Those who are unfamiliar with the term polyphonic overtone singing will be utterly astounded with what it refers to. It refers to the capability of a person producing two or more singing notes at the same time. A choir unto him- or herself, polyphonic overtone singing is a vocal range that even Mariah Carey would be envious of.
Demonstrating her polyphonic overtone singing abilities is Anna-Marie Hefele, a German singer whom blogger Marilyn Bellamy aptly describes as "a human theremin." Neatorama writes, "When she performs, she can pair notes however she wishes in a vast variety of combinations. The musical effect is stunning and unlike anything I've ever heard from a human voice." Otherworldly is a great way to summarize her polyphonic overtone singing skills.