Voice source, formant frequencies and vocal tract shape in overtone singing. A case study by Johan Sundberg, Björn Lindblom & Anna-Maria Hefele, published in Logopedics Phoniatrics Vocology, 2021

One Singer, Two Voices – Johan Sundberg, Björn Lindblom, and Anna-Maria Hefele – published in Acoustics Today, 2021

Polyphonic Overtone Singing: an acoustic and physiological (MRI) analysis and a first-person description of a unique mode of singing
written by Anna-Maria Hefele, Robert Eklund, Anita Mc Allister, 2019
Link to official publication >here

Obertongesang- eine besondere Stimmtechnik als Kunstform 
veröffentlicht in: Vox Humana, 01/2017, written by Anna-Maria Hefele

Western Techniques of Overtone Singing and their Application in Classically Notated Music 
published in: International Choral Bulletin, 01/2015, written by A-M Hefele

Westliche Obertongesangstechnik und ihre Anwendung in der klassisch notierten Musik
veröffentlicht in: International Choral Bulletin, 1/2015, von A-M Hefele

Eine neue Dimension für die Stimme
von Eva Runkel, veröffentlicht in der Süddeutschen Zeitung, 19. 10. 2018

Voice ranges of Anna-Maria Hefele in overtone singing
green notes work well, red ones are impossible, orange does work, but doesn’t really sound good.

Overtone focusing in biphonic tuvan throat singing
written by Christopher Bergevin & associates, 2020
Link to official publication >here

Speech Biomechanics: Shaping new sounds
this is an introduction article to: “Overtone focusing in biphonic tuvan throat singing”; written by Timothy D Griffiths & associates
Link to official publication >here

Sheetmusic: arrangements in polyphonic overtone singing for female voices (singable by male voices as well) by Beate Eckert and Barbara Lübben. The collection contains well-known folk songs that can be sung over two or three fundamental notes and is available for download for free: >here.

Here are more resources about overtone singing available to download for free on Wolfgang Saus’ webpage, including voice ranges for male voices.

The software I use to make sound visible is called Overtone Analyzer by Bodo Maass, it can be downloaded >here

Very interesting is the DVD “The Voice” by the institute for musician’s medicine, where I participated as overtone singer for MRI recordings. Published at Helbling in 2017; available >here.