Found this really interesting:
In Mongolia, in the Republic of Touva, in certain monasteries of Tibet and among the Xhosa people of South Africa, one can hear an amazing vocal technique in which one singer can produce two stable notes at the same time while a third note varies on top. In other words, one person can sing in chords and can also provide a melody. The lowest note is so extremely low it sounds hoarse and raucous, stretching the limits of the human voice.
The vocal technique is based on a very precise control over the vocal resonators so as to amplify specific overtones. The vocal cords are used in a special way that allows them to vibrate at two different frequencies at the same time. Western voice specialists are still perplexed by this technique.
THINKING OF STAYING IN MONGOLIA AND LEARN IT, I AM SURE IT WILL BE SOMETHING FASCINATING. STILL TO FIND A SAMPLE OF IT.