Multiphonics is a term that describes extended techniques that involves production of several voices on monophonic musical instruments. In brass instruments the most commonly used multiphonic technique is playing one note while singing another note, thus producing one or more additional notes, here referred to as summation tones and difference tones, and these tones will represent specific frequencies depending on the played and sung notes. The sum of the frequencies of the sung and the played note constitutes the summation tone. The subtraction of the lowest from the highest note constitutes the difference tone.
There are several ways to notate multiphonics. In some works one can find the whole chord notated, including summation and difference notes. In these cases it is important to note which note that are supposed to be played and sung. The summation and difference tones may then be notated as dots, the sung note as a small note, and the played note as a normal size note. The easiest way to read multiphonics is if only the played and sung note are present in the score, and normally with the instruction “hum” by the first sung note.
The execution of multiphonics can be demanding for a brass player. One important consideration is airflow. It is advisable to use less air because the singing voice is normally not as loud as the euphonium sound, and this affects the balance of the chords. Playing softer can affect the intonation, and it is normally advisable to play with a bit firmer embouchure. It is also better if one draws the voice towards a euphonium like timbre, normally with a bit darker sound than usual.
The rigorous work with multiphonics in the Wikiphonium project revealed a great deal of discrepancy between theory and practice in this field. For instance are the summation tones often louder than the difference tone. It is also difficult to play some of the multiphonics combinations because the voice also divides the partials in the instrument, and this can conflict with the played note.
It seems as if the quality of the voice affects the multiphonic timbre, and the summation tones are more prominent if the voice is as close to the euphonium timbre as possible. The performer needs to adjust voice timbre, the speed of air, balance, embouchure, and the opening of the throat in order to balance a multiphonic chord.
These considerations has led to the conclusion that it is necessary to present an excessive number of examples of multiphonics in order to be able to have an overview over the great many possibilities of compository uses of multiphonics.