively clean signal of the tuba/euphonium through, while minimizing the audibility of the acoustic sound of the instrument. As such, it is a very useful technique for the use of analog effects pedals.
Necessary information
In the author’s experience, the best results are achieved with using a powered contact mic. This has a greater amount of sensitivity, but that comes with the risk of a very high amount of gain. Non-powered contact mics do work well, though, and are a perfectly suitable tool for this kind of work.
When fastening the mics to the body of the tuba/euphonium, it is best to use a non-residue-leaving adhesive like painter’s tape or scotch tape.
Notation
The notation of contact miking methods is not usually a concern, but if this technique is notated, the usual rules for clear and consistent methodology apply. This is due to the simple fact that contact mics are almost always placed beforehand, and left on during performance. If that is not the case, any clear notation that indicates the movement of the mics is recommended. It may be helpful also to draw diagrams indicating the place to fasten the contact mics; as usual with such specificity, though, it is once again important to remember that not all tubas have the same shape.
Relative Difficulty
Professional
Works to consider (bolded titles are particularly representative examples of this technique)
The Composer's Guide to the Tuba 