In the practice of overtone singing, which is best known in Mongolia and Southern Siberia, a single person sings two voices at the same time : a low-pitched sound with his vocal cords and in addition a melody using selected harmonics in the buccal cavity.
An introductory course conducted by Tran Quang Hai, a concert and the interviews of Mongolian singers, X-ray and spectrographical pictures, in real time and synchronous sound, show how this unique vocal technique operates.