What are overtones?

Overtone singing involves the highly refined articulation of vowels, resulting in audible overtones in the form of quasi-autonomous whistling tones. Depending on the position of the mouth, certain overtones are formed which we automatically recognize as vocals (vowels).
Try, for example, singing a ‘u’, keeping the tone constant. You should able to distinguish an overtone quite easily, in the form of a rather sharp whistling tone. Now change this u sound into an ‘ooh’ very gradually, and you will find the sound becomes darker while the overtones get lower. A gradual change from a u to an e, however, will give the opposite effect. In principle, all the vowel sounds can be used to create overtones.
Reciting a text using the same single note or a simple melody will always create overtones. Examples of this include Gregorian singing or the recitation of mantras. Certain ethnic traditions also use overtones in music, such as the Mongolians and the Tuvan people in Siberia.
The European tradition of overtone music is somewhat more recent, dating back to Karl-Heinz Stockhausen’s work ‘Stimming’ from 1968. Having been introduced to the phenomenon of overtones by Stockhausen, Michael Vetter went on to develop a number of overtone techniques suited to the European voice.

THE OVERTONE SEQUENCE

The physical structure of overtones is rather different than our own hearing would lead us to believe. The sequence of frequencies looks like a multiplication table: if the fundamental note has a frequency of 100 Hertz, all multiples of 100 Hz are potential audible overtones. If – for example – the fundamental note, number 1, has a frequency of 100 Hz, number 2 will have a frequency of 200 Hz, and so on. In this example, the distance between two consecutive overtones will always be 100 Hz. Overtone number five will thus have a frequency of 500 Hz, while number 16 will have a frequency of 1600 Hz.
c c g c e g b flat c
100 200 300 400 500 600 700 800

The human ear does not experience these intervals as proportionate. To our ears, octaves (c-c-c-c) sound like proportionate distances. In reality, however, an octave is a doubling in frequency; the octave of 100 Hz is 200 Hz, but the octave of 200 Hz is 400 Hz. The higher we move up the overtone scale, the more overtones the human ear actually hears, which means the intervals between the overtones become increasingly smaller. This is illustrated by the notated overtone sequence on page 00. From the lower staff upwards, the notation shows how many overtones go in an octave. The number of overtones becomes increasingly greater; the first overtone octave (from number 1 to number 2) does not contain any overtones; the second octave (from 2 to 4) contains overtone number 3; the third octave (from 4 to 8) contains three overtones, and so on.
Moving up the scale, every subsequent interval is smaller than the last, while every higher octave contains more overtones than the previous one. As a result, the overtone sequence is a scale unlike all others, which simply repeat the same notes in every octave.

Scientists about overtones