“Overtone singing is when a vocalist sings two different notes at the same time. It sounds like something a higher form of intelligence (think Aliens, but without the scariness) might have to try and communicate with earth. The mid and high range keys are in harmony with each other, yet there is a resemblance of some form of old time radio wave.”
Listen here! http://www.cmuse.org/anna-maria-hefele-does-overtone-singing/