Anna-Maria Hefele is a German singer who specializes in polyphonic overtone singing, a technique that allows her to produce two or more notes at the same time with her voice. She has been practicing this technique since 2005 and has written compositions and arrangements for polyphonic solo voice. She also teaches workshops and online courses on overtone singing and has performed in various concerts and festivals around the world.

Polyphonic overtone singing is based on the principle that every sound consists of a fundamental frequency and a series of higher frequencies called overtones or harmonics. By manipulating the shape of the vocal tract, the singer can amplify certain overtones and make them audible as distinct pitches. The result is a fascinating sound that resembles a combination of singing and whistling.

Anna-Maria Hefele demonstrates this technique in her YouTube videos, where she explains visually how she produces different overtones and how she changes them while singing. She also shows how she can sing in parallel with herself, creating harmonies and melodies with her voice alone.

Anna-Maria Hefele is one of the few singers who can master this complex and rare technique of polyphonic overtone singing. She has developed her own style and repertoire, blending traditional and contemporary elements.