Overtone Singing by Anna-Maria Hefele.
Overtone singing is known from the Mongols.
Anna-Maria Hefele combines it with Western classical music.
Explains the concept:
Sings Mozart – in an MRI scanner. She cut her hair for the experiment. She is not ill.
Recording was done in Freiburg. university website
What she does is , while singing, she shapes a resonant band pass filter with her tongue that amplifies a part of the overtone spectrum of her voice.
Male mongol singers use a guttural deep voice, leading to a very dense spectrum of harmonics, allowing them to have many more possible frequencies to amplify than there are notes on a piano.
Here she shows the spectrum.